# Help relativity exam in in 5 hours

1. Mar 7, 2005

### nemzy

urgent help relativity exam in in 5 hours

an electrion with mass 9.11e031 kg has momentum 4e-22. what is its kinetic energy

the answer is 6.3e-14 J but i have no idea how to get here

relativity kinetic energy = (y-1)mc^2 where y = 1/(square root of 1-(v^2/c^2)

momentum = ymu

thats as far as i can get , how the hell can u solve this type of problem if they dont give u the velocity of the electron?

Last edited: Mar 7, 2005
2. Mar 7, 2005

### thomate1

We have a soln .
We have K.E = (P^2)/2m;
where KE is kinetic energy,P is momentum

So solving eqn u can get the answer

3. Mar 7, 2005

### nemzy

i got 8.78e-14

when plugging into that equation?

4. Mar 7, 2005

### dextercioby

Use this
$$E^{2}=m_{0}^{2}c^{4}+p^{2}c^{2}$$ to find the total energy and then from that simply subtract the rest energy...

Daniel.

Last edited: Mar 7, 2005
5. Mar 7, 2005

### nemzy

thank u!! i was stuck on that problem forever

6. Mar 7, 2005

### dextercioby

U'd better memorize that formula.It's the most important in the Special Relativity,at least,not in Minkowski space.

Daniel.