# physics

posted by .

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 -

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.

• physics -

thank you. but i don't think this is the answer they are looking for :(. hypothetically, can we assume elasticity?

• physics -

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 through the big mass (and V' of course is zero), ignore that solution.

Have fun.

• physics -

thank you so much. i'll get working on this.

## Similar Questions

1. ### Physics

Car A with mass of 1250 kg is traveling at 30 m/s to the east. Car B is a truck with mass of 2000 kg, traveling to the west at 25 m/s. Assume these two vehicles experience an inelastic collision but do not stick together and car a …
2. ### physics

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 care has a mass of 650 kg, and is traveling at 3.7m/s immediately before it hits the truck. Both the car …
3. ### Physics

A truck travels at a constant speed of 28.0 m/s in the fast lane of a two-lane highway. It approaches a stationay car stopped at the side of the road. When the truck is still 1.2x10^2 m behind the car, the car pulls out into the slow …

A small car of mass 985 kg is parked behind a small truck of mass 1551 kg on a level road. The brakes of both the car and the truck are off so that they are free to roll with negligible friction. A 83 kg woman sitting on the tailgate …
5. ### physics

Car A, with a mass of 1250 kg, is traveling at 30 m/s to the east. Car B is a truck with a mass of 2000 kg, traveling to the west at 25 m/s. Assume these two vehicles experience an inelastic collision but do not stick together, and …
6. ### scienc

I'm driving my car of mass 1000 kg at 100 km/hr when I accidentally hit the rear end of a truck of mass 5000 kg traveling in the same direction at 90 km/hr. My car and the truck lock together and move as a single object. Let F1 be …
7. ### physics

You're quietly sitting by a street one day, enjoying the sun, when you notice a speeding car traveling at 20 m/s down the street. The driver notices that there's a stop sign in the street and hits the brakes 40 m before the stop sign, …
8. ### physics

Car A with a mass of 1250 kg, is traveling at 30 m/s to the east. Car B is a truck with a mass of 2000 kg, traveling to the west at 25 m/s. assume these two vehicles experience an inelastic collision but do not stick together, and …
9. ### physics

Car A with a mass of 1250 kg, is traveling at 30 m/s to the east. Car B is a truck with a mass of 2000 kg, traveling to the west at 25 m/s. assume these two vehicles experience an inelastic collision but do not stick together, and …
10. ### physics

a car and a truck starts from rest at the same instant with the car is behind the truck initially at the same distance. the truck has a constant acceleration of 4 ft/s^2 and the car has an acceleration of 6 ft/s^2. the car overtake …

More Similar Questions