# Electric field, charged balls, conservation of energy, velocity

• thermocleanse
In summary, the two balls have different speeds when they hit the ground due to the difference in their kinetic energy. The ball with more kinetic energy will hit the ground first.

## Homework Statement

near the surface of the Earth there is an electric field of about 150 V/m which points downward.

2 identical balls with mass 0.540 kg are dropped from a height of 2.0 m, but one of the balls is positively charged with q1 = +650 uC and q2 = -650 uC.
use conservation of energy to find difference in speed of 2 balls when they hit ground (neglect air resistance).

## The Attempt at a Solution

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Here's a hint. You don't want to use kinematics for this, so forget about the
y = y0 + v0t +(1/2)at2
from your list of relevant equations. Instead use conservation of energy to solve this.

There are three aspects of energy that relate to each ball (and not necessarily the same way for each ball).

Gravitational potential energy. This can be expressed as mgh.

Kinetic energy. This can be expressed as (1/2)mv2.

Work done by the electric field. I'm going to let you form this equation yourself. But it's related to
W = Fs
where W is the work done by a constant force F, over a distance s.

i worked out the 3 aspects of energy related to each ball, but i get two ways to get the velocity. one way is via use of KE equation, whereas the other way is via conservation of energy. i don't even see how it can be 2 different velocities!

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thermocleanse said:
i worked out the 3 aspects of energy related to each ball, but i get two ways to get the velocity. one way is via use of KE equation, whereas the other way is via conservation of energy. i don't even see how it can be 2 different velocities!