# Why do things stop accelerating?

Before $$t=0$$: object is not moving

At $$t=0$$: I push object with sufficient force to overcome static friction

After $$t=0$$: Object accelerates. Resultant force is greater than $$0$$. Dynamic friction is less than the push. So why does the object stop accelerating after say $$t=2$$ (either starts to decelerates or reaches constant velocity)? $$F_f = \mu N$$ so it does not depend on the velocity of the object so as the object accelerates it, the push force and frictional force stay constant so the resultant force also does.

Forces are interactions between two objects. You push on an object, and the object pushes back with an equal but opposite force. When the interaction stops, the force disappears. No exception to this has ever been observed. That is why it is called a law.

The object never stops accelerating as long as you keep pushing with a force greater than the friction force. If you decrease your pushing force to be equal to the friction force it will move at constant velocity. If your pushing force becomes less then the friction force, the object decelerates. These are simple statements about how Newton's First and Second Laws apply.

• I meant that once you push it you leave it. When you have pushed it it has a net force. According to Newtons 2nd law that should stay constant unless either the frictional force or the push force changes. The frictional force shouldn't change since the surface is always the same. Would the push force decrease? If so, why? Commented Oct 28, 2020 at 16:04
• Your misconception is that the force stays with the object. I'll revise my answer. Commented Oct 28, 2020 at 16:21

the push force and frictional force stay constant so the resultant force also does.

Your scenario is not very explicitly stated, but I am assuming that you are talking about a human arm pushing an object over a rough surface. If so, then the quoted statement is actually incorrect. The push force does not stay constant. As the push force decreases it becomes less than the friction force and the object decelerates.

The push force decreases because animal muscles and limbs produce a very non-constant force output. The force decreases with speed and also changes with joint angle. The force drops sharply to 0 as the arm becomes fully extended (joint angle reaches 180 degrees).

If you were to actually provide a constant push force then indeed the object would continue accelerating until other forces (e.g. air resistance) became non-negligible.

• Thanks, but I don't understand why does the push force decreases? You said animal limbs give a very non-constant force so if a machine supplied the force would it be different? What I meant was that someone pushes an object and leaves it (does not keep supplying the force. Also, the rough surface part doesn't matter it could be on any surface all the steps happen just quicker or slower. Commented Oct 28, 2020 at 16:00
• If you stop supplying the force then the force obviously goes to 0. Why would you think that the force is constant if you do not keep supplying the force?
– Dale
Commented Oct 28, 2020 at 16:49