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Question on momentum

Hey guys, I am a little confused by this question, any help would be much appreciated!

A 0.2g leaf begins to fall and soon reaches a downward terminal velocity of 0.50 m/s. Find the change in momentum of: (a) the leaf ( I found this to be 1 x 10^-4 kg m/s)
(b) the Earth

I'm not sure how you're meant to work out the change in momentum of the Earth! :redface:

Reply 1

AlphaNumeric

Remember conservation of momentum, total change in momentum is zero. So if one object gets X amount of momentum, the other must get -X.

Reply 2

giggsy

just use p(momentum) = mass times velocity for qu 1 dont know about 2

Reply 3

Gregorian IV

Momentum is alwyas conserved

1) 0.5 * 0.0002 = 0.0001 kg m/s
2)Since momentum is conserverd:

Delta P leaf = Delta P Earth
0.0001 = 0.0001

I think that's right

Reply 4

AlphaNumeric

No,

\Delta P_{1} + \Delta P_{2} = 0

not

\Delta P_{1} - \Delta P_{2} = 0

as you have. Remember Newton's 3rd Law being about equal and opposite forces.

A simple example would be a rocket in space. The rocket goes forwards because the exhaust goes backwards, thus keeping