Is mechanical energy conserved in car collisions and bicycle coasting?

[jayadds]

Hi,

I was just wondering, is mechanical energy conserved when two cars of equal mass and moving at equal speed collide head-on and in the process both come to rest?

I'm inclined to say it isn't conserved because kinetic energy is converted into other forms of energy (e.g. heat and sound) during the collision.

And how about when the bicycle rider ceases to pedal and her bicycle coasts along the path until it comes to rest? Is mechanical energy conserved there? I think because of friction, it isn't conserved...

Please correct me if I'm wrong.

Many thanks.

 

[NascentOxygen]

Energy is not a vector quantity, it's a scalar. Each car has KE before the collision, and after the collision when the dust has settled each can be seen to have zero KE, so there is not much that has been conserved.


... unless a wheel went into orbit, or something...

 

[Whovian]

Hi,

I was just wondering, is mechanical energy conserved when two cars of equal mass and moving at equal speed collide head-on and in the process both come to rest?

I'm inclined to say it isn't conserved because kinetic energy is converted into other forms of energy (e.g. heat and sound) during the collision.

And how about when the bicycle rider ceases to pedal and her bicycle coasts along the path until it comes to rest? Is mechanical energy conserved there? I think because of friction, it isn't conserved...

Please correct me if I'm wrong.

Many thanks.

In the car collision, some's converted into sound, but for the purposes of this discussion, the energy's turned into heat.

Same with the bicycle.

Note that total energy is conserved here, but mechanical energy isn't.