rollling friction?

Question

what happens to the friction as 'a box on 4 wheels' gradually slows down and comes to a stop? does the friction increase/decrease or remain the same? why? what about the net force?

Answer 1

Friction doesn't change, because the box's mass and the earth's acceleration remains constant. Of course, I'm also assuming that the surface it's rolling on doesn't change either. Net force causes an acceleration, so that doesn't change either. If a net force is applied to the object against its motion, it will eventually slow down and stop.

Answer 2

Actually, the coefficient of rolling (kinetic) friction is greater than the coefficient of static friction for otherwise the same conditions. This is why, in the olden days, we were taught to "pump your brakes" to slow a car down quickly without getting into a skid by locking the brakes and resulting in a static friction situation. Pumping the brakes kept the tires rolling to take advantage of the higher rolling coefficient of friction. Nowadays, ABS brakes do the pumping for us, but the physics is the same.

So, to your point, the coefficient of rolling friction remains the same until your box actually stops. At that time, the static coefficient takes over and the rolling and static coefficients are not the same. The net force decelerating the rolling box remains fixed because the rolling friction force remains fixed a R = rN; where r is the rolling coefficient for a box weighing N, and as you can see, this stays the same until the box stops. S = sN when the box is static where s is the static coefficient for the same N weight.

So as soon as you push on the box with some force F > S, the box will start to move (accelerate) and rolling friction R becomes the new resisting force. Thus, when the rolling starts, F > R and the box acceleration would decelerate a bit because (F > R) < (F > S).

Bottom line, if F is the pushing force throughout, the net force will be greater on the static box, but only for an instant...until the box starts to roll. Then the net force lessens because rolling friction is greater than static friction, all things equal.

Answer 3

Friction depends on the surface. If the surface does not change, the friction remains the same.

As for the net force, the car is moving, so the net force is +ve until it comes to a stop, which will become zero.

Answer 4

When a body rolls over other then the friction comes in play is rolling friction. The friction applies remains the same but the energy lost due to friction, so body comes to stop.