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When one surface slides over another or has a tendency to, a force parallel to the interface acts on the two surfaces which resists the sliding. This force is called friction.

This force is due to the roughness of the surfaces, the tiny bumps and valleys on a surface that prevent one surface from smoothly sliding over the other ( Fig. 1 ), and the inter-molecular attractive forces between the surfaces.

Figure 1

The inter-molecular forces are forces between the molecules of one body and those of another. So friction is ultimately electromagnetic in nature. The normal reaction is also the result of all the electromagnetic forces between the molecules at the surface of one body with those of another. In fact, if one body exerts a force F on another , the component perpendicular to the surface of contact is called the normal reaction and the component parallel to the surfaces is called friction ( Fig. 2 ).

Figure 2

Friction opposes motion. You have heard that many times. You have to be careful how you understand that. Let's say you are standing still, and then you start to walk or run ( Fig. 3 ). You have accelerated in the forward direction.

Figure 3

What is the force that accelerated you forward ?

If you are tempted to say, "My foot!", keep in mind that we need an external force to accelerate an object. An object cannot act on itself.

Your feet can push on the rest of your body and accelerate it, but if I considered your entire body as one system, then we need an external force to accelerate its center of mass.

There is only one external force in the horizontal direction: friction. Hence, it must be acting in the forward direction. How can that be?

Let's consider another example. A car accelerates forward. What is the force that provided this acceleration? If you are tempted to say, "Why, the engine!", imagine the same car out in space, drifting along. You hit the accelerator, the wheels spin, but you go nowhere. Your engine is not any good. Put the same car on a frictionless surface ( a lake of ice ), the wheels spin, and again you go nowhere. Put the car on a road with plenty of friction, and you can watch it go.

Friction is the external force that accelerates the car forward. It must then point in the forward direction. How can that be?

To understand these two examples, you must look not just at the objects that moved, but at the surfaces where the friction acts.

Figure 4

When you walk, you place your foot on the ground and push backwards ( Fig. 4 ). The foot tends to slide backward. The ground exerts an opposing frictional force, in the forward direction. Friction opposes the relative sliding of two surfaces in contact.

Figure 5

In the case of the wheel too ( Fig. 5 ), you can see that the bottom surface of the wheel tends to slide backwards along the road when we accelerate. The road resists this with friction, which acts forwards on the wheel. Of course, the reaction to this is the backward acting frictional force on the road.

There is much more to the acceleration of a car. What happens when we hit the brakes? The car is now decelerating. Which means that the frictional force on the wheels is now backwards. However, the wheels are still spinning forwards, since the car is still moving forwards. I'm afraid you will have to wait till the chapter on Rigid Body Dynamics for the full explanation. In the meantime, ponder over it.

We need an external force to accelerate. We use Newton's 3rd law, cleverly pushing the floor back so that it pushes us forward. A flat floor can push us forward only if there is friction.

Moral of this story: Friction opposes the relative sliding of two surfaces against one another, it does not oppose motion.

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