Answer:
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
