Static Friction

Static Friction

Static Friction

Static Friction

If we didn't have static friction...

...then we wouldn't be able to walk.

Static friction prevents your shoe from sliding at the start and end of a step.

 f_smax = μ_sF_N

• The coefficient of static friction is a measure of the relative difficulty of sliding two surfaces across each other.

• The easier it is to slide one surface on the other, the smaller the coefficient is.

• This coefficient is unit-less and typically has a value between 0 - 1.

Why is friction dependent on normal force and not on mass?

Think back to the friction experiment. If we were to press down on the block with a greater and greater force, the mass of the block wouldn't change, but the normal force would increase...

...thus increasing the maximum static friction force.

Kinetic Friction

• Kinetic indicates that the surfaces in contact are moving relative to each other.

• A similar relationship exists as between the friction force and the normal force, but with two important differences:

• Under the same conditions, the magnitude of the kinetic friction force is always lower than the maximum static friction force.

• The resistive force exerted by the surface on the moving object has a constant value.

Kinetic Friction

What causes friction?

Even the slickest surfaces have bumps that can hook onto the bumps on another surface.

Smoother surfaces should have

reduced friction, consistent with our previous findings.

Fun fact:

If two surfaces are too smooth, friction increases again (for example, two pieces of plastic wrap). 

This is due to attraction between particles at the surface that are too close together without typical surface bumpiness.