Question

A projectile is fired upward with an initial speed vo on an airless world. A short time later, it comes back down and has a final speed vo (just before it hits the ground).
What was the sign of the total work done by the force of gravity during the flight?

a.Wgrav=0

b.Wgrav < 0

c.Wgrav >0

Answer 1

Answer:

$W_{grav} < 0$

Explanation:

When a projectile is fired upwards with some initial speed then the it reaches the top of the projectile and then falls back to the ground.

According to the question we need to find the work done by the gravity which is acting downwards for the projectile when it is at a position just about to hit the ground in course of falling down.

As we know that work is given as:

$W=F.s\cos\theta$

here:

$F=$ force of gravity on the object (which is acting downwards)

$s=$ displacement of the object

• Here the work done by the gravity at an instant just before the projectile hits the earth will be negative as the displacement is in the direction opposite to the force of gravity.