Conservation of momentum, Elastic car collision.

AI Thread Summary
In an elastic collision between a 1689 kg car and a 2000 kg truck, the initial velocity of the truck is 17 km/h while the truck is stationary. The conservation of momentum equation is applied to find the initial speed of the car, leading to a calculation of approximately 20.13 km/h. However, there is confusion regarding the final velocity of the car, with some assuming it to be zero, which is incorrect in elastic collisions. The discussion emphasizes the need to consider both conservation of momentum and kinetic energy in elastic collisions. Ultimately, the initial speed of the car is estimated to be around 19 km/h based on the provided options.
Zsmitty3
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A 1689 kg car collides head on with a 2000 kg truck. The collision is elastic. If the velocity of the truck is 17km/h in the same directions as the car's initial velocity, what is the initial speed of the car in km/h?



I'm getting an answer but doesn't seem to make sense.



So I'm looking at the conservation of momentum equation to set this up.

(1689-kg)(x)+(2000-kg)(0 km/h)=(1689-kg)(0)+(2000-kg)(17 km/h)

Then: (1689)(x)=(34000) or x= 20.13

I'm assuming that the final velocity of the car is 0 but it doesn't specifically say in the questions. Can I assume that since it's head on? Also it's a multiple choice question and the closest answer I can find is 19 km/h.
 
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Zsmitty3 said:
A 1689 kg car collides head on with a 2000 kg truck. The collision is elastic. If the velocity of the truck is 17km/h in the same directions as the car's initial velocity, what is the initial speed of the car in km/h?
There's not enough information. Below, you assume the truck is initially stationary. Is that true? (Not what I would call 'head on'.)
I'm assuming that the final velocity of the car is 0
You should not. You are told the collision is elastic. What additional conservation law does that allow you to use?
 
The Truck is parked so yes its initial is 0. Sorry I forgot to add that. Conservation of KE?
 
Last edited:
Zsmitty3 said:
The Truck is parked so yes its initial is 0. Sorry I forgot to add that. Conservation of KE?
Yes.
 

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