Elastic Collision

  • Thread starter LSarah6
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Homework Statement


A collision occurs between two football players. Player 1 has mass of 75 kg and a velocity of 6 m/s. Player 2 has mass of 150 kg and a velocity of -3 m/s. Assuming an ELASTIC collision, what is the final velocity of player 2?


Homework Equations


They gave us this equation in class to calculate the final velocity:
m = mass
Vi = initial velocity
Vf = final velocity
a = player 1
b = player 2

Vfb = [(2*ma)/(ma+mb)]*Via + [(mb-ma)/(ma+mb)]*Vib


The Attempt at a Solution


Using the equation above and plugging in the numbers:

Vfb =
[(2*75kg)/(75kg + 150kg)]*(6 m/s)
+
[(150kg - 75kg)/(75kg + 150kg)]*(-3 m/s)

which gives me:

Vfb = 4 m/s + -1 m/s = 3 m/s


The "online" homework solution says it is -3 m/s. Am I incorrect? Did I miss a negative sign somewhere, or is there a principle of momentum that I am not getting?
 

Answers and Replies

  • #2
Doc Al
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The "online" homework solution says it is -3 m/s.
That answer implies that the velocity of the 2nd player is unchanged by the collision--which makes no sense.
 

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