Solving Elastic Collision Homework: Player 2's Final Velocity

[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. 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?

 

[Doc Al]

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.

 

[m2003]

Your solution is correct. The final velocity of player 2 should be 3 m/s, not -3 m/s. It is possible that the online homework solution is incorrect. It is also possible that there is a mistake in the given equation or in the values provided for player 2's mass and initial velocity.

It is always important to double check your calculations and make sure you are using the correct units in your equations. If you are still unsure, you can consult with your teacher or a classmate to verify your solution. Remember, in science, it is important to always question and critically analyze the information presented to us. Keep up the good work!