# Homework Help: Two SR Questions

1. Jul 29, 2008

### cscott

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

In frame S , particle 1 is at rest and particle 2 is moving to the right with velocity v.
Consider frame S ′ , moving relative to S , with speed u. Find the value of u such that
the two particles appear in S ′ to be approaching each other with equal but opposite
velocities.

I don't see here how you can get the particles to approach each other.

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

Calculate the range of speeds for a particle of mass m in which the classical relation
for kinetic energy is within 1% of the correct relativistic value.

For this can I use K/T = 0.01 and solve for the velocity of the mass, where K is newtonian KE and T is relativistic KE?

2. Jul 29, 2008

What equations do you think you should use on the first question? You're kind of close on the second part, but you want the difference in K and T to be 0.01 of the relativisitic..

3. Jul 29, 2008

### cscott

The velocity transforms could be used for the first question. Can I just set the S' frame velocity to -1/2v or -v (not sure here) then solve for the frame speed u given that the S frame speed is v?

Thanks, I'll work on the second one.

4. Jul 29, 2008

### Dick

Be careful! If v=c then there is no such frame. So the answer clearly isn't c/2. You have to use the relativistic formula for velocity addition.