Originally Posted By: paul
then tell me what is a unit of momentum.
unit kg·m/s, or, equivalently, N·s


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let me ask you a question , if a object that is not accelerating and has a velocity
of 40 m/s , what would its momentum be in your world.

Use p=mv



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it just so happens that I have one.
ok , now let me go outside to my truck.
........... time passes .....................
nope the truck didnt move !!!

Do you know why? It's not directly because of the large mass of the truck. There are other factors which we don't have in the tube.


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40N = 40 kg m / s^2
So no, those expressions are not equal.


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that would be a force of 40N.s

Ararrh13!#@ Force does not have units of Ns!!!! That's momentum. Come on! Your own reference to Wikipedia agrees with me.


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kallog do you have any springs that you can apply a force to and get 2 times that force back by allowing the spring to expand in 2 seconds?

Expand? Why is the spring expanding? If you apply enough force to compress it, it'll remain compressed as long as the force is applied. It'll also move with the sail so you have to keep expending energy running after it applying that force.

Unless the force is reacted against the sail itself, then it won't keep accelerating the sail and won't keep consuming energy. Something like a latch that clamps the spring in compression.



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isnt accelerating (but has a velocity) that collides with another object would generate?
you pick a word for us to use.

Not enough information. It depends how much the objects deform. If the two bodies are rigid then the force would theoretically be infinite. If they deform (plasticly or elastically) then the force will be less. It'll typically increase gradually as the collision starts, then diminish again. That's why we use soft padding to reduce the forces on things when they're dropped or walked into.

Although it's very complex to model the (changing) force during a collision, we can easily find the overall effect using impulse. I think impulse is what you're looking for. But it's not a force, look it up if you aren't sure.