# Why does friction create rolling?

1. Aug 10, 2008

### giladbr

Consider a round object given a push at the middle. The push generates linear velocity, and If there wasn't any friction, the object would start sliding; but the friction in the contact point with the ground causes the object to start rolling.

Intuitively I can explain it: the friction causes the linear speed of the contact point to diminish, and since the other parts of the object still move in the same velocity, rolling is created.

But could somebody provide me with a more thorough physical explanation of what actually happens when the object starts rolling?

Thank you.

2. Aug 10, 2008

### Staff: Mentor

The friction at the edge (surface) applies a moment (force at distance from rotational axis or center of mass) which induces a torque, which induces a change in angular momentum of the mass about the rotational axis.

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