Kinetic Friction

Friction

Everything you ever wanted to know (and more).

Poll

In the experiment, you observed an object that is not yet being pulled. Does your force diagram look like this?

• Yes

• No

Static Friction

Static Friction

Static Friction

Poll

In the experiment trials where you pulled on the block with greater force but it did not move, did you draw your horizontal vectors longer and longer but still balanced as shown on these previous slides?

• Yes, I drew them correctly.

• No, I did not draw them correctly and I don't understand why.

• No, I did not draw them correctly but I understand what I did wrong.

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.

Multiple Choice

Which of the following statements is correct about static friction?

• Static friction does not depend on the surface roughness.

• Static friction always has the same magnitude for a given object on a given surface no matter what external forces are applied.

• Static friction is always parallel to the object-surface interface.

• The magnitude of the static friction force depends solely on which horizontal external forces are acting on the object.

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.

Multiple Choice

Which of the following statements is correct about kinetic friction?

• Our model works for rolling objects.

• Our model works for objects moving at very high speeds.

• Our model works for rigid surfaces and objects.

• Our model depends on the mass of the object.

Poll

Did you find this helpful?

• No, it was so boring that I could barely stay awake.

• Sort of, but I'm still confused.

• Yes, it was pretty helpful.

• Absolutely! This really clarified the concept of friction for me.