Posted by **aok** on Wednesday, November 10, 2010 at 7:42pm.

A delivery truck with a mass of 1700 kg is stopped in the street w/o its brakes on, when a small car hits it from behind. The car has a mass of 650 kg, and is traveling at 3.7m/s immediately before it hits the truck. Both the car and the truck have good bumpers, so assume the collision is elastic.

Find the final velocity of the two vehicles. Consider the direction that the car was originally traveling to be the forward direction

Detailed solutions, with step by step instructions are greatly appreciated. THANK YOU!

- physics -
**bobpursley**, Wednesday, November 10, 2010 at 7:50pm
YOu cant assume elasticity in this. THe brakes are on on the truck. What do you think brakes do? ANS: they absorb energy.

So the KE total cannot equal incoming energy, before the truck could ever move at all, the brakes are holding it, absorbing energy. Don't let the teacher convince you the truck gets an "initial velocity". It doesn't, if it moves, the brakes are absorbing energy.

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**aok**, Wednesday, November 10, 2010 at 7:53pm
thank you. but i don't think this is the answer they are looking for :(. hypothetically, can we assume elasticity?

- physics -
**bobpursley**, Wednesday, November 10, 2010 at 8:09pm
oops, the it says w/0 the brakes. darn.

so you have two conditions:

a) conservation of momentum

intialmoment=final momentum

M*0+ m*3.7=MV' + mv'

now, conservation of energy.

1/2 m *3.7^2=1/2 M V'^2 + 1/2 mv'^2

two equations, two unknowns (V' and v')

To solve, start with the first equation, solve for one of the unknowns.

as in V'=(3.7m-mv')/M

now, the hard and messy part. Put numbers for m and M . then, put that expression in the second equation for V'

Yes, you have to square that term, so FOIL it.

after that, it is just algebra to solve for v', use the quadratic equation. Check both solutions, usually one of them will be the starting velocity as if it passed thru the big mass (and V' of course is zero), ignore that solution.

Have fun.

- physics -
**aok**, Wednesday, November 10, 2010 at 8:10pm
thank you so much. i'll get working on this.

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