Notes

NET FORCE:
According to Newton's Second Law, when an object accelerates, then there must be a net force acting on it. Conversely, if a net force acts on an object, that object will accelerate. If the net force acting on an object is zero, then the object is not accelerating and is in a state that we call equilibrium. When an object is in equilibrium, then two things can be true: either the object is not moving at all, or the object is moving at a constant speed.
Example: Consider a hypothetical situation in space. You are doing a spacewalk and are fixing something on your shuttle. While working on the issue with a wrench, you get angry. You throw the wrench away from you. What happens? Well, once the wrench leaves your hand, it will continue moving with the same speed that you gave it when you threw it. This is an example of a zero net force situation. The wrench will move with the same speed and will not accelerate in space. If you throw the same wrench on Earth, the wrench will fall to the ground and eventually stop. Why did it stop? There is a net force acting on the wrench causing it to slow down and stop.

In another example, let's say you are on an ice rink. You grab a hockey puck and slide it across the ice. Eventually, the hockey puck will slow down and stop, even on the smooth and slippery ice. This is another example of a situation with a non-zero net force.