# Warm-Up Exercise for lecture 8

## Due 8:00 am, Thurs, 24 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

When you carry an object across the room, without lifting it or setting it down, you do no "mechanical work" on it.
true
false

You need to carry a suitcase up a flight of stairs. In which case will you do the most mechanical work?
You carry the suitcase up quickly.
You carry the suitcase up slowly.
Both cases involve the same amount of work.

The amount of potential energy possessed by an elevated object is equal to
the distance it is lifted
the force needed to lift it
the work done in lifting it
its acceleration due to gravity

Ralph asked me a question about today's reading assignment. According to the reading assignment, a car coasting from rest down two hills, one steeper than the other, would arrive at the bottom of each hill with the same speed, as long as the two hills have the same vertical height. (Of course, this is true only if we neglect friction and air resistance.) Ralph wondered how this could be possible, since as we learned in class the acceleration of the car down the steep hill will be greater than down the other hill. What should you 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?