7.2
Frames of Reference and Relative Velocity
(1.5)
vac = vab + vbc
 A soccer ball is fired at 80 km/h out the back of a truck moving at 80 km/h. How fast will it move
relative to the ground? Mythbusters - Soccer Ball Shot from Truck
http://www.youtube.com/watch?v=BLuI118nhzc
 A man walks up and down stairs beside an escalator. He also walks down the down escalator and up the
up escalator. How long will it take him to walk up the down escalator? Boat in the river question
http://vimeo.com/13739329
1) A person walks up a stalled escalator in 90 s. When standing still on the moving escalator he is carried
up in 60s. How much time would it take to walk up the moving escalator? How much time does it take
to walk down the moving escalator? Hint: Write an equation for each d = vt.
2) Two boys race across a river 40-m wide with a current of 1.0 m/s. Both swim at 2.0 m/s but one points
himself upstream so that he lands directly opposite and the other points himself directly across and lands
downstream. After landing, he runs at 6.0 m/s up the shore. Who wins and by how much? Hint: Draw
vector diagrams of the velocities.
3) A power boat travels down the St. Lawrence River from Montreal to Quebec City at full throttle. The
trip takes 3.0 h. The boat then heads back to Montreal, again at full throttle. This time the trip takes 15 h.
With no gas left, the boat drifts with the current back to Quebec City. How long does the third trip take?
4) A sunbather, drifting on a raft, dives off just as it passes under a bridge and swims against the current for
15 min. She then turns and swims just as hard downstream and overtakes the raft 1.0 km downstream
from the bridge. What is the speed of the current? (p. 67) Hint: What does the problem look like to a
person sitting on the raft?
5) An athlete rows at a constant speed with respect to the water, upstream. Just as he passes a bridge his
lunch falls off the seat of the boat into the water and floats downstream. He continues upstream for 20
minutes. He reverses and finds his lunch 2 km below the bridge. How fast is the river flowing?
6) Lucy and Jean are swimming in the Ottawa River which flows at 1.2 m/s. Lucy swims 50% faster than
Jean. When Lucy swims upstream and Jean swims downstream, they meet halfway. They then angle
themselves so that they swim directly across the river, which is 750 m wide. By how much time does
Lucy beat Jean?