Acceleration is the rate of change of the velocity. So if you are told that an object is stationary or is moving at constant velocity, this means the acceleration is zero. From Newton’s Second Law (F = ma), if the acceleration is zero, this means the net force is zero. So whenever velocity is constant, all the forces in the x and y directions must cancel out! Here’s a problem that shows how this concept is a powerful tool for solving free body problems:
Example: Jack pushes a large crate across a level surface at a constant speed. The crate has a mass of 50 kg and Jack exerts a horizontal force of 200 N. What is the coefficient of friction, μ?
Answer: It seems that we don’t have enough information because we don’t know the velocity of the crate. But the fact that the speed is constant tells us that the net force is zero. So the solution is straightforward. First draw the free body diagram:
Because the net force is zero, the weight Fg is equal in magnitude to FN.
And because the net force is zero, the force Jack exerts is equal in magnitude to the frictional force.
