What is the Solution to a Perfectly Inelastic Collision Problem?

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In a perfectly inelastic collision problem involving a 90-kg halfback and a 120-kg opponent, the initial velocities are 9 m/s north and 3 m/s south, respectively. The correct approach uses the conservation of momentum equation, leading to the calculation of the final velocity after the tackle. The initial attempt yielded an incorrect final velocity of 5.57 m/s due to not accounting for the direction of the velocities. After clarifying the positive direction, the correct final velocity is determined to be 2.14 m/s. The discussion emphasizes the importance of considering direction in momentum calculations.
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Homework Statement


A 90-kg halfback running north with a speed of 9 m/s is tackled by a 120-kg opponent running south with a speed of 3 m/s. If the collision is perfectly inelastic and head-on, calculate. (a) the velocity of the players just after the tackle and (b) the total energy lost as a result of the collision.


Homework Equations


m1v1i + m2v2i = (m1 + m2)vf


The Attempt at a Solution


I plugged the masses and velocities into the equation for perfectly inelastic collisions and solved for vf. I got 5.57 m/s and the answer is 2.14 m/s.
 
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Which direction have you picked as positive, north or south?
 
OK I see what I did. I didn't take into account negative velocity. Thanks!
 

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