It is said that in a spaceship, you need to spend as much energy to brake as you spent for accelerating. An electric car, however, charges its batteries while braking, thus it actually recovers energy by braking.
Both facts somehow seem intuitive to me, but aren't these two observations contradicting each other?
Addendum
Looking at the answers, I realize the quastion might not have been clear enough. So let me pose the question in a different way:
Do you absolutely need an outside object moving at a different speed (the road for a car, slamming into an atmosphere as a space ship) to convert kinetic energy into another form? What is the fundamental principle?
Answer
The main point is that the space-ship is a closed system and the car is not
Consider that to conserve momentum we need to give something else the momentum our decelerating object had before.
- In the case of the space-ship this requires ejecting something in the opposite direction to the direction of travel. We need to put energy in to do this.
- In the case of the car we have been connected to the road the whole time and because of this friction we need to continuously provide energy in order not to decelerate. So our wheels are turning and because of the connection with the road friction will decelerate us, what electric cars do is to add an extra resistive force to the turning of the wheels (which is needed to keep going) and make use of the energy gained from this.
So because space-ships don't require any further thrust to maintain a constant speed we have no process to steal the energy from. If you could provide a resistive force on the space-ship you could regain some of the energy but it would have to be outside the space-ship (a magnetic field emitted from a series of space stations for example).
You have to be moving relative to something else which you can impart energy to.
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