Two rubber balls undergo an elastic collision. Ball 1 has a mass of 10 kg and an initial velocity of 5 m/s in the positive x direction. Ball 2 has a mass of 10 kg and is stationary. What is the final velocity of the two balls, v1f, and v2f?
Using conservation of momentum
m1v1i + m2v2i = m1v1f + m2v2f
as second ball was at rest, v2i = 0
m1v1i = m1v1f + m2v2f
from conservation of energy
m1v1i2 = m1v1f2 + m2v2f2
Now, put in the values, we have
50 = 10*v1f + 10*v2f
divide by 10, we get
5 = v1f + v2f ------------ (1)
250 = 10*v1f2 + 10*v2f2
divide by 10, we get
25 = v1f2 + v2f2 --------------(2)
solve for v1f from momentum equation (1)
v1f = 5 - v2f
put this in (2)
25 = (5 - v2f)2 + v2f2
25 = 25 + v2f2 - 10v2f + v2f2
0 = v2f2 - 10v2f + v2f2
0 = 2v2f2 - 10v2f
10v2f = 2v2f2
v2f = 5 m/s
v1f = 0 m/s
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