Question

You throw a ball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down.

a) Work is + on the way up and + on the way down
b) Work is + on the way up and - on the way down
c) Work is - on the way up and + on the way down
d) Work is - on the way up and - on the way down

Answer 1

Work is negative on the way up and positive on the way down.

Step-by-step explanation:

Work is positive if displacement and the force are in the same direction.

Work is negative if displacement and the force are in opposite directions.

(Note that work is zero if displacement is perpendicular to the force.)

In this question, the gravity on the object points downwards at all times.

On the way up, the position of the object is above where it was launched. Hence, displacement would point upwards.

Since the direction of gravity is opposite to that of displacement on the way up, the work on the object would be negative.

In contrast, displacement of the object points downward on the way down. Since displacement is in the same direction as the force of gravity, the work on the object would be positive.