# Homework Help: Find Torque given Angular momentum and time

1. Jun 23, 2012

### btlogan2

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

A particle is to move in an xy plane, clockwise around the origin as seen from the positive side of the z axis. In unit-vector notation, what torque acts on the particle at time t = 5.3 s if the magnitude of its angular momentum about the origin is (a) 5.7 kg·m2/s, (b) 5.7t2 kg·m2/s3, (c) 5.7t1/2 kg·m2/s3/2, and (d) 5.7/t2 kg·m2*s?

Additionally, the answers have to be in terms of i, j, and k.

2. Relevant equations

Torque= r X F

L= r x p = m(r x v)

Tnet = dL/dt

3. The attempt at a solution

I don't understand how to use what is given into any of the formulas. Torque is equal to the time derivative of the angular momentum. So how would I go about solving this.

2. Jun 23, 2012

### vela

Staff Emeritus
Like you said, the torque is the time derivative of the angular momentum, so differentiate the angular momentum. First, you need to understand the angular momentum of the object. You're given the magnitude of the angular momentum as a function of time. What about its direction?

3. Jun 23, 2012

### btlogan2

How do you find the direction with the information provided?

4. Jun 23, 2012

### vela

Staff Emeritus
The direction of the angular momentum is directed along the rotational axis, according to the right-hand rule. The first sentence provides the needed info.

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