Did I Make a Calculation Error in the Elastic Collision of Two Football Players?

[LSarah6]

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 (meaning going in an opposite direction). 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?

 

[chemisttree]

The answer is obviously wrong by inspection. For the answer to be true, the velocity of Player 2 is unchanged both in sign and magnitude... no way!

I think the equation you were given is wrong. The equations should be:

http://hyperphysics.phy-astr.gsu.edu/hbase/elacol2.html#c2