# Warm-Up Exercise for lecture 7

## Due 8:00 am, Tues, 22 Sep 2009

Physics 105, Fall 2009

Enter your 3-digit class identification number:

Enter the last 4 digits of your BYU ID:

Did you complete the reading assignment? (Typically this means at least 20-30 minutes looking over the assigned sections of the book.)
☐ yes ☐ no

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 to the right. 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?

The comments in the next next two boxes go into a big, mostly anonymous text file that I skim through before the morning lecture, using the comments to help me plan class discussion. ("Mostly anonymous", because I can track down who made what comment, but it takes some effort on my part to do so.) Therefore, if you really want to make sure I see your question/comment and answer it individually, you should send it to me via email and not through this form.

Which part of today's assignment was particularly hard or confusing? What would you like to spend extra time on in class?