# Homework Help: Impulse Momentum Method for Rotational

1. Aug 5, 2012

### freshbox

1. The problem statement, all variables and given/known data
For impulse momentum, I know that collision between two object, impulse = 0.
Can somebody explain to me how come for this question the impulse is 0 "shaft A engaged to shaft b" has the same meaning as collision?

And I assume that it is right for me to add shaft A and shaft b moment of inertia together in order to solve the question but can somebody explain the reason behind it?

Thanks alot.

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2. Aug 5, 2012

### ehild

impulse momentum = angular momentum. For a rigid body, angular momentum = (moment of inertia) multiplied by the (angular velocity ω).
In a collision of point masses, the momentum (mv) stays constant. In case of interaction between rigid bodies, the moment of inertia stays constant if there is no external torque.
The two shafts rotates around the same axis, but slipping one on the other. That is some interaction between them, like in a collision. The interaction will speed up the rotation of the faster shaft and slow down the rotation of the other one, till they both rotate with the same angular velocity.
The angular momentum of a system can change if some external torque acts on it. There is no external torque in this case, so the sum of the angular momenta of the shafts stays constant.

ehild

3. Aug 5, 2012

### freshbox

Can you give some example of external torque?

4. Aug 5, 2012

### ehild

You need an external force F which is not parallel to the axis of rotation and its line does not go through the axis. You get the torque τ as force times the length of its arm d, τ=Fd. The "arm" is the distance of the line of force from the axis. When you push a door open, you exert a torque on it. The door interacts with the hinge, but your force causes an external torque.

ehild

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5. Aug 5, 2012

### freshbox

Thank you ehild. For the next question, how do you determine which momentum is -ve or +ve? Bigger momentum = +ve?

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6. Aug 5, 2012

### ehild

In this problem you decide which direction you take positive. You can choose the greater one.

ehild

7. Aug 5, 2012

ok thank you