# Initial speed of the proton?

• kristjan

## Homework Statement

The moving proton hits the second proton, which we consider to be stationary. In the moment of central strike gap between the protons is 10(-13)m. What was the initial speed of the moving proton? Proton mass is 1.67*10(-27)kg and charge 1.6*10(-19) C.

## Homework Equations

Electric potential energy U=kQq/r
Kinetic energy 1/2 mv2

## The Attempt at a Solution

kinetic energy is transformed into electric potential energy at the point of closest approach:
electric potential energy=kinetic energy of moving proton
From there I find initial speed of moving proton to be v=1.66*10(6) m/s, in book answer is 2.35*10(6) m/s

## Homework Statement

The moving proton hits the second proton, which we consider to be stationary. In the moment of central strike gap between the protons is 10(-13)m. What was the initial speed of the moving proton? Proton mass is 1.67*10(-27)kg and charge 1.6*10(-19) C.

## Homework Equations

Electric potential energy U=kQq/r
Kinetic energy 1/2 mv2

## The Attempt at a Solution

kinetic energy is transformed into electric potential energy at the point of closest approach:
electric potential energy=kinetic energy of moving proton
From there I find initial speed of moving proton to be v=1.66*10(6) m/s, in book answer is 2.35*10(6) m/s
Perhaps, the other proton was stationary at the beginning, but free to move. Then momentum is conserved.

Remark by ehild above is right. If it is asumed that the target proton is (somehow) held stationary, then I am also getting the same answer that kristjan got. If you assume that the target proton is free to move, and that it is a 1-dimensional collision problem, using (non-relativistic) momentum and energy conservation, I get the book answer.