1) A 40 kg girl gets on her 10 kg wagon on level ground with two 5 kg bricks. She throws the bricks horizontally off the back of the wagon one at a time at a speed of 7 m/s relative to herself. a) How fast does she go after throwing the second brick

1.4 m/s

Explanation:

Assuming that the system is an isolated one, then we will conserve the linear momentum of the system so as to be able to solve the system for required quantities:

Using the conservation of momentum, we have,

initial momentum = final momentum

P(i) = P(f)

And from the question, we see that the system has no initial momentum

After throwing the first brick we have

(40 + 10 + 5)v2 + 5 * -7 = 0

55v2 - 35 = 0

55v2 = 35

v2 = 35/55

v2 = 0.63 m/s

Now, considering that she conserves momentum when she throws the second brick again, we then say

(40 + 10 + 5) * 0.63 = (40 + 10)v(f) + 5 * -7

55 * 0.63 = 50v(f) - 35

35 = 50v(f) - 35

50v(f) = 35 + 35

50v(f) = 70

v(f) = 70/50

v(f) = 1.4 m/s