# Freezing of water inside a hollow sphere which is rolling

haruspex
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One which passes through both CM and point of contact. ..
Why through CM? What force has a torque about that?

• Jnan
Why through CM? What force has a torque about that?
No force would produce torque about that. I think any axis passing through the point of contact should work fine.

haruspex
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No force would produce torque about that. I think any axis passing through the point of contact should work fine.
Yes, but it does not even need to pass through the point of contact. You just need that at no time does friction torque have a torque about it.
Also, to be useful, the rotating ball does need to have angular momentum about it.

• Jnan
Okay now I understand it quite well. Let us now initiate a discussion about the linear momentum.

haruspex
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Okay now I understand it quite well. Let us now initiate a discussion about the linear momentum.
There are two ways to solve this problem.
1. Let the frictional force be F, write one equation for linear momentum and one for angular momentum about the centre of the ball.
2. Choose the axis for angular momentum such that friction has no torque about it. Angular momentum about that axis is conserved, so you only need one equation. You do not to consider linear momentum.

• Jnan