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I know that the (magnitude of) the tangential velocity of a planet in an elliptical orbit decreases while approaching the aphelion and increases while approaching the perihelion because the centripetal force is not perpendicular to the motion of the planet.

However, would the same phenomenon occur in all situations where an object is traveling in an elliptical path? For example, would it be possible for a car traveling in an elliptical path to maintain a constant magnitude of tangential velocity (speed)?

Vincent Thacker
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Varshil MVH Pets
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2 Answers2

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An elliptical orbit can only be performed around something, relative to which the perihelion and aphelion distance can be measured. A car driving in an ellipse can do so irrespective of what it's driving around, there is no analogous "perihelion" in the car scenario. If you draw an elliptical path for a car to drive on, there is no "preferred side" where it should slow down or speed up.

There's no reason a car would have to slow down or speed up when approaching any point in its trajectory - if the car would change speed, just press the gas or brake so that it doesn't. It's trivial to drive a car on any trajectory you want while maintaining a constant speed of 5mph, for example.

A space craft in an elliptical orbit does not have this ability, since it passively remains in its orbit under the force of gravity alone. A spacecraft with plenty of fuel that could hit the "gas" or "brakes" in space would also not have to change speed in an elliptical orbit.

Nuclear Hoagie
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A planet in orbit slows as it approaches aphelion because of conservation of energy. It is "coasting uphill" against gravity as it moves away from its star.

A car has an engine and brakes to adjust its kinetic energy. A car can drive at constant speed on a curved road. (Up to a point. Too fast and the tires won't hold you on the road.) Or it can speed up or slow down.

controlgroup
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mmesser314
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