# Friction between objects?

1. Apr 30, 2015

### mohamed el teir

if a body is static on horizontal rough ground it will not experience friction, but if it starts moving it will experience friction opposing its motion, this is between object and ground, but let's suppose 2 rough objects each has its coefficient of friction, suppose one of them is at rest and the other moved with its velocity tangent to the rest object (the object moving just touched the rest one and didn't collide with it), each object will experience friction due to its contact with other, the friction on the moving object will oppose its motion, but what about the friction on the rest object, will the friction make it move ?

2. Apr 30, 2015

### Svein

No, When the objects collide, the law of conservation of momentum applies. Afterwards, both objects slow down due to friction.

3. Apr 30, 2015

### Philip Wood

Bodies resting (that is stationary) on surfaces often experience frictional forces. For example if I push horizontally on a big box resting on the floor, friction will exert a force in the opposite direction on the box, even if the box isn't moving.

It depends on the other forces (if any) acting on the rest object and on the mass of the rest object. As usual, the acceleration of the rest object = resultant of forces acting on the rest object divided by mass of rest object. If the rest object is the floor (in the example I gave above) you won't notice the floor accelerating on account of the frictional force from the box, because the floor is (we hope) attached to the Earth, whose mass is rather large.

4. Apr 30, 2015

### A.T.

Yes. Friction can transfer momentum from one object to the other. It probably will also start spinning.

5. Apr 30, 2015

### USeptim

Friction force will transfer momentum to the object at rest as it must be conserved. It will also transform kinetic energy into heat since the moving object loses energy and the object at rest almost does not increase its energy.

6. May 1, 2015

### jbriggs444

If I have a box lying still in the middle of my flatbed truck as I accelerate gently when the light turns green, it is the force of friction between the box and the bed that is responsible for the box gaining momentum.

The force of static friction acts to oppose the relative motion that the box and truck would otherwise have, were it not for the friction. It acts forward on the box and backward on the truck bed.

If I accelerate more strongly, the box may begin to slide backward.

The force of kinetic friction acts to oppose the relative motion that the box and truck actually do have. It acts forward on the box and backward on the truck bed.