a spacecraft in outer space increases velocity by firing its rockets. How can hot gases escaping from its rockets change the velocity of the craft when there is nothing in sapce for the gases to push against?
i just learned momentum and impulse today. But i didn't really understand it. can somebody help please??
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The gases push against the rocket. As the gas is expelled, the force at which it is expelled is the same force that is added to the velocity of the rocket. It's a basic "every action has an equal and opposite reaction" action.
Think about standing on ice wearing skates and throwing a heavy medicine ball away from you. The force that you applied to throwing the medicine ball would push you backward, albeit slowly--this is the very same principle.
How do rockets move in space?
If space is basically a vacuum and void of atmosphere, how do rockets alter the direction and speed of space craft? In other words, how do they "push off" against nothing?
This is a very good question. Isaac Newton worked out the solution and published it in 1687 in his Principia Mathematica. It is phrased as Newton's 3rd law. I'll include all 3 below just in case!
1st: A body will remain at rest or at motion with a uniform speed unless it is acted on my an external force.
2nd: The acceleration of a body with a force acting on it is that force divided by the mass of the body (F=ma)
3rd: Every action has an equal and opposite reaction.
So the third law basically says that if you shoot out stuff in one direction you will move in the other direction. This is how rockets work in a vacuum. They have a source of fuel which is heated up so that it expands and is pushed out of the rocket. In order to change direction in space rockets have to have little 'thrusters' on all sides (you need 6 in total to maneuver completely in 3 dimensions).
Newton's 3rd law seems contrary to our intuition because on Earth there are lots of sources of friction - providing much easier methods of propulsion, however you might have seen it in action if you have ever blown up a balloon and then let go of it before tying it up. What pushes the balloon all around the room is the air you blew into in escaping.
Sure, here's how it works:
The expanding gases don't push against space. They push against the rocket. They expand out of the back of the rocket, but work by pushing up INTO the rocket (and then out). They push against the rocket, and the rocket pushes back - so, the rocket moves.
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