Momentum, Energy, Work

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A 50 kg teenager just got his license and is driving his brand new Escalade (mass 1100 kg) at 30 m/s. As he is texting and driving, he crashes into a stopped minivan, which has a mass of 980 kg. Since he is not wearing a seatbelt, he flies out of the window after the crash. If the Escalade and Minivan move together with a speed of 13 m/s after the perfectly elastic crash, with what velocity is the teenage (former) driver now flying through the air?

May 23rd, 2015

1/2*1150*13*13=1/2*50*v*v

v=sqrt 3887= 62.35 m/s

May 23rd, 2015

let u1 & u2 be the velocity of the car and mnivan before
collision.let v1 & v2 be the velocities after collision.


initial momentum of the system before collision is

Pi = m1*u1 + m2*u2

final momentum of the system after collision is

Pf = m1*v1+m2*v2

elastic collisions  the total momentum is conserved .


the momentum before collision = momentum after collision


according to conservation of linear momentum


m1*u1 + m2*u2 = m1*v1+m2*v2

m1*(u1-v1) = m2*(v2-u2)........(1)


initial KE of the system KEi = 0.5*m1*u1^2 + 0.5*m2*u2^2

final KE of the system KEf = 0.5*m1*v1^2 + 0.5*m2*v2^2

according to conservation of energy


0.5*m1*u1^2 + 0.5*m2*u2^2 = 0.5*m1*v1^2 + 0.5*m2*v2^2


m1*(u1^2 - v1^2) = m2*(v2^2-u2^2).....(2)

from 1 &2

u1 + v1 = u2+v2


u1 - u2 = v2 - v1


v2 = u1 - u2 + v1........(3)

3 in 1


m1*(u1-v1) = m2*(u1 - u2 + v1 - u2)


v1 = u1*(m1-m2)/(m1+m2) + 2*m2*u2/(m1+m2)


v2 = u2*(m2-m1)/(m1+m2) + 2*m1*u1/(m1+m2)

m1 = 1150 kg  m2 = 980 kf


u1 = 30...........u2 = 0


v1 = ?...........v2 = ?


v1 = ((1150-980)*30)/(1150+980) = 2.39 m/s <-----------answer


v2 = (2*1150*30)/(1150+980) = 32.39 m/s  <-----------answer


May 23rd, 2015

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