Filling in the Variables

In many cases, determining which of the five variables are known is pretty easy. "The initial velocity of the car is 6m/s" doesn't leave much room for guesswork.

There are some phrases, however, which tell us something about the motion, but which aren't quite so obvious. Here are a few:

What it says: "The car starts from rest."

What it means: The initial velocity is zero.

What it says: "When the ball reaches its highest point"

What it means: When the velocity is zero

The ball is at a turning point, a maximum or minimum in its position. As we saw in our discussion of graphs, the velocity is zero at turning points.

What it says: "I throw a ball into the air and it returns to my hand."

What it means: The displacement is zero.

The displacement $\Delta y$ is the distance between the starting point and the eneding point. In this case, the ball starts and ends in my hand, and so the displacement is zero.

What it says: "When the book hits the ground"

What it means: The moment before the book hits the ground

Collisions are complicated things which the tools we've developed in this chapter are not equipped to handle. Since we can't deal with what happens between the moment the book makes contact with the ground and the moment the book stops, we must be content to ask what happens right before the book hits the ground.

What it says: "The box is in free fall."

What it means: The box has an acceleration of $g=9.8\u{m/s^2}$ downward.

Also remember

that the acceleration is related to the forces: if you know the net force, then the acceleration is $a=F_{net}/m$.