# The power delivered by gravity?

## Homework Statement

Consider a projectile fired vertically with an initial velocity vi. This can be thought of as the gravitational force doing work Wg = Fg∆y on the projectile as it travels through some vertical displacement ∆y. Which of the following statements is true?

Question options:
a) The power delivered to the projectile from the gravitational force is zero.

b) The power delivered to the projectile from the gravitational force is constant.

c) The power delivered to the projectile from the gravitational force is P(t) = –mgvy(t)

d) The power delivered to the projectile from the gravitational force is P = mgvi

## Homework Equations

Power = work/time

## The Attempt at a Solution

I started with work = force x displacement, with the force of gravity being the mass of the object (m) times the acceleration due to gravity (g), since the object is moving in only the vertical direction. With only mg/time, the power delivered would be constant over time, which was incorrect.

I'm now thinking the answer could be c) since gravity would be acting in the opposite direction from the initial velocity with the equation from power = force x velocity but am not sure. I assume the power would not be constant because kinetic energy would be constantly decreasing due to the negative work done by gravity?

berkeman
Mentor

## Homework Statement

Consider a projectile fired vertically with an initial velocity vi. This can be thought of as the gravitational force doing work Wg = Fg∆y on the projectile as it travels through some vertical displacement ∆y. Which of the following statements is true?

Question options:
a) The power delivered to the projectile from the gravitational force is zero.

b) The power delivered to the projectile from the gravitational force is constant.

c) The power delivered to the projectile from the gravitational force is P(t) = –mgvy(t)

d) The power delivered to the projectile from the gravitational force is P = mgvi

## Homework Equations

Power = work/time

## The Attempt at a Solution

I started with work = force x displacement, with the force of gravity being the mass of the object (m) times the acceleration due to gravity (g), since the object is moving in only the vertical direction. With only mg/time, the power delivered would be constant over time, which was incorrect.

I'm now thinking the answer could be c) since gravity would be acting in the opposite direction from the initial velocity with the equation from power = force x velocity but am not sure. I assume the power would not be constant because kinetic energy would be constantly decreasing due to the negative work done by gravity?