Physics Lesson 10/27

Physics Lesson 10/27

Friction

Open Ended

Journal - Chop Wood Carry Water

Kinetic and Static Friction

• What happens to the book as I slide it across the table?

• The friction force of the desk acting on the book actually accelerated the book.

• It accelerated it in the direction opposite to which the book was moving.

• Up to this point we haven't considered friction in solving problems, now we will look into it.

Types of Friction

• There are two main categories of friction

• When I moved the book across the desk it experience that type of friction that acts on moving objects.

• Kinetic Friction - the force exerted on on surface by another when the two surfaces rub against each other because one or both surfaces are moving.

Types of Friction

• Imagine trying to push a couch across the floor.

• You push on it with a small force, but it does not move.

• Because it's not accelerating, Newton's laws tell us the net force on the couch must be zero.

• Therefore we know there is a second horizontal force acting on the couch that opposes your force and is equal in size.

Types of friction

• This force is static friction

• Static friction is the force exerted by one surface on another when there is no motion between the two surfaces.

• If you continue to push on the couch with more force and it still doesn't move, the force of static friction must be increasing.

Poll

Is there a limit to the static friction force?

• Yes

• No

Types of friction

• Absolutely, there is a limit to the static friction force.

• If you kept pushing the couch harder, it would eventually begin to move.

• When this occurs, your applied force is greater than the static friction force and kinetic friction begins to act on it.

Mathematical Models for Friction Force

• On what does friction force depend?

• The materials the surfaces are made of definitely play a role.

• For example, is there more friction between running shoes and concrete or between socks and a polished wood floor?

Poll

Does the force of friction depend upon the surface area in contact or the speed at which surfaces move past each other?

• Yes

• No

Mathematical Models for Friction Force

• Surprisingly, no. Experiments have shown this is not the case.

• The normal force between the two objects does matter however.

• The harder one object is pushed against the other, the great the force of friction.

Kinetic Friction

• 
  

   $F_{f,\ kinetic}\ =\ \mu_kF_N$ 

• This is the formula for kinetic friction force

•  $\mu_k$  is the coefficient of kinetic friction and it relates the friction force to the normal force.

• This value changes depending on the surfaces.

Coefficients of Static Friction

• Cast iron on cast iron = 1.1

• Glass on Glass = 0.94

• Leather on oak = 0.61

• Nonstick coating on steel = 0.04

• You get the idea, the possibilities are endless.

Open Ended

Exit ticket - Are there any surfaces on Earth that do not cause friction? Explain.