# Warm-Up Exercise for lecture 8

## Due 8:00 am, Thurs, Sep 25

Physics 105, Fall 2008

You start to push against a refrigerator. It initially does not budge, but then after you push hard enough it starts sliding along the floor. Which is correct?
☐ The friction force was smallest just after the refrigerator started to move.
☐ The friction force was largest just after the refrigerator started to move.
☑ The friction force was largest just before the refrigerator started to move.
☐ The friction force was constant the whole time.

A block is sliding down a ramp with constant speed. There is friction between the block and the ramp. The friction force is:
☐ smaller than the component of gravity down the ramp
☐ larger than the component of gravity down the ramp
☑ equal to the component of gravity down the ramp

A small block is "riding" on top of a large block, which is being pulled to the right with a rope. Both blocks accelerate at the same rate. The force of friction acting on the small block is:
☐ to the left
☑ to the right
☐ nonexistent

Ralph noticed that in the equation for the force of static friction, there's a "less than or equal to" sign, but in the equation for kinetic friction, there is a plain equals sign. He wants to know why they are different. What should I tell him?

Friction opposes motion. In the case of static friction, it opposes potential motion. The friction force required to keep an object from moving depends on how much force you are using to try to move the object. Thus, the static friction force is variable, up to some maximum amount.