A car of mass m moving at a speed v1 collides and couples with the back of a tru

Question

A car of mass m moving at a speed v1 collides and couples with the back of a truck of mass 4m moving initially in the same direction as the car at a lower speed v2.
(a) What is the speed vf of the two vehicles immediately after the collision? (Use any variable or symbol stated above as necessary.)

vf = (mv1 + 4mv2)/5m

(b) What is the change in kinetic energy of the car–truck system in the collision? (Use any variable or symbol stated above as necessary.)

Change in KE = KEf - KEi

KEf = 1/2(m1+m2)vf^2
KEi = 1/2m1v1i^2 + 1/2m2v2i^2

KE = 1/2(5m)((mv1 + 4mv2)/5m)^2 - (1/2(m)(v1^2) + 1/2(4m)(v2^2))

I dont see how this is wrong? and i do not trust web assign.... so whats wrong?

Explanation / Answer

in K.E. part in the last there will be - instead of +. so verify that. the correct answer would be K.E. = 1/2(5m)((mv1 + 4mv2)/5m)^2 - (1/2(m)(v1^2) - 1/2(4m)(v2^2))

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