# Why Do Objects move?

Let's imagine a block resting on horizontal table. When We apply a force which is greater than frictional force it moves.

But why does this happen? Why can't friction withstand any force? As we know that one cause of friction is due to interlocking of irregularities, I think there is a opposing force as irregularities apply a normal force .

So why if applied force is increased the normal force can't withstand the increased force? Also is there any possibility that in a situation the frictional force could always cancel the applied force?

Your image shows what is going on at the microscopic level between two surfaces. To understand why friction works, you have to look smaller, at the atomic level: and when you get to that point you're no longer taking about "friction" as we know it, but about physiochemical interactions between atoms and molecules. Those interactions are mediated by electromagnetic force and tend to be quite weak at the boundaries between two solid objects. As one example demonstrating the fundamentally chemical nature of the interactions, note that the reason it's easier to grip paper if your fingers are slightly damp is because the polar water molecules form bonds between your skin, the paper and each other, somewhat "gluing" everything together.

To add a little to Asher and Gert's answers: The atoms on surfaces are constantly in motion.

If you're used to thinking of solids as being really rigid and difficult to deform, try this viewpoint instead: think of them like a bunch of grains of sand held together by tiny Slinkies. The atoms can move out of the way and slide back into place, or even be replaced and find another home elsewhere.

The wikipedia article for Surface Diffusion has some nice animations showing some related phenomena.

Why can't friction withstand any force? Because the amount of friction that inter-surface interactions can provide is limited, not unlimited.

In the idealised force diagram below, only the object's weight $mg$ provides a vertically downward (aka 'Normal') force, causing a friction force between the object and the plane.

This friction force is macroscopically modelled by $F_2=\mu mg$, where $\mu$ is a coefficient of friction (this is only a simple model of friction in which $\mu$ is considered independent of any movement).

According Newton's Laws of Motion, if $F_1 > \mu mg$, the friction force $F_2$ is overcome and the object will start sliding over the surface. $F_1$ is an externally applied force.

• Friction is not modelled $F_2=\mu mg$ but rather $F_2=\mu n$. In this simple case that you have set up, $n$ just happens to be equal to the weight $n=w=mg$ – Steeven Aug 20 '15 at 19:13

So why if applied force is increased the normal force can't withstand the increased force?

Also is there any possibility that in a situation the frictional force could always cancel the applied force?

In that case you would need a material (surface) which is infinitely strong.

• If you put a heavy stone on a table, the table applies the normal force to hold it up.
• Put on a heavier stone, and the table applies an even larger normal force to still keep it up.
• Put on a too heavy stone, and the table will apply its largest possible normal force; but since this is not enough the table will break. The limit of the strength of the table material has been reached.

The strength of materials depends on direction, microstructure, composition, density etc. and depends on molecular bonds in the end. The point is simply that everything existing and any material has a strength limit.

Each and every small peek in the close-up image you give have their own individual strength in this horizontal direction (also depending on how far deep into the gaps the objects falls). Summing it all up gives you some large force that they are able to withstand before breaking.

So, even if the bonding would never let go, there would still be a material strength limit where the peeks on the microscopic level simply break apart. This is like glue; the glue bondings with the surface might be stronger than the peeks of the surface material themselves. Then those peeks will break before the glue lets go.