Momentum and Energy


How do rockets accelerate in space?

If you’re trying to stop a lineman, you need to exert a force in the direction opposite the direction of his motion. The greater the force and the longer you exert it, the greater the momentum change. The product of force and the time the force is applied is called the impulse. Because force is a vector, so is impulse. Thus the direction of the impulse that stops the lineman is opposite his motion. The larger the impulse the greater the change in momentum.

When a rocket’s motor is fired it expels gas at a high velocity backward. Thus the gas, originally at rest in the rocket, is given a large momentum backward. With no external forces on the rocket-gas system, the rocket’s momentum must increase in the forward direction. It will speed up.

When the rocket is on the launch pad there is an external force on it, the force of gravity. How can the rocket take off? Now the momentum of the gas and rocket isn’t conserved, but still the impulse the rocket gives the gas in pushing it backward is equal and opposite to the impulse the gas gives the rocket. Thus the rocket rises, just more slowly than it would if there were no gravity.


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