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20 June 2019 00:39

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Asked by: Momentus
Subject: Where does the energy go?
Question: Where does the energy go?

Imagine a heavy train, running along at 100 kph. A small electric car accelerates from rest to catch up with the train, taking 10 units of energy from the battery. The car then uses regenerative braking to bring the car back to rest, recovering the stored kinetic energy, recharging the battery.

Accelerate the car again, this time taking the car on board the moving train when their speeds match.

Now whilst the car is on the train, it is accelerated to 100kph along the train corridor, using a further 10 units of energy. 20 units of energy have been taken from the battery.

Now for the catch.

a) If the car accelerates from the back of the train and exits from the front, it will be travelling at 200 kph when it touches the track. Then regenerative braking can bring it to rest, recharging the battery, recovering the 20 units of energy.
b) If however the car accelerates from the front of the train and exits from the rear, it will be at rest when it touches the track. No regenerative braking, no energy recovery.

As a gedankenexperiment, scenario b) can be reiterated until the amount of energy transferred becomes very large. How will that manifest itself? Will the mass be heated up?

Where does the energy go?
All thoughts are welcome.

Date: 21 January 2015
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Answers (Ordered by Date)

Answer: Glenn Turner - 19/03/2016 13:25:33
 The required amount of energy to keep the train at a steady speed would increase/decrease as the car accelerated and unaccelerated.

In other words as long as the car and trains energy is taken into account that nothing odd appears to happen.

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