What is Friction?
- Friction is a difficulty in movement caused by imperfections in surfaces.
- If surfaces were perfectly smooth, then there would be no friction and everything would go sliding forever (ignoring air resistance) at the slightest nudge.
- Friction and similar forces are all contact forces. This means that they have no effect over distance.
- Field forces for example, are non-contact forces as they can act over a distance.
Normal Force
- Normal force is the force that acts directly perpendicular to an object on a surface.
- For example, as you are sitting in your chair, a normal force, equal to your own weight (in Newtons) acts against you.
- If the normal force didn't act against you, you'd fall right through your chair!
- Normal force always acts perpendicularly to the object, even if the surface is tilted.
Frictional Force
- Ever noticed how even if an object is at an incline it can still manage to stay still?
- This is due to the frictional force preventing it from movement.
- Frictional force always acts against movement. For example if you are pushing a box, the frictional force will be acting opposite to the direction you are pushing it.
- There are two forms of frictional force, static and dynamic.
- Static frictional force prevents still objects from moving.
- Dynamic frictional force slows down moving objects.
- Static friction is always stronger than dynamic friction, which is why its easier to keep an object moving than to make it start moving.
- Frictional force for static friction can be calculated via the formula below.
- Frictional force for dynamic friction can be calculated via this other formula below.
- Frictional force is the force required to move the object, or keep it moving in the case of dynamic friction.
- The reason for the differences in the formulas is that if frictional force is higher than the static friction, the object begins to move. If frictional force is less than the dynamic friction, then the object begins to decelerate and come to a stop once again.