# Homework Help: Mass oscillating on spring

1. Dec 3, 2008

### veitch

1. The problem statement, all variables and given/known data

The position of a mass that is oscillating on a spring is given by x(t) = (14.5cm)cos[(14.0s-1)t].
What is the speed of the mass when t = 0.820 s?

2. Relevant equations

A= Amplitude
W= Angular frequency

x(t) = ACos(Wt)
v(t) = -WASin(Wt)

3. The attempt at a solution

v(0.820) = (-14.0)(14.5)Sin[(14.0)(0.820)]
= (-203)Sin[11.48]
= -40.4 cm/s

This seems so easy and yet apparently I have the wrong answer... any insight? :/ Thanks in advance

2. Dec 3, 2008

### fluidistic

What you did seems right to me.

3. Dec 3, 2008

### timmay

Check the units of your frequency. It looks like you have the value in Hertz (or oscillations per second); try converting the instances of frequency to radians per second (and if you're carrying out the sine calculation on a calculator then change the mode to radians instead of degrees).

4. Dec 3, 2008

### veitch

Okay, that makes more sense...

W=2 x pi x freq

thus v(t) = -(2 pi 14)(14.5)Sin[(2 pi 14)(0.82)]

which gives -160 cm/s (with 3 sig figs)... but it still isn't right.
I suppose there shouldn't be a negative should there? Since speed is scalar... but 160 isn't the right answer either. :/