# Homework Help: Calculating velocity at a certain time

1. Apr 3, 2012

### phyyy

1. The problem statement, all variables and given/known data
The following describes the position of a body as a function of time: $X(t)=9\cdot cos(1.2t)\cdot e^{-t/10}$.

What is the velocity of the body at t=17sec ?

2. The attempt at a solution
I'm trying to use the formula $v(t)=\frac{dr}{dt}$ in order to get the velocity and I get: $-9\cdot e^{\frac{-17}{10}}\cdot [sin(1.2\cdot 17)\cdot 1.2+\frac{1}{10}\cdot cos(1.2\cdot 17)]=-1.975$

I guess negative velocity means the body is travelling in the oposite direction but is seems a little odd. Could this be the right answer?

2. Apr 3, 2012

### collinsmark

Hello phyyy,

Welcome to Physics Forums!

That's what I got (ignoring any rounding differences). But that assumes that the number 1.2 in the cosine function has units of rad/sec (as opposed to deg/sec).
That shouldn't seem that odd. Velocity is vector. It has both magnitude and direction.

3. Apr 4, 2012

### phyyy

Thanks collinsmark!