1. Jun 20, 2013

The Coin(Green) in lying on a paper which is lying on a box.When I pull the paper with a higher force so that I could remove it easily,the coins stays about the same place.
When I remove the paper slowly,the coin comes with the paper.
Why?

I think that objects can react(Move) to smaller forces easily but for larger forces,it doesn't.
What do you think?

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Last edited: Jun 20, 2013
2. Jun 20, 2013

### Staff: Mentor

Because the coefficient of static friction is higher than the coefficient of kinetic friction.

No, that is wrong.

3. Jun 20, 2013

What?I don't Understand

4. Jun 20, 2013

### jbriggs444

In addition to the difference between static and kinetic friction, there is the difference between fast and slow.

If you pull the paper with high force, you pull it fast. There is not much time for the coin to speed up. If you pull the paper with low force, you pull it slow. There is more time for the coin to speed up.

The momentum delivered by a fixed force over a time interval is proportional to the duration of that interval.

5. Jun 20, 2013

### Staff: Mentor

Static friction force is given by $f_s\le\mu_s N$ and kinetic friction force is given by $f_k=\mu_k N$. In both cases N (the normal force) is the same, so $f_s$ (the friction force without slipping) can be larger than $f_k$ (the friction force with slipping).

6. Jun 20, 2013