# Differentiation Help

1. Aug 20, 2008

### RoryP

Differentiation Help!!

1. The problem statement, all variables and given/known data
A particle is moving along a straight line so that, at time t seconds after leaving a fixed point O, its velocity vms-1 is given by v=10sin(1/2 t).
Find the time when the acceleration, given by dv/dt, is first zero.

2. Relevant equations
sinx differentiates to cosx

3. The attempt at a solution
Ok i know they are asking me to find the time where there is a stationary point so therefore the gradient or dv/dt = 0.
From differentiating 10sin(1/2 t) i got 5cos(1/2 t). so therefore, 5cos(1/2 t)=0 so i thought you multiply by 2 to get rid of the half then divide by 10 so the Right hand side= 0 and then inverse cos of 0 = 1/2 Pi but the answer in the textbook gives Pi.

2. Aug 20, 2008

### Defennder

Re: Differentiation Help!!

You've got the steps right, just note that it's 1/2t = arc cos 0 and not t = arc cos 0.

3. Aug 20, 2008

### RoryP

Re: Differentiation Help!!

ohhh yeah of course!! thank you very much =]