What a force is
When you push something, what you are doing is applying a force. When you pull something, that is applying a force too. Stretching, squeezing — all of these happen because a force is applied.
What does a force do?
Applying a force to an object can do any of these:
- Start a stationary object moving
- Increase the velocity of a moving object
- Decrease the velocity of a moving object, or stop it
- Change the direction it is moving in
- Change the shape of the object
The link back to chapter 2
The first four have something in common — every one of them is a change in velocity. And you learned in chapter 2 that the rate of change of velocity is acceleration.
So a force is the thing that causes an acceleration. Chapter 2 described motion; this chapter says why it changes.
Is applying a force enough?
To move a stationary object you have to apply a force in the direction you want it to move. But does applying a force, on its own, start it moving?
Try pushing a table gently. Usually nothing happens. As you steadily increase the force you apply, at some point the table starts to move.
The question this leaves
Where did that first push go? Why did it not move the object? On the next page we identify the force that answers this.