Gravity and inertia

If you want to move a car to some speed on the Earth's surface you will need some energy. The path is horizontal, air resistence and friction neglected. Now if you wish to repeat the same experiment and reach the same speed on the Moon (which has 6 times less mass), you will need: a) more energy, b) less energy, c) the same energy as on Earth?
Question asked by: dariovid

Asked on: 04 Mar 2010

Common sense would suggest to me that the answer is less energy needed in order to accelerate an object to a certain speed on the moon because there are less forces that you need to overcome in order to reach that speed.
By: artiste
Replied at: 07 Mar 2010
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Dear artiste, You must make difference between gravity and inertia, although both of them are directly dependent on the mass (as Einstein did it in his famous mental experiment with a lift). In this case gravity is perpendicular to the movement of the car and does not have any influence on it. Inertia is always the same in the whole universe, no matter if the mass is inside a gravitational field or not. So you will need the same amount of energy as on the Earth. (If this was not so, spaceships could be accelerated almost without any energy when they are far from gravity fields.) Do not worry, I was astonished how many graduated physicists could not give the right answer. dariovid
By: dariovid

Date of comment: Mon, Mar 8th 2010

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inertia gravity

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