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## Homework Statement

Two frictionless pucks are placed on a level surface with an initial distance of 20.0 m. Puck 1 has a mass of 0.80 kg and a charge of + 3 E-4 C while puck 2 has a mass of 0.4 kg and a charge of +3 E-4 C. The initial velocity of puck 1 is 12 m/s [E] and the initial velocity of puck 2 is 8 m/s [W]. Find the minimum separation of the two pucks?

## Homework Equations

[/B]

KE 1 + KE 2 + EE = EE' + KE'

Pt=Pt'

## The Attempt at a Solution

*First I used conservation of momentum to calculate v for each mass at the min separation.*

Both pucks have the same velocity.

Both pucks have the same velocity.

*Then just some algebra to isolate v'.*

m

_{1}v

_{1}+m

_{2}(-v

_{2})=(m

_{1}+m

_{2})v'

v'=(0.80)(12)+(0.40)(-8)/0.80+0.40

v'=5.33 m/s [East]

This part I can use conservation of energy (kinetic and electric potential). I don't think the energy here would be negligible and doing calculations I see big differences in the answer.

KE 1 + KE 2 + EE = EE' + KE'

½ m

_{1}v

_{1}² + ½ m

_{2}(-v2²) + kq²/r = kq²/r' + ½ (m

_{1}+ m

_{2})(v')²

½ (0.80)(12)² + ½ (0.40)(-8)² + (9.0 E+9)(+3.0 E-4)²/20 = (9.0 E+9)(+3.0 E-4)²/r' + ½ (0.80 + 0.40)(5.33)²

r' = 7.34 m

I've found so many attempts at this and there is so many answers.. Many I know are wrong and this one I think is right.

Please can you review this and see what you think?

Thank you

Julian

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