Force, Mass, and Motion

142 questions

All types142 Multiple choice73 True / False11 Numeric17 Matching27 Ordering14

In order for a heavy freight locomotive to maintain a constant, steady speed along a straight, level track, the forward pulling force it exerts must be greater than the combined forces of friction and air resistance acting against it.

True

False

Show answer & explanation

Answer: False

Answer

False

The statement is false because maintaining a constant velocity requires all forces to be completely balanced (a net force of zero). The locomotive's forward pulling force must exactly equal the opposing forces of friction and air resistance, not exceed them. If the forward force were greater, the train would accelerate.

Step-by-Step Solution

Identify the locomotive's state of motion as described in the scenario.

The locomotive is traveling at a 'constant, steady speed along a straight, level track', which means it has a constant velocity (no change in speed or direction).

Understanding the state of motion is required to determine which of Newton's laws applies to the situation.

Apply Newton's First Law (Inertia) to the state of constant velocity.

An object traveling at a constant velocity must have a net force of exactly zero acting upon it.

Inertia ensures that an object will maintain its current velocity indefinitely unless acted upon by an unbalanced (net) force.

Determine the necessary relationship between the opposing forces to achieve a zero net force.

The forward pulling force must perfectly balance the backward forces of friction and air resistance.

If the forward force were greater than the opposing forces, there would be a net forward force, which would cause the locomotive to continuously accelerate rather than maintain a steady speed.

Key Concept

An object moving in a straight line at a constant speed has a net force of zero acting on it, meaning all applied driving forces perfectly balance all resisting forces.

Estimated Time:45s

Question 82Question →

A heavy transport truck is traveling East on a straight highway at a constant speed of $90 \text{ km/h}$ . The driver applies the brakes, producing a constant deceleration until the truck comes to a complete stop in exactly $10 \text{ seconds}$ . Which of the following correctly describes the truck's average acceleration vector during this braking period?

2.5 \text{ m/s}^2

West

2.5 \text{ m/s}^2

East

9.0 \text{ m/s}^2

West

9.0 \text{ m/s}^2

East

Show answer & explanation

Answer: $2.5 \text{ m/s}^2$ West

Answer

The correct acceleration vector is

2.5 \text{ m/s}^2

West.

To find the acceleration, the initial speed must first be converted from km/h to m/s by multiplying by

1000/3600

, resulting in

25 \text{ m/s}

. The acceleration magnitude is the change in velocity (

25 \text{ m/s}

) divided by the time (

10 \text{ s}

), giving

2.5 \text{ m/s}^2

. Because the truck is braking while moving East, the acceleration vector must oppose the motion, meaning it points West.

Step-by-Step Solution

Convert the initial velocity from km/h to m/s.

90 \text{ km/h} \times \frac{1000 \text{ m}}{3600 \text{ s}} = 25 \text{ m/s}

East.

Standard kinematic equations require consistent metric units, specifically meters and seconds, to yield an acceleration in

\text{m/s}^2

Calculate the magnitude of the average acceleration using the formula

a = \frac{v_f - v_i}{t}

a = \frac{0 \text{ m/s} - 25 \text{ m/s}}{10 \text{ s}} = -2.5 \text{ m/s}^2

The final velocity (

v_f

) is zero because the truck comes to a complete stop.

Determine the direction of the acceleration vector.

The acceleration is

2.5 \text{ m/s}^2

West.

Because the truck is slowing down, the acceleration vector must point in the opposite direction of its velocity (East). The negative sign in the calculation confirms this opposing direction.

Key Concept

Kinematic equations with unit conversions and the vector nature of acceleration

Estimated Time:1m 30s

Question 83Question →

A washing machine's spin cycle rotates wet clothes at high speeds inside a drum with small holes. This process effectively removes water from the fabric. Which of the following statements best explains the mechanical principles involved in this process?

The drum exerts an inward centripetal force on the clothes, while the water escapes through the holes by continuing in a straight line due to inertia.

The high-speed rotation generates an outward centripetal force that actively pulls the water toward the edge of the drum.

The circular motion creates an outward reaction force to balance the drum's spin, pushing the water away from the center.

The water has more mass than the fabric fibers, which causes it to experience a greater outward acceleration during the spin.

Show answer & explanation

Answer: The drum exerts an inward centripetal force on the clothes, while the water escapes through the holes by continuing in a straight line due to inertia.

Answer

The drum exerts an inward centripetal force on the clothes, while the water escapes through the holes by continuing in a straight line due to inertia.

For an object to move in a circle, there must be an inward force (centripetal force). In a washing machine, the solid walls of the drum provide this inward force on the clothes. When water droplets reach the holes, there is no wall to provide an inward force, so the water continues moving in a straight line (tangent to the drum) because of its inertia.

Step-by-Step Solution

Identify the actual forces acting on the objects inside the spinning drum.

The solid drum walls push inward on the clothes.

An inward net force (centripetal force) is required to make any object move in a circular path.

Determine what happens to the water when it encounters a hole in the drum.

The water is no longer pushed inward by the drum wall, so it moves in a straight line tangent to the circular path.

According to Newton's First Law (inertia), objects will continue moving in a straight line unless acted upon by a net force.

Key Concept

In circular motion, centripetal force always acts inward. Objects that lose this inward restraining force will travel in a straight tangential path due to inertia.

Question 84Question →

A heavy steel block must be moved along a level track. Rank the following methods or conditions based on the amount of frictional resistance they generate, from LEAST resistance to GREATEST resistance.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct order from least to greatest resistance is: floating on oil (fluid friction), moving on rollers (rolling friction), sliding continuously (kinetic friction), and overcoming the initial standstill (static friction).

Fluid friction separates the surfaces entirely, offering the least resistance. Rolling friction prevents surfaces from dragging, making it the second lowest. For the dry track, sliding (kinetic) friction is less than the static friction required to break the object's initial grip, making static friction the greatest resistance.

Step-by-Step Solution

Identify the primary type of friction acting in each scenario.

Oil film involves fluid friction; steel rollers involve rolling friction; continuous sliding involves kinetic friction; breaking free from a standstill involves static friction.

Categorizing the physical interaction is required to compare their relative magnitudes.

Determine the lowest sources of friction.

Fluid friction creates the least resistance, followed closely by rolling friction.

Fluids deform easily to allow movement and separate the surfaces entirely, while rollers prevent direct surface scraping.

Compare the resistance of a moving object versus a stationary object on the exact same dry surface.

Kinetic (sliding) friction is always less than the maximum static (standstill) friction.

Once the microscopic bonds between the resting surfaces are broken, it takes less force to keep the object moving than it did to start it.

Arrange the scenarios from least to greatest based on these physical principles.

The correct sequence is: Fluid, Rolling, Kinetic, Static.

This correctly orders the magnitudes of frictional resistance from lowest to highest.

Key Concept

Friction and Resistance

Question 85Question →

A deck winch on a naval vessel applies a constant net horizontal force of $4,500 \text{ N}$ to drag a heavy supply pallet across a level deck. If the pallet has a mass of $1,500 \text{ kg}$ , what is the acceleration of the pallet?

Show answer & explanation

Answer: 3

Answer

The correct answer is 3.

By applying Newton's Second Law (

F = ma

) and rearranging for acceleration (

a = \frac{F}{m}

), we divide the net force of

4,500 \text{ N}

by the mass of

1,500 \text{ kg}

to find an acceleration of

3 \text{ m/s}^2

Step-by-Step Solution

Identify the given information

Force (

F

) =

4,500 \text{ N}

, Mass (

m

) =

1,500 \text{ kg}

Extracting the known variables is the first step in solving a physics calculation problem.

Select the appropriate formula

F = ma

Newton's Second Law relates force, mass, and acceleration.

Rearrange the formula to solve for acceleration

a = \frac{F}{m}

We need to isolate the unknown variable, acceleration (

a

Substitute the values and calculate

a = \frac{4500}{1500} = 3 \text{ m/s}^2

Plugging in the given numbers yields the final numerical answer.

Key Concept

Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (

F = ma

Question 86Question →

A $500\text{ kg}$ steel pipe is suspended by a vertical cable from a motorized overhead bridge crane. The crane carriage and the suspended pipe are traveling horizontally along a straight track at a constant speed of $2\text{ m/s}$ . If the crane operator suddenly engages the emergency brakes and the crane carriage stops instantly, what will initially happen to the suspended steel pipe?

It will continue moving forward, swinging in the original direction of travel, due to its inertia.

It will swing backward in the opposite direction of travel as a reaction to the crane's braking force.

It will stop moving horizontally, but its downward weight will momentarily increase.

It will stop immediately with the crane because its large mass causes it to naturally decelerate faster.

Show answer & explanation

Answer: It will continue moving forward, swinging in the original direction of travel, due to its inertia.

Answer

The pipe will continue moving forward, swinging in the original direction of travel, due to its inertia.

According to Newton's First Law, an object in motion will stay in motion at a constant velocity unless acted upon by an external net force. The emergency brakes apply a stopping force to the crane carriage on the track, but not directly to the suspended pipe hanging below. Therefore, the pipe's mass strongly resists the change in velocity (inertia), causing it to continue moving forward and swing in its original direction of travel.

Step-by-Step Solution

Identify the state of motion before the brakes are applied.

Both the crane carriage and the suspended pipe are moving horizontally at a steady speed of 2 m/s.

Establishing the initial conditions is necessary to properly apply Newton's First Law.

Determine where the stopping force is applied when the emergency brakes are engaged.

The braking force is applied directly to the crane carriage on the track, causing the carriage to stop abruptly.

To predict the motion of the suspended pipe, we must identify if an external horizontal force acts directly on it.

Apply Newton's First Law of Motion (Inertia) to the suspended pipe.

Because no direct horizontal stopping force is applied to the pipe, its inertia causes it to resist the change in motion and it attempts to continue moving forward at 2 m/s.

Newton's First Law states that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Evaluate the resulting physical behavior of the pipe.

Since the crane carriage stops but the pipe continues forward, the flexible cable allows the pipe's continued momentum to manifest as a forward swing, like a pendulum.

Visualizing the mechanical connection explains the exact physical outcome resulting from the object's inertia.

Key Concept

Newton's First Law (Inertia)

Question 87Question →

A high-speed rail train is traveling at an initial velocity of $216 \text{ km/h}$ . To safely navigate a sharp curve, the train must reduce its velocity to $108 \text{ km/h}$ . If the train applies its braking system to produce a constant deceleration of $1.5 \text{ m/s}^2$ , how many seconds will it take to reach the required speed?

Show answer & explanation

Answer: 20

Answer

It will take 20 seconds for the train to reach the required speed.

The correct answer is found by first converting both velocities to meters per second to match the acceleration's units.

216 \text{ km/h}

is equivalent to

60 \text{ m/s}

, and

108 \text{ km/h}

is equivalent to

30 \text{ m/s}

. The change in velocity is

-30 \text{ m/s}

. Using the formula

t = \Delta v / a

, dividing

-30 \text{ m/s}

by the deceleration of

-1.5 \text{ m/s}^2

results in exactly

20 \text{ seconds}

Step-by-Step Solution

Convert the initial velocity (

v_i

) from km/h to m/s.

216 \text{ km/h} \div 3.6 = 60 \text{ m/s}

Standardizing units to match the acceleration given in m/s^2 is required before calculation.

Convert the final velocity (

v_f

) from km/h to m/s.

108 \text{ km/h} \div 3.6 = 30 \text{ m/s}

All velocity variables must be in meters per second to be compatible with the acceleration.

Calculate the change in velocity (

\Delta v

\Delta v = 30 \text{ m/s} - 60 \text{ m/s} = -30 \text{ m/s}

Acceleration is the rate of change of velocity, so finding the total change is the necessary intermediate step.

Apply the acceleration formula

a = \frac{\Delta v}{t}

to solve for time (

t

t = \frac{-30 \text{ m/s}}{-1.5 \text{ m/s}^2} = 20 \text{ seconds}

Dividing the total change in velocity by the rate of deceleration yields the elapsed time.

Key Concept

Kinematics: Calculating time from constant acceleration and velocity change while managing unit conversions.

Question 88Question →

When a solid object is swung in a circular path on a tether and the tether suddenly snaps, the object will immediately fly directly outward, straight away from the center of the circle.

True

False

Show answer & explanation

Answer: False

Answer

False

The statement is false because an object released from circular motion will follow a straight trajectory tangent to the circle, dictated by its instantaneous velocity, rather than flying radially outward.

Step-by-Step Solution

Identify the forces acting on the object before the tether breaks.

The tether provides a continuous inward force (centripetal force) that constantly changes the object's direction, keeping it in a circular path.

Circular motion requires a net inward force directed toward the center of rotation.

Analyze the state of the system at the exact moment the tether snaps.

The physical connection is broken, causing the inward centripetal force to instantly drop to zero.

The tether is the only source of the inward force in this scenario.

Apply Newton's First Law of Motion (inertia) to determine the object's resulting path.

The object will travel in a straight line at a constant speed in the exact direction it was moving when the tether broke.

Without a net force acting upon it, an object in motion stays in motion in a straight line, which is mathematically tangent to the edge of the circle.

Key Concept

Inertia and Tangential Velocity in Circular Motion

Question 89Question →

A mason is pushing a solid, uniform rectangular clay brick across a horizontal wooden scaffold plank at a constant speed. Initially, the brick is positioned so it slides while resting flat on its widest face. If the mason stands the brick up so it slides on its narrowest face across the same section of the plank, how will the kinetic frictional resistance change?

It will remain unchanged because kinetic friction depends on normal force and surface materials, not contact area.

It will decrease because there is less surface area rubbing against the wooden plank.

It will increase because the brick's total weight becomes concentrated, increasing the total normal force.

It will decrease because altering the brick's orientation reduces its resistance to changes in motion.

Show answer & explanation

Answer: It will remain unchanged because kinetic friction depends on normal force and surface materials, not contact area.

Answer

The kinetic frictional resistance will remain unchanged.

For standard rigid objects, the kinetic frictional force is determined solely by the nature of the materials in contact (the coefficient of friction) and the total force pressing them together (the normal force). Because the brick's mass and the materials involved do not change when it is rotated, the overall frictional resistance remains exactly the same, regardless of how much surface area is touching the plank.

Step-by-Step Solution

Identify the formula for kinetic friction.

F_k = \mu_k \times N

To determine exactly which physical variables affect the frictional force.

Analyze the normal force (

N

The normal force equals the weight of the brick (

N = mg

), which remains constant regardless of its orientation.

To see if rotating the brick changes the total downward force pressing the surfaces together.

Analyze the coefficient of kinetic friction (

\mu_k

The coefficient remains constant because the materials (clay and wood) and their surface properties have not changed.

To verify that the fundamental physical interaction between the two specific surfaces remains the same.

Evaluate the effect of surface contact area.

Surface area is not a variable in the standard kinetic friction formula.

To conclusively confirm that resting the brick on a narrower face does not alter the total frictional resistance.

Key Concept

Independence of Friction from Surface Area

Question 90Question →

A combat engineer squad is using a motorized winch to pull a $2,500 \text{ kg}$ concrete barrier horizontally across rough ground to construct a defensive position. The winch applies a constant tension of $12,000 \text{ N}$ to the towing cable. If the friction between the barrier and the ground opposes the motion with a force of $7,000 \text{ N}$ , what is the acceleration of the barrier?

0.5 \text{ m/s}^2

2.0 \text{ m/s}^2

4.8 \text{ m/s}^2

0.2 \text{ m/s}^2

Show answer & explanation

Answer: $2.0 \text{ m/s}^2$

Answer

2.0 \text{ m/s}^2

The correct answer is found by first calculating the net force, which is the applied tension minus the opposing friction (

12,000 \text{ N} - 7,000 \text{ N} = 5,000 \text{ N}

). Then, using Newton's Second Law (

a = \frac{F}{m}

), the net force is divided by the mass of the barrier (

\frac{5,000 \text{ N}}{2,500 \text{ kg}}

) to yield an acceleration of

2.0 \text{ m/s}^2

Step-by-Step Solution

Identify the applied force and the opposing force acting on the barrier.

Applied Force =

12,000 \text{ N}

, Opposing Friction =

7,000 \text{ N}

To find the net force, all horizontal forces acting on the object must be accounted for.

Calculate the net force (

F_{net}

) acting on the concrete barrier.

F_{net} = 12,000 \text{ N} - 7,000 \text{ N} = 5,000 \text{ N}

Net force is the vector sum of all forces; opposite forces are subtracted.

Apply Newton's Second Law (

F = ma

) to find the acceleration.

a = \frac{F_{net}}{m} = \frac{5,000 \text{ N}}{2,500 \text{ kg}} = 2.0 \text{ m/s}^2

Acceleration is directly proportional to net force and inversely proportional to mass.

Key Concept

Newton's Second Law (F=ma) and Net Force

Question 91Question →

When a person pushes forward against a stalled car to move it, the force the person exerts on the car is greater than the force the car exerts backward on the person.

True

False

Show answer & explanation

Answer: False

Answer

False

The statement is false because Newton's Third Law states that the mutual forces of action and reaction between two interacting bodies are always equal in magnitude and opposite in direction. The person and the car exert exactly equal forces on each other.

Step-by-Step Solution

Identify the two interacting objects in the given scenario.

The two interacting objects are the person pushing and the stalled car.

Newton's Third Law describes the mutual forces between two interacting bodies.

Determine the action and reaction forces occurring between the objects.

The action force is the person pushing forward on the car. The reaction force is the car pushing backward against the person's hands.

Identifying the specific action-reaction pair is necessary to evaluate their magnitudes.

Apply Newton's Third Law to compare the magnitudes of the two forces.

Newton's Third Law dictates that these two forces must be equal in magnitude and opposite in direction.

Because the forces must be equal, the statement claiming the person's force is greater is false.

Key Concept

Newton's Third Law (Action/Reaction)

Estimated Time:45s

Question 92Question →

An automated cargo tug applies a forward thrust of $24,000 \text{ N}$ to a $1,000 \text{ kg}$ supply sled. If the ground surface creates a constant opposing frictional force of $4,000 \text{ N}$ , what is the resulting acceleration of the sled?

20 \text{ m/s}^2

24 \text{ m/s}^2

2 \text{ m/s}^2

0.05 \text{ m/s}^2

Show answer & explanation

Answer: $20 \text{ m/s}^2$

Answer

20 \text{ m/s}^2

The sled experiences a forward thrust of

24,000 \text{ N}

and a backward frictional force of

4,000 \text{ N}

, resulting in a net force of

20,000 \text{ N}

. Using Newton's Second Law (

F = ma

), dividing the net force (

20,000 \text{ N}

) by the mass (

1,000 \text{ kg}

) gives an acceleration of

20 \text{ m/s}^2

Step-by-Step Solution

Calculate the net force acting on the sled.

F_{\text{net}} = 24,000 \text{ N} - 4,000 \text{ N} = 20,000 \text{ N}

Acceleration is determined by the net force, which is the applied force minus any opposing forces like friction.

Apply Newton's Second Law (

F = ma

) to find the acceleration.

a = \frac{F_{\text{net}}}{m} = \frac{20,000 \text{ N}}{1,000 \text{ kg}} = 20 \text{ m/s}^2

Rearranging the formula to solve for acceleration gives

a = \frac{F}{m}

Key Concept

Newton's Second Law (

F = ma

)

Estimated Time:45s

Question 93Question →

Match each mechanical adjustment to its primary physical effect on frictional resistance.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Motor oil decreases the coefficient of friction; adding cargo increases the normal force; ball bearings replace sliding friction with rolling friction; and changing the resting face of a crate leaves the frictional resistance unchanged.

Each adjustment demonstrates a fundamental principle of friction. Oil modifies the surface interaction (lowering the coefficient of friction). Added mass increases the normal force (increasing total resistance). Bearings change the physical mechanism from sliding to rolling. Finally, rotating the crate tests the principle that friction is independent of the contact area.

Step-by-Step Solution

Analyze the effect of motor oil.

It creates a fluid boundary layer.

Lubrication physically separates the sliding metal surfaces, which directly reduces the coefficient of friction.

Analyze the effect of heavier cargo.

The downward force on the floor increases.

Friction is proportional to normal force. More weight means a greater normal force, increasing total frictional resistance.

Analyze the effect of ball bearings.

Sliding motion is converted to rolling motion.

Rolling friction relies on point or line contact and avoids the continuous shearing of surface asperities seen in sliding friction.

Analyze the effect of changing the crate's contact area.

The frictional force does not change.

According to Coulomb's laws of friction, kinetic friction is independent of the apparent area of contact as long as the mass (and thus normal force) remains constant.

Key Concept

Friction and Resistance

Question 94Question →

Match each described action force with its corresponding reaction force according to Newton's Third Law of Motion.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Each action force must be paired with a reaction force that is equal in magnitude, opposite in direction, and acting on the original body.

Newton's Third Law states that for every action, there is an equal and opposite reaction. To correctly identify the corresponding pair, you simply reverse the 'pusher' and the 'receiver' objects, and reverse the physical direction of the force.

Step-by-Step Solution

Identify the two interacting objects in each scenario.

The paired objects are identified (e.g., swimmer/water, hammer/nail, rocket/gas, box/floor).

Reaction forces always occur between the exact same two interacting bodies.

Determine the direction of the action force.

The initial force directions are identified (e.g., backward, downward).

The reaction force must be in the exact opposite direction.

Select the reaction force that swaps the acting object and reverses the direction.

The correct matches are successfully established.

This satisfies the fundamental definition of action and reaction pairs.

Key Concept

Newton's Third Law (Action/Reaction)

Question 95Question →

An astronaut is performing maintenance outside a space station in deep space, far from any significant gravitational fields. The astronaut and a heavy metal wrench are both drifting together in a straight line at a constant velocity of $0.5\text{ m/s}$ relative to the station. If the astronaut simply lets go of the wrench without pushing it, which of the following best describes the resulting motion of the wrench?

It will continue moving in the exact same direction at

0.5\text{ m/s}

, as its inertia maintains its state of constant velocity.

It will rapidly decelerate and come to a complete stop, because the active force from the astronaut is no longer pushing it forward.

It will immediately begin falling downward away from the astronaut, as the heavy weight of the metal causes it to drop when no longer supported.

It will shoot forward at a faster speed than the astronaut, because its smaller mass requires less force to accelerate.

Show answer & explanation

Answer: It will continue moving in the exact same direction at $0.5\text{ m/s}$ , as its inertia maintains its state of constant velocity.

Answer

The wrench will continue moving in the exact same direction at

0.5\text{ m/s}

, as its inertia maintains its state of constant velocity.

According to Newton's First Law of Motion, an object will remain in its current state of uniform motion unless acted upon by an external, unbalanced force. Since the wrench is released in a frictionless vacuum with no significant gravity, there are zero external forces acting on it. Therefore, its intrinsic inertia dictates that it will continue traveling indefinitely at its initial constant velocity.

Step-by-Step Solution

Analyze the forces acting on the wrench immediately after release.

In deep space, there is no air friction and no significant gravitational pull. Therefore, the net force acting on the released wrench is zero.

Newton's First Law requires us to evaluate the net forces acting on an object to predict its motion.

Apply Newton's First Law (Inertia) to the object's initial state.

Before being released, the wrench was traveling at a constant velocity of

0.5\text{ m/s}

. Because the net force is zero, the wrench must maintain this exact same state of motion.

An object in motion stays in motion with the same speed and direction unless acted upon by an unbalanced force.

Key Concept

Newton's First Law (Inertia)

Question 96Question →

A delivery truck is attempting to climb a slippery, snow-covered hill, but its rear drive wheels begin to spin in place. The driver stops and shifts several heavy crates from the front of the trailer to a position directly over the rear drive axle. What is the primary physical reason this action helps the truck regain traction?

It increases the normal force pressing the drive tires against the road surface, which increases the maximum available frictional force.

It causes the tires to flatten out and expand their contact surface area with the road, which is the main factor that creates more friction.

It increases the inertia over the rear wheels, which generates a forward reaction force to push the truck up the hill.

It adds mass to the drive axle, allowing the constant rotational force of the engine to produce a greater forward acceleration.

Show answer & explanation

Answer: It increases the normal force pressing the drive tires against the road surface, which increases the maximum available frictional force.

Answer

The action increases the normal force pressing the drive tires against the road surface, which increases the maximum available frictional force.

Traction is the practical application of static friction between a tire and a road surface. Frictional resistance depends on two primary factors: the coefficient of friction (the slipperiness of the snow and rubber) and the normal force (the force pressing the two surfaces together). By shifting heavy cargo directly over the drive axle, the downward force on those specific wheels increases. This increases the normal force, which directly increases the maximum available friction, allowing the tires to grip the snow rather than spin.

Step-by-Step Solution

Analyze the cause of the wheels spinning.

The driving force exerted by the engine exceeds the maximum static friction available between the tires and the snow, causing kinetic friction (slipping) instead.

To move forward without slipping, static friction must be greater than or equal to the driving force.

Determine the physical effect of shifting the crates.

Moving the heavy crates directly over the rear drive axle increases the concentrated downward weight on those specific wheels.

Weight distribution affects how much force is applied to different parts of the vehicle.

Relate the increased weight to frictional force.

The increased weight increases the normal force (

F_N

) pushing the tires into the ground. Since friction (

F_f = \mu F_N

) is directly proportional to normal force, the maximum static friction increases.

Frictional resistance relies on the normal force; greater normal force yields greater traction.

Key Concept

The maximum force of static friction (traction) is directly proportional to the normal force pressing two surfaces together.

Question 97Question →

A person wearing roller skates stands facing a solid concrete wall. When they push forward against the wall with their hands, they immediately roll backward. According to Newton's Third Law of Motion, which of the following best explains why the person rolls backward?

The person's inertia resists their forward push, which creates a backward force that moves them away from the wall.

The wall exerts a reaction force on the person that is equal in magnitude and opposite in direction to the person's push.

Because the person has less mass than the wall, they experience a greater reaction force than they originally applied.

The person moves backward because their body weight is much less than the overall weight of the concrete wall.

Show answer & explanation

Answer: The wall exerts a reaction force on the person that is equal in magnitude and opposite in direction to the person's push.

Answer

The wall exerts a reaction force on the person that is equal in magnitude and opposite in direction to the person's push.

According to Newton's Third Law, forces always occur in pairs. When the person applies an action force forward onto the wall, the wall simultaneously applies an equal reaction force backward onto the person. This backward force acts on the person and causes them to roll backward on their low-friction skates.

Step-by-Step Solution

Identify the two interacting objects in the given scenario.

The interacting objects are the person (specifically their hands) and the concrete wall.

Newton's Third Law analyzes forces as pairs between two distinct objects.

Identify the action force.

The action force is the person pushing forward against the solid wall.

This is the initial physical interaction described.

Determine the reaction force based on Newton's Third Law.

The wall exerts a reaction force pushing backward on the person. This force is equal in strength to the person's push.

Newton's Third Law dictates that every action force generates an equal and opposite reaction force on the interacting body.

Key Concept

Newton's Third Law states that when one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.

Estimated Time:45s

Question 98Question →

A passenger train is traveling forward at a constant velocity on a straight, level track. If a passenger places a smooth cup of coffee on a perfectly level table inside the train, the cup will continuously slide backward relative to the table due to the train's high forward speed.

True

False

Show answer & explanation

Answer: False

Answer

False

Because the train is traveling at a constant velocity, the cup maintains its state of uniform motion along with the train. Due to inertia, it requires an unbalanced force (such as the train accelerating or braking) to change its speed relative to the table. Therefore, it will not slide backward simply because the train is moving fast.

Step-by-Step Solution

Analyze the motion of the train system.

The train is moving at a constant velocity on a straight, level track, meaning its acceleration is zero.

Identifying the state of motion (acceleration vs. constant velocity) is necessary to apply Newton's Laws correctly.

Determine the state of motion of the cup.

The cup is inside the train, so it already shares the train's constant forward velocity.

Objects within a moving reference frame share the velocity of that frame.

Apply Newton's First Law of Motion (Inertia).

Since there is zero acceleration, there is no unbalanced net force acting horizontally on the cup. The cup will remain at rest relative to the table.

Newton's First Law dictates that without an unbalanced external force, an object's velocity remains constant.

Key Concept

Newton's First Law (Inertia) and Constant Velocity

Question 99Question →

A heavy steel crate must be moved across a horizontal concrete floor. Rank the following conditions based on the magnitude of frictional resistance opposing the applied force, from LEAST resistance to GREATEST resistance.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence from least to greatest frictional resistance is: using wheels (rolling friction), sliding on oil (fluid friction), sliding at a constant speed (kinetic friction), and pushing just prior to movement (static friction).

Frictional resistance follows a predictable hierarchy based on how surfaces interact. Rolling friction is the lowest because there is no sliding action. Fluid friction is next, as the fluid layer prevents solid-to-solid contact. Dry kinetic (sliding) friction is higher because the solids scrape directly against each other. Finally, static friction is the highest because the microscopic peaks and valleys of the stationary surfaces have settled deeply into one another.

Step-by-Step Solution

Identify the type of friction present in each condition.

Wheels = rolling friction; Oil = fluid friction; Sliding = kinetic friction; Stationary just before moving = static (breakaway) friction.

Different physical mechanisms govern resistance in each scenario.

Compare rolling friction to all forms of sliding friction.

Rolling friction is ranked lowest (least resistance).

Rolling mechanisms are specifically designed to eliminate the dragging forces characteristic of sliding.

Evaluate the lubricated surface against the dry surfaces.

Fluid friction is ranked second lowest.

Lubricants separate the solid surfaces, drastically reducing resistance compared to dry contact, but generally offer more resistance than precision wheel bearings.

Compare the two dry conditions: moving at constant speed vs. about to move.

Kinetic friction is ranked third, and static friction is ranked highest.

The coefficient of static friction is typically greater than the coefficient of kinetic friction for the same materials; it takes more force to start an object moving than to keep it moving.

Key Concept

Types of Friction (Static, Kinetic, Rolling, Fluid)

Question 100Question →

When a heavy, fast-moving bowling ball strikes a light, stationary wooden pin, the impact force exerted by the ball on the pin is greater than the reaction force exerted by the pin on the ball.

True

False

Show answer & explanation

Answer: False

Answer

The statement is false. The action and reaction forces are exactly equal in magnitude.

The statement is false because Newton's Third Law dictates that for every action, there is an equal and opposite reaction. The force the bowling ball exerts on the pin is exactly equal in magnitude to the force the pin exerts back on the bowling ball. The reason the pin flies away while the ball continues moving forward is due to their differences in mass (inertia), not a difference in the force applied.

Step-by-Step Solution

Identify the two interacting objects in the collision scenario.

The two objects are the heavy bowling ball and the light wooden pin.

To determine the action-reaction force pair correctly, you must isolate the interacting bodies.

Apply Newton's Third Law of Motion to the interaction.

The law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

This fundamental physical law applies to all mechanical interactions regardless of size, weight, or speed.

Evaluate the statement's claim about differing forces based on mass.

The mass and velocity only affect how much each object accelerates (Newton's Second Law), but the impact force itself is identical for both.

Force is an interaction between two objects; they share the exact same force magnitude.

Key Concept

Newton's Third Law of Motion (Action and Reaction)

Estimated Time:45s

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