1.A 45 kg girl stands on a 7 kg wagon holding two 21 kg weights. She throws the

Question

1.A 45 kg girl stands on a 7 kg wagon holding two 21 kg weights. She throws the weights horizontally off the back of the wagon at a speed of 8 m/s relative to herself.

a) Assuming that the wagon was at rest initially, what's the speed of the girl relative to the ground after she throws both weights at the same time?

b) If the wagon was initially at rest, what's the speed relative to the ground with which the girl will move after she throws the weights one at a time with a speed of 8 m/s each relative to herself?

Explanation / Answer

a)

here

(8m/s - v) * (21kg + 21kg) = (45 kg + 7 kg)*v

v = 3.57 m/s

b)

Total momentum before throw = (45 + 7 + 21) * 2 = 146 kgm/s

Total momentum after throw = Momentum of thrown weight + Momentum of Girl + Wagon.

Momentum of thrown weight = 21 * (-8+3.57) = -93.03 kgm/s

Momentum of Girl + Wagon = (45 + 21) * V2 = 52 * V2.  

146 = -93.03 + (52 * V2)

239.03 = 52 x V2

V2 = 4.596 m/s

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