Friction What causes Friction? Static, Kinetic and Rolling Newton’s 2nd Law with Friction Outline

What Causes Friction?

Static Friction [ in the direction that prevents slippage] This is the friction that keeps things from starting to move Notice that this is an inequality! FA

Kinetic Friction [ in the direction that reduces the relative motion between the two surfaces] This is the friction that you get when the two surfaces are already moving relative to each other This one is an equality v

Rolling Friction [ in the direction opposite the motion] Rolling friction is what makes a rolling object eventually stop, not what makes a tire stop when the brakes are applied (static friction does that) [ in the direction opposite the motion] The coefficient of rolling friction is much smaller than the coefficient of kinetic friction. Dragging a bicycle with its brakes on is much harder than rolling it along Rolling friction is caused by the flexing of the object that is rolling. It constantly has to change its shape such that the bottom portion of the rolling object is more flat than the rest. Flexible objects have the highest rolling friction.

Copper on steel 0.53 0.36 Aluminum on aluminum 1.5 1.1 Teflon on Teflon 0.04 0.04 Values depend on smoothness, temperature, etc. and are approximate Usually m < 1, but not always Usually, mk is less than ms , and never larger The coefficients depend on the materials, but not on the surface areas, contact pressure, etc.

QQ38: Inequality Ex 1: A 10 kg box on a level floor. The coefficient of static friction between the box and the floor is 0.5, and kinetic friction 0.3. What is the force required to make the box move?

QQ38: Inequality Ex 2: A 10 kg box is moving over a level floor. The coefficient of static friction between the box and the floor is 0.5, and kinetic friction 0.3. What is the force of friction ?

QQ38: Inequality Ex 3: A 10 kg box is pushed over a level floor with a force of 100N. The coefficient of kinetic friction between the box and the floor is 0.3. What is the acceleration of the box?

Example: You are pulling a sled across a level, snowy field. QQ39: NII plus Friction Example: You are pulling a sled across a level, snowy field. There is a package on the sled, which you do not want to have slip off. The coefficient of static friction between the package and the sled is 0.5. What is the maximum acceleration of the sled for which the package does not slip?

Do for next class: Read: Sections 5.6 Suggested problems: 5.50, 5.51, 5.53 Do as many more of the problems from the end of the chapter as it takes you to realise that they are all the same. If this takes you more than two full problems, make sure that you are following the procedure that I followed for the examples in class.