# Homework Help: Acceleration time problem

1. Mar 7, 2010

### slpnsldr

1. The problem statement, all variables and given/known data
An electron is accelerated uniformly from rest to a velocity of 2.0x10^7m/s(E) over the displacement .10m(E).

How long does the electron take to reach its final velocity?

2. Relevant equations

This is what i want to know

3. The attempt at a solution

I know the answer, but can not figure out how to get it 1x10^-8

2. Mar 7, 2010

### rock.freak667

Maybe try using one of the kinematic equations like v=u+at or s=ut+1/2at2

3. Mar 7, 2010

s=(u+v)/2*t
put u=0

4. Mar 7, 2010

### slpnsldr

ok more precisely the problem is that i dont know acceleration or time. all the general equations i know, require either acceleration or time to figure out one or the other. So, not knowing either, how do I proceed?

and, dont those three equations require me to know time??

5. Mar 7, 2010

### lightgrav

you almost know the average speed :
half the ending speed , because the starting speed was zero.

6. Mar 7, 2010

### benhou

When you don't know how to proceed, it's better if you list all the know data. Like this:

$$v_{1}=0, v_{2}=..., \Delta d=..., a=?, \Delta t=?$$

Now, since you know v1,v2, Delta d, which formula can you use?

7. Mar 7, 2010

### slpnsldr

of the 5 general equations for uniform acceleration that i know, all of the equations require us to know one of the two acceleration, or time.

8. Mar 7, 2010

### lightgrav

well, if the equation doesn't have TIME in it , you'll never be able to solve for the time .

But don't you have an equation withOUT acceleration?

9. Mar 7, 2010

### benhou

Interceptor has told you the answer.

10. Mar 8, 2010

### slpnsldr

Ok, so if interceptors equation is,

s=(u+v)/2*t

s is Speed = 1x10^7
u is ? but = 0
v is Velocity = 2x10^7
t is time? = unknown

Which means I cant proceed because I have an equation that requires one of my unknowns.

Sorry for being so much trouble. But Im really havin a hard time with this lol.

11. Mar 8, 2010

### rock.freak667

s is displacement.

12. Mar 8, 2010

### benhou

It's alright. By s=(u+v)/2*t, Interceptor meant, s for displacement, u for initial speed and v for final speed. u=0 because the electron started from rest; other than that, you know s and v. Therefore, you can solve for t.

13. Mar 8, 2010

### slpnsldr

ummm.. t= ((u+v)/2)/s ??? does that make sense? ahhhh I suck at this

14. Mar 8, 2010

### slpnsldr

ahhhh Nice!

t=s/(u+v)/2

Thanks so much guys! My heros!