Understanding Angular Momentum Conservation

[mathdude88]

Homework Statement

Homework Equations

dl/dt= r*f=t

The Attempt at a Solution

I think the answer is b or c but I'm not sure what the difference in the two statements are.

 

[Hootenanny]

The difference is that if a force acts through the centre of mass of the body then there is no torque, but there is still an external force acting on the body.

Does that make sense?

 

[D H]

In contraposition to Hootenanny's answer, there can be a force couple that acts on the body. The net force is zero, but there is still an external torque acting on the body.

 

[mathdude88]

So 0 torque can result from 0 position vector or 0 force acting on it.? and no net external force acts means 0 force? If change in angular momentum is equal to torque and torque is equal to position vector times force wouldn't 0 torque and 0 force equal constant angular momentum?

 

[mathdude88]

how can 0 net force result in torque?

 

[D H]

how can 0 net force result in torque?

Put a book on a table. Apply forces equal in magnitude but opposite in sign to two corners of the book. For example, push left on the far right corner and right on the near left corner. What is the net force on the book (hint: the forces are equal in magnitude but opposite in sign)? What happens to the book?

 

[mathdude88]

the book would rotate counter clockwise. That's pretty cool torque is created from 0 net forces just because they act over a distance.

 

[mathdude88]

so the answer has to be choice B because change in torque would result in slowing down or speeding up. Increasing opposing forces would still result in no net force but also would result in speeding up.

 

[D H]

Excellent.