# physics question

• June 6th, 2017, 09:41 AM
physics question
Hello, new member here. Here's a physics question: Let's say you have a 1,000,000 pound spaceship on a launch pad, and you're trying to launch it with a model rocket engine that produces 1 pound of thrust. Seeing as how thrust is measured in terms of acceleration, would the rocket engine technically be producing zero thrust, because it cannot move the heavy spaceship? I guess I could also ask it this way: does force necessarily require the element of motion to be considered a force?
• June 6th, 2017, 10:28 AM
Janus
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Hello, new member here. Here's a physics question: Let's say you have a 1,000,000 pound spaceship on a launch pad, and you're trying to launch it with a model rocket engine that produces 1 pound of thrust. Seeing as how thrust is measured in terms of acceleration, would the rocket engine technically be producing zero thrust, because it cannot move the heavy spaceship? I guess I could also ask it this way: does force necessarily require the element of motion to be considered a force?

Let's put it this way: assume your rocket is being suspended by a cable from a very large scale. Before firing your rocket engine, the scale will register 1,000,000 lb. While firing your engine the scale will register 999,999 lbs. So yes, the engine is providing thrust.
Don't confuse force with work. The rocket engine can exert a force on the rocket even if it does not move it and performs no work on it.
• June 6th, 2017, 10:31 AM
Dywyddyr
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Seeing as how thrust is measured in terms of acceleration

This is incorrect.
Thrust is measured in terms of force applied (e.g. lbf or N).

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would the rocket engine technically be producing zero thrust
No, it would technically be producing 1 lbf of thrust.

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I guess I could also ask it this way: does force necessarily require the element of motion to be considered a force?
What? No. If you sit on chair you apply a force to it, and it applies one to you.