In a football came, a running back (mass 45 kg) is running in one direction (say

Question

In a football came, a running back (mass 45 kg) is running in one direction (say, in the positive x-direction) at 7.5 m/s. Brian Urlacher (55 kg) is running directly at him in the opposite direction (say, in the negative x-direction) at a speed of 10 m/s to tackle the running back They collide and bounce off each other with the running back moving in the opposite direction he was initially going with a speed of 2 m/s. Assume both football players move only in the x-direction (and not in the y-direction) before and after the collision. Using conservation of momentum, in which direction does Brian Urlacher end up going? What is his speed? Is energy conserved? Show your calculation proving whether energy is conserved or not. Is this an elastic or inelastic collision? Explain your reasoning.

Explanation / Answer

Using momentum conservation for collision,

initial momentum = final moemntum of two player system

45 x 7.5 + 55 x -10 = 45x-2 + 55v

337.5 - 550 = -90 + 55v

v = - 2.23 m/s

so Brian will keep moving with 2.23 m/s in the direction he was running initially,.

c) initial energy = ( 45x 7.5^2 /2 ) + (55 x 10^2 /2 ) = 4015.625 J

final energy = (45 x 2^2 /2 ) + (55 x 2.23^2 /2 ) =226.75 J

initial and final energies are not same .

hence energy is not conserved.

d) in elastic collision, enegy is conserved.

but here energy is not conserved.

hence collision is not elastic.

(collision is defined by coefficients of restitution )

e = velocity of separation / velocity of approach

e = ( 2.23 - 2) / ( 10 + 7.5) = 0.013

for elastic collision, e = 1

hence this is inlelastic collision,

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.