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# OCR mechanics friction watch

1. Hi

The equation for the frictional force for OCR M1 is:

F=uR

Where;

F= the maximum frictional force
u= coefficient of friction
R= normal reaction force

We are taught that the frictional force is equal to the driving force unless the driving force is greater than uR, in which case the frictional force is equal to uR.

My question is that if I apply a driving force to an object (greater than uR) and then release that force, there will be an initial frictional force but then won't the friction be equal to zero because my driving force is now zero? And therefore the object just moves at a constant velocity forever?

Have I missed something or is this just a simplified equation?
2. (Original post by JAVM)
Hi

The equation for the frictional force for OCR M1 is:

F=uR

Where;

F= the maximum frictional force
u= coefficient of friction
R= normal reaction force

We are taught that the frictional force is equal to the driving force unless the driving force is greater than uR, in which case the frictional force is equal to uR.

My question is that if I apply a driving force to an object (greater than uR) and then release that force, there will be an initial frictional force but then won't the friction be equal to zero because my driving force is now zero? And therefore the object just moves at a constant velocity forever?

Have I missed something or is this just a simplified equation?
What you have been taught doesn't always work, as your example shows. You can probably imagine that if you were to release the force on a moving object then it would obviously decelerate due to friction.

In M1 for an object in motion (accelerating, deccelerating or constant speed), the friction will always be at its maximum, independedent of what the driving force is. So for a moving object.

In reality static friction and kinetic/dynamic friction are often different but in M1 you can assume that when an object is moving and for a stationary object.
3. Ok great, thank you. That makes sense.

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