# Warm-Up Exercise for lecture 9

## Due 8:00 am, Tues, Sep 30

Physics 105, Fall 2008

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, or
the value of the acceleration of 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 the acceleration of the car down the steep hill is much greater than down the other hill. What should you tell him?