Definition

Objects will remain moving with a constant velocity unless acted upon by a net force.


This is saying that, unless there is a force that acts upon an object, the object will keep moving until there is something that stops the object. This also applies to objects that are staying still. They will stay still until there is something that makes the object move. The main reason that objects stop is because of friction. Friction is a net force that has effect on almost every moving object, and makes the object slow down and eventually come to a stop. Friction also applies to objects that are staying still. A force that acts on an object, to make an object move, will have to overpower the static friction. And if a ball is moving across the floor with a constant speed, the force will have to be greater than the kinetic force, to make the object accelerate.
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So, if F is a net force and A is acceleration, if F=0 then A=0. This is because an object will not accelerate if there is no net force. It will either choose to remain at rest or remain at a constant velocity and with both of these, there is no acceleration. An object, if it has a choice, will choose to keep a constant velocity or remain still because of Newton's first law and the lack of a net force. Although you might be wondering why a ball doesn't keep rolling forever at a constant speed because of Newton's first law, it is because of friction. It actually does have a force acting upon it.

Here is a sample problem.

A block is being pulled by 50,000N by a string parallel to the horizontal. If the block is moving at a constant velocity. What is the amount of friction force applied to the block?

The answer is -50,000N. If you use Newton's 1st law you know that the forces must be equal because the object is not accelerating. If the object is not accelerating then there is no net force. (50,000N - 50,000N = 0N)This picture represents an unbalanced force. The force from the piston will change the objects movement for a instant until the piston removes contact from the block. This change in motion should continue at a constant pace assuming that the surface has negligible friction, like it says in the video.


unbalanced_force.png

This picture represents an unbalanced force. The force from the piston will change the objects movement for a instant until the piston removes contact from the block. This change in motion should continue at a constant pace assuming that the surface has negligible friction, like it says in the video.