# The earth surface or a point at infinity from the earth can be chosen as zero reference level of? (a) Electric P.E. (b) Kinetic Energy (c) Gravitational P.E. (d) All of the above.

Aug 17, 2017

I think $\text{C}$.

#### Explanation:

• We often define the surface of the earth as a point of $0$ gravitational potential energy when dealing with objects near the earth's surface, such as a book sitting on a shelf, which has GPE $U = m g h$, where $h$ is defined as the height of the book above Earth's surface.

• For GPE between two massive bodies, we further apply Newton's laws of gravitation. The way that gravitational potential energy is defined here is negative.

${U}_{g} = - \frac{G {m}_{1} {m}_{2}}{r}$

The negative potential energy means that the potential energy of two masses at separation r is less than their potential energy at infinite separation. The zero point of potential energy is defined at $r = \infty$.

So it is certainly applicable to answer $\text{C}$.

• Kinetic energy is $0$ for objects at rest, as $v = 0$, and kinetic energy is defined by:

$K = \frac{1}{2} m {v}^{2}$

regardless of the object's position relative to the earth.

$- \Delta V = E$

Aug 18, 2017

I think (a) Electric P.E.

#### Explanation:

I at first thought GPE. Then I reread the question. Since it says, that the zero point can be the Earth or a point at infinite distance. That is done with Electric P.E. It is true that a point at infinity from the earth could be chosen. However I see no advantage to that.

This well respected website discusses both options:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html
Look at the last 3 sentences in the section titled Zero Potential. Also look at the section titled Potential Reference at Infinity . Where they are letting the distance ${r}_{b}$ increase to infinity, they are setting the reference equal to a point at infinity from the charge $Q$.

I hope this helps,
Steve