Describe the angular momentum of the ball and net torque on

[heartshapedbox]

Homework Statement

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.

Homework Equations

torque= (F)x(r)
Tension in rope= (mv^2/r)+qvb

The Attempt at a Solution

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

 

[bigguccisosa]

which best describes the angular momentum of the ball

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

 

[heartshapedbox]

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

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

 

[bigguccisosa]

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

 

[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?

 

[heartshapedbox]

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

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

 

[heartshapedbox]

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?

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?

 

[bigguccisosa]

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.

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.