The Answers
Note that our descriptions of the moving objects reflect our own imaginations. You might have come up
with some very different descriptions, and that’s fine ... provided that your answers are conceptually the
same as ours.
1 . The average speed over the first 5 s is 10 m/s, or about 22 mph. So:
Someone rolls a bowling ball along a smooth road. When the graph starts, the bowling ball is moving
along pretty fast, but the ball encounters a long hill. So, the ball slows down, coming to rest after 5 s.
Then, the ball comes back down the hill, speeding up the whole way.2 . This motion only lasts 1 s, and the maximum speed involved is about 5 mph. So:
A biker has been cruising up a hill. When the graph starts, the biker is barely moving at jogging speed.
Within half a second, and after traveling only a meter up the hill, the bike turns around, speeding up as
it goes back down the hill.3 . The maximum speed of this thing is 30 cm/s, or about a foot per second. So:
A toy racecar is moving slowly along its track. The track goes up a short hill that’s about a foot long.
After 2 s, the car has just barely reached the top of the hill, and is perched there momentarily; then, the
car crests the hill and speeds up as it goes down the other side.