a) He simultaneously throws both stones horizontally off the back of the wagon.

Question

a) He simultaneously throws both stones horizontally off the back of the wagon. The stones fly out of his hand with a speed of 8 m/s relative to Taqwaan. How fast is the wagon moving forward afterwards?

b) He stops and gets out of the wagon, gathers up the stones, and sits in the motionless wagon again. This time, he throws the stones out one at a time; each stone again leaves his hand at a speed of 8 m/s (relative to him).

i. How fast is he moving after he has thrown the first stone?

ii. How fast is he moving after he has thrown the second stone?

The answers should be:

a) 1.45 m/s

b) i. 0.73 m/s

ii. 1.53 m/s

Please explain how to get to set up the equation to get these answers.

Explanation / Answer

Your answers are wrong:

a)
Before he throws the stone the net momentum is 0.
So after throwing the stone, his momentum shold be 0.
So,
momentum of stone + momentum of taqwan = 0
2*5*(-8) + (35)*v =0
v = 80/45 m/s
=1.78 m/s
Answer: 1.78 m/s
b)
i)
for 1st stone:
momentum of stone + momentum of taqwan = 0
5*(-8) + (35+10+5)*v1 = 0
v1 =0.8 m/s
Answer: 0.8 m/s
ii)
After 1st stone is thrown, the velocity of taqwan is 0.8 m/s
Speed of 2nd stone relative to ground = 8-0.8= 7.2 m/s
consider 2nd stone,wagon and taqwan as system
momentum of system before throwing 2nd stone = momentum of system after throwing 2nd stone
50*0.8 = 5*(-7.2) + 45 * v2
v2 =1.69 m/s
Answer: 1.69 m/s

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