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Difficulty: MediumNewton's First Law (Inertia)

A tower crane is lowering a massive concrete beam to the ground. For a brief period, the beam descends at a constant, steady velocity. According to Newton's First Law, what must be true about the primary forces acting on the beam during this steady descent?

  1. The upward tension in the cable exactly equals the downward weight of the beam, resulting in zero net force.Answer
  2. B
    The downward weight of the beam must exceed the upward tension in order to overcome the beam's inertia and maintain its downward motion.
  3. C
    The upward tension in the cable must be exactly equal to the beam's mass to prevent it from accelerating.
  4. D
    The net downward force acting on the beam is proportional to its steady downward velocity.

Answer

The upward tension in the cable exactly equals the downward weight of the beam, resulting in zero net force.
According to Newton's First Law, an object in motion will maintain a constant velocity as long as the net external force acting on it is zero. Since the beam descends at a steady, unchanging speed, its acceleration is zero. Therefore, the upward force (cable tension) and the downward force (beam's weight) must be perfectly equal and balanced.

Step-by-Step Solution

1
Identify the object's state of motion described in the scenario.
The concrete beam is moving at a constant, steady velocity.
Determining the state of motion (constant velocity vs. accelerating) is the first step in applying Newton's Laws.
2
Apply Newton's First Law of motion to the object's state.
Because the velocity is constant, the acceleration is zero. Therefore, the net external force acting on the beam must be zero.
Newton's First Law dictates that an object maintains a constant velocity if and only if the forces acting upon it are balanced (net force is zero).
3
Evaluate the specific forces acting on the beam.
The primary forces are the downward force of gravity (weight) and the upward force from the cable (tension). To achieve a net force of zero, these opposing forces must be exactly equal in magnitude.
For vertical forces to cancel out and produce a zero net force, the upward pulls must perfectly equal the downward pulls.

Key Concept

Objects moving in a straight line at a constant velocity have zero acceleration and therefore experience a net force of zero, meaning all opposing forces perfectly balance each other.
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Newton's First Law (Inertia)

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