High School Mousetrap Car Energy efficiency

[JLT]

TL;DR

define "useful" energy of a car.

For simple comparison, I think the same thought process can be followed as a block slides down a hill, - for block down hill, simple starting PE of mgh to final max KE 0.5mv^2 - comparing PE1 to max KE2 would result in finding the work friction did through the process. efficiency is just 100*KE2/PE1.

If a mousetrap car travels along a flat surface, a starting PE of 0.5 k th^2 can be measured and maximum velocity of the car can also be measured.

If energy efficiency is defined by starting energy → useful energy, would you agree that "useful" for a car is just the translational motion of the car going forward (not rotating tires or internal inertia etc. calculations). For a simple calculation, would you agree the energy efficiency of a mousetrap car would simply be 100 X final maximum translational energy of the car divided by initial PE in the mousetrap spring?

 

[Baluncore]

TL;DR: define "useful" energy of a car.

For a simple calculation, would you agree the energy efficiency of a mousetrap car would simply be 100 X final maximum translational energy of the car divided by initial PE in the mousetrap spring?

That would seem to be correct, but don't forget the final rotational energy of the wheels.

When the trap is sprung, the centre of mass of the trap moves towards the bait. You need to place the trap on the car so that the bait end of the trap is at the back, as that will further advance the car along the track, by the distance the centre of mass of the trap moves.

 

[JLT]

That would seem to be correct, but don't forget the final rotational energy of the wheels.

When the trap is sprung, the centre of mass of the trap moves towards the bait. You need to place the trap on the car so that the bait end of the trap is at the back, as that will further advance the car along the track, by the distance the centre of mass of the trap moves.

Yes the wheels are rotating and lever arm might still be moving - but the main goal is just the car going forward - what is "useful" is the car going forward however that happens. I guess for semi-trucks/trains - what is really "useful" is the load they carry traveling - not even the mass of the truck/train, but just use the mass of the load?

I admit I tried AI with this question, and it kept trying to add in energy of all the pieces moving, when I really think that is not the "useful" part. Is there a definitive definition of what "useful" energy is when it comes to energy efficiency?

 

[Ibix]

I think what "useful" is depends on your metric for "good". For example, if you are being judged on top speed then the KE of the car is the only thing that is useful. But if you're being judged on distance travelled, rotational KE of the wheels is useful too because it's energy that needs to be dissipated to bring the car to a stop.

You may find that energy efficiency isn't quite the thing you want to know. If you're being judged on distance then energy may be dissipated from different "stores" at different rates, and the same nominal efficiency may produce different performances.

So the first question is: what is "good" for you?