posted by Ty on .
Jack, jill and spot go out for a picnic at the side of the river. jack takes spot for a jog. jack and spot are on the wast bank of the 30m wide river when jill is on the east side. the rivers current is 0.75m/s south.
a) spot jumps into the river and swims straight east at 1.8m/s relitive to the water. once across spot runs 4m/s on the shore to get to jill. How long does to take spot to get to jill?
b)jack dives in the river and aims slightly upstream and swims 2m/s relitive to the water to go straight across. How many seconds does jack beat spot by?
a) time for spot to cross river: 30/1.8
spot goes this distance downstream*.75*30/1.8
time for spot to reach jil:30/1.8+distancdownstream/4
b> Jack swims at arctan .75/2 upstream. His velocity across the river is 2*cosine of that angle, and time across is 30/vacrossriver.
Do some calculating, and you have it.