Electric Potential/Potential Energy problem

[jawhnay]

Homework Statement

a) From a large distance away, a particle of mass 6 grams and charge 54 μc is fired at 25 m/s to the right straight toward a second particle, originally stationary but free to move, with mass 13 grams and charge 22 μc. Find the distance of closest approach between the charges.

b) Four identical particles each have charge of 4 μC and mass 2 grams. They are released from rest at the vertices of a square of side 42 cm. How fast is each particle moving when their distance from the center of the square triples?

Homework Equations

a) ΔU + ΔK = 0
b) U=(kQ₁Q₂)/r , ΔU + ΔK = 0, K = (1/2)mv², V = (KQ)/r

The Attempt at a Solution

a) not sure how to do this problem other than knowing that V_i = 25 m/s for the 6 gram particle and V_i = 0 for the 13 gram particle.

 

[mfb]

Homework Statement

a) From a large distance away, a particle of mass 6 grams and charge 54 μc is fired at 25 m/s to the right straight toward a second particle, originally stationary but free to move, with mass 13 grams and charge 22 μc. Find the distance of closest approach between the charges.

b) Four identical particles each have charge of 4 μC and mass 2 grams. They are released from rest at the vertices of a square of side 42 cm. How fast is each particle moving when their distance from the center of the square triples?

Homework Equations

a) ΔU + ΔK = 0
b) U=(kQ₁Q₂)/r , ΔU + ΔK = 0, K = (1/2)mv², V = (KQ)/r

The Attempt at a Solution

a) not sure how to do this problem other than knowing that V_i = 25 m/s for the 6 gram particle and V_i = 0 for the 13 gram particle.

For (a), how can you describe the energy at their closest approach?

(b) can be solved in a similar way.

 

[voko]

I think (a) also requires conservation of momentum.

 

[jawhnay]

For (a), how can you describe the energy at their closest approach?

(b) can be solved in a similar way.

i'm not sure... my knowledge of energy is kind of rusty since i haven't taken mechanics in a while...

 

[mfb]

I think (a) also requires conservation of momentum.

Right

i'm not sure... my knowledge of energy is kind of rusty since i haven't taken mechanics in a while...

Then you should try to fix that. While it is possible to solve it without the concept of energy, this would require solving differential equations, which is NOT easier.