I'm trying very hard to understand the following concept:

Object A is placed on object B and is pulled forward with a certain force (F). There is no friction between object B and the surface it is on, but there is a certain friction betweem A and B.

If F is less than fsmax then the two objects will move forward together.

However, I'm tring to understand why the friction should increase as the force increases. How does the increase in force change anything in the relation between A and B.

Whilst sitting, troubled over this question - I pondered whether perhaps I have a fundamental misunderstanding of the concept?

Anyhow, I would be glad of any help.


Daniel.

Frictional forces are "lazy" forces. A frictional force won't exert a force until it has to, else you would accelerate as a result of friction. When you pull the boxes and the force is less than the max, friction will not be at its max; therefore, as you increase F, the frictional force required to keep the box from sliding will increase. However, there is a maximum force that can be exerted by friction, and once the force caused by F gets too large, the box will slide.

Two types of friction. Friction between two sliding sufaces and friction between non-sliding sufaces. Friction between non-sliding surfaces is usually greater.

But why, before reaching the max, doesn't the friction match the force exerted?

You also must consider inertia. If Fmax is applied quickly (an impulse) then the inertia of block B will be enough to overcome the friction between block A and B. The magnitude of the friction between blocks A and B is also dependent on the weight of block A.