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Difficulty: MediumNewton's Third Law (Action/Reaction)

A motorized boat is accelerating forward across a body of water. According to Newton's Third Law of Motion, which specific force is the reaction force that directly propels the boat forward?

  1. The force of the water pushing forward against the propeller bladesAnswer
  2. B
    The force of the propeller blades pushing backward against the water
  3. C
    The internal rotational force generated by the boat's engine
  4. D
    The forward momentum of the boat overcoming the water's resting resistance

Answer

The force of the water pushing forward against the propeller blades directly propels the boat.
According to Newton's Third Law, forces always come in pairs. When the boat's propeller pushes water backward (action), the water simultaneously exerts an equal and opposite force forward on the propeller (reaction). Because the propeller is attached to the boat, this reaction force is what propels the entire vessel forward.

Step-by-Step Solution

1
Identify the interacting bodies in the mechanical system.
The two primary interacting bodies are the boat's propeller and the surrounding water.
Newton's Third Law requires identifying the two distinct bodies exerting forces on each other.
2
Determine the action force created by the boat's mechanism.
The turning propeller exerts a force backward against the water.
To move forward, a marine propulsion system must push fluid in the opposite direction.
3
Apply Newton's Third Law to find the equal and opposite reaction force.
If the propeller pushes the water backward, the water must push the propeller forward with an equal magnitude of force.
Forces always occur in simultaneous pairs acting on different objects.
4
Isolate the force that acts specifically on the boat.
The forward force from the water acting on the propeller blades is what accelerates the boat forward.
An object's acceleration is determined only by the external forces acting upon it, not the forces it exerts on other things.

Key Concept

Newton's Third Law requires that for every action force, there is a simultaneous reaction force equal in magnitude and opposite in direction, acting on two different bodies.
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Newton's Third Law (Action/Reaction)

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