Since the friend stops pulling on the box, and friction is now present, let’s make another diagram.
The box was moving before it hits the rough section. When the friend let go, the boy’s push and friction are suddenly equal, but in opposite directions. This is NOT an application of Newton’s third law of motion, since the forces are of different types.
So we go to Newton’s first law of motion. Since the forces are equal in magnitude and directed opposite to each other, they cancel out. The box then moves at a constant velocity. In other words, it keeps moving at a constant speed in one direction.
EXTRA: If the boy stop pushing as well, the box would decelerate (reduce speed) since the frictional force is directed opposite to the motion. This means that the acceleration is also directed in the opposite way, so the velocity should decrease in magnitude.
Forces and Newton's Laws of Motion Answers to Question 4
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