Calculating velocity at a certain time

[phyyy]

Homework Statement

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 traveling in the oposite direction but is seems a little odd. Could this be the right answer?

 

[collinsmark]

Hello phyyy,

Welcome to Physics Forums!

Homework Statement

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

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).

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

That shouldn't seem that odd. Velocity is vector. It has both magnitude and direction.

 

[phyyy]

Thanks collinsmark!