# Help with frictional force concept

1. Feb 21, 2012

### Jetview

Salutations everyone!

I'm sure this is a simple question for many of you, but I don't understand why doesn't the force of friction immediately stop an object after an applied force? And for an instance when an object is moving and eventually comes to a halt, was it because the force of friction increased or the applied force gradually decreased to a point the object is stopped by the bigger frictional force??

2. Feb 21, 2012

### bp_psy

The kinetic friction force is determined by the normal to the contact surface and the kinetic friction coefficient.The friction is constant if the normal or contact surfaces do not change.If the kinetic friction force is greater then the other force on the object it will do negative work on the object until it stops.At that point we no longer talk about kinetic but by the static friction force which is now equal to the sum of the other forces on the object but smaller then the max fs.

3. Feb 22, 2012

### Lsos

Force never stops an object "immediately". Force accelerates (or decelerates) an object at a certain rate, depending on its mass.

If there was no friction, the object would just keep on moving forever after we stop applying force to it. Because there is friction, the object slows down and eventually stops. But this always takes a finite time. Even if it looks immediate for some very light objects, it still takes a measurabe amount of time.

4. Feb 22, 2012

### tiny-tim

Welcome to PF!

Salutations Jetview! Welcome to PF!
I think you're confused about applied forces.

When friction reduces something to a halt, there's usually no applied force …

with no force, it will carry on at the same speed forever (good ol' Newton's first law) …

the friction reduces its momentum slowly and steadily to zero.

If there is an applied force (eg from an engine), that applied forces is usually constant, and so is the friction, so the acceleration (which may be negative) is also constant.​