# What is the GPE of the 10-kg rock at the top of the house, at the top of the cliff and at the bottom of the cliff?

## NOTE: the house is 5m tall and the cliff is 25m tall. The answers should be: a. 0 b. 490 J c. 2900 J What I don't understand is why the GPE is 0 at (a)?? It doesn't make sense to me that an object above ground should have a GPE = 0. Please clarify!! Thank you.

Jan 22, 2017

See explanation.

#### Explanation:

I think there are mistakes in those answers.

The GPE of a body is given as:

## ${E}_{G} = m \times g \times h$ 

where $m$ is mass (here $10 k g$), $g$ is gravitational acceleration (here $g = 9.8 \frac{m}{s} ^ 2$), $h$ is height above the ground.

So

In point $a$ (at the top of the house) the energy is:

${E}_{P} = 10 \times 9.81 \times 5 = 490$ (so here the value from $b$ is correct)

In point $b$ we have:

${E}_{p} = 10 \times 9.81 \times 25 = 2450$ (not in the given values).

At the bottom the energy should be $0$ (as you suggested)

Note:

The energy close to answer $c$ is possible if we assume that the house stands on the top of the cliff. Then you would have to add the heights of the house and the cliff, so you would have:

${E}_{p} = 10 \times 9.8 \times \left(25 + 5\right) = 10 \times 9.8 \times 30 = 2940$