hi,I already solved the problem but my question is partly related to it(adsbygoogle = window.adsbygoogle || []).push({});

A person with mass m=80kg is climbing up the helicopter ladder that is

hovering in the air.Person climbs with speed 0.5m/s relative to the ladder.

With what speed is helicopter descending?

If we think of helicopter and a person together as isolated system then any forces the two exert on each other are considered internal and as such can't change the momentum of the system.

Since person has momentum p = m*v=40 helicopter must also have same momentum in opposite direction

40=mass_helicopter * speed

speed = 1.6cm/s

Although I solved it, I'm not totally shure I understand what is going on. If we consider both the helicopter and a person as one object with some net momentum, then net momentum will stay the same as long as there are no external forces acting on an object.

I solved it as if net momentum was always zero and I considered mass_person * 0.5m/s as a change of momentum (previously both the person and a helicopter each had momentum=0), so helicopter also had to have the same change in momentum.

But couldn't net momentum actually be the momentum person climbing up the ladder has? In that case for net momentum to stay the same helicopter's momentum should stay zero.

Also confusing is the fact that if this was to be isolated system,then once person has momentum p=40 it should remain 40 even if person doesn't try to push itself up anymore. But in reality as soon as it does that its momentum will be zero due to gravity. So why call this isolated system ?

And finally,as you can see from above I have a hard time figuring out what is isolated system and what not. Two cars going with constant speeds and crashing with each other could be considered one, if it were not for friction with the road.

thank you

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# Homework Help: Momentum and isolated system

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