Electron-Proton Relative Speed: Solving for 0.9c and -0.0994c

[najisalem2009]

Homework Statement

A laboratory experiment shoots an electron to the left at 0.9c. What is the electrons speed relative to a proton that is moving to the right at 0.9c?

I have no idea how to attempt this problem i know the answer is -.0994c but I am not sure how to get this answer?

 

[najisalem2009]

anyone?

 

[Redbelly98]

Welcome to Physics Forums.

If you were assigned a question like this, then you would have been given the equation for combining relativistic velocities. It should be in your textbook or class lecture notes -- it's a pretty standard equation.

 

[najisalem2009]

So i found the equation Its s= (v+u)/1+(vu/c²)

s=(0.9c+0.9c)/1+(0.9c)(0.9c)/(2.98*10⁸)²
i get s=.18c but the answer is -0.994c

 

[yoshtov]

Question: Why did you leave "0.9c" as it was, but then expressed "c" later as 2.98*10^8 m/s?

(This is where your problem resides.)

 

[Vipho]

So i found the equation Its s= (v+u)/1+(vu/c²)

s=(0.9c+0.9c)/1+(0.9c)(0.9c)/(2.98*10⁸)²
i get s=.18c

It's wrong. That must be s=(0.9c+0.9c)/(1+0.81)=1.8c/1.81=0.994c

 

[najisalem2009]

oh ok thanks a lot guys for clearing that up for me