# Describe the angular momentum of the ball and net torque on

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1. Mar 13, 2016

### heartshapedbox

1. The problem statement, all variables and given/known data
1. At the instant illustrated, which best describes the angular momentum of the ball and net torque on the ball, as measured around the origin?

L⃗ is in the kˆ direction, ⃗τ is 0.
2. Relevant equations
torque= (F)x(r)
Tension in rope= (mv^2/r)+qvb

3. The attempt at a solution
I am at a loss, I do not understand this word problem. Can this please be explained?

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2. Mar 13, 2016

### bigguccisosa

Is this a multiple choice problem? I'm not sure I understand what you mean by which best describes the angular momentum.

3. Mar 13, 2016

### heartshapedbox

This is the complete problem, I do not know how to do #3. :) Thanks!

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4. Mar 13, 2016

### bigguccisosa

Do the three stars indicate anything in particular (***)?

5. Mar 13, 2016

### bigguccisosa

Anways, what you have is a problem where you must determine what forces are acting on the particle, and how they influence the torque and angular momentum. Recall that $\vec{\tau} = \vec{r} \times \vec{F}$ and $\vec{L} = \vec{r} \times \vec{p}$. The right hand rule will come in handy. There is a rope, so that will apply a force of tension on the particle. There is also a magnetic field, which way will the force point due to that?

6. Mar 13, 2016

### heartshapedbox

the correct answer is marked by "***" :)

7. Mar 13, 2016

### heartshapedbox

Ok thank you, I believe I understand. Right hand in direction of velocity, curl towards r, L is out of the page, so k direction.
Right hand in direction of velocity, curl towards F (there is the force of B and the centripetal force) they point in opposite directions, so they cancel, making torque zero?

8. Mar 13, 2016

### bigguccisosa

Yes right hand in direction of velocity (and so linear momentum), curl towards r, so L points in positive k. For torque you should be looking at the direction of the Forces, and crossing them with r. Note that the tension points towards the centre, and the magnetic force points away (F =qv x B). So if you cross them with respect to the r vector, do they contribute to torque? But yes in the end the torque is zero.

Last edited: Mar 13, 2016
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