Chapter 3, Problem 36A

To pull a wagon across a lawn with constant velocity, you have to exert a steady force. Does this fact contradict Newton’s first law, which tells us that motion with constant velocity indicates no force?

To determine

Whether Newton’s first law contradicts the fact that a constant force is applied to pull the wagon across the lawn with constant velocity.

Answer to Problem 36A

No

Explanation of Solution

Introduction: Newton’s first law states that if an object is in a state of rest then it will stay at rest or if any object is in motion then it will keep moving unless and until an external force is applied on it.

Constant velocity implies that the wagon is in constant motion. Now, if the external force applied varies, then it will not change its velocity but will change its course of motion. In the absence of external force, the wagon would come to halt eventually due to friction. This constant motion implies that Newton’s first law of motion is obeyed to keep moving the wagon. The force is still zero as the wagon is not accelerating. The constant applied force is required to balance the frictional force.

Conclusion: Thus, constant force applied to the wagon in a specific direction will make sure that Newton’s first law is obeyed.