Velocity of insulating sphere before colliding with other insulatin spherre

1. The problem statement, all variables and given/known data

Two insulating spheres having radii 0.47 cm and 0.67 cm, masses 0.13 kg and 0.89 kg, and charges −4 μC and 2 μC are released from rest when their centers are separated by 1.3 m. How fast is the smaller sphere moving when they collide? Answer in units of m/s.

2. Relevant equations

What techique or conceptual understading can help me attack this problem? Am I missing something?

3. The attempt at a solution

I tried to use the conservation of energy. But there are two problems, because they don't hit and stop, so they must both be independet from each other (unless the problem would tell me that they collide and their final velocity is 0. I don't know how to relate the two spheres. I do know that they have charges that attract each other and that I could use the force of the electric field if I treat them as point charges. But with the force of the electric field I don't know if it can be useful because I can't use it to find the velocity. If I use kinematics it won't work since there is no time given.
Last edited:


Science Advisor
Homework Helper
I tried to use the conservation of energy. But …
Hi phymateng! :smile:

Yes, you do need an extra equation …

use conservation of total momentum …

momentum is always conserved. :wink:

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