Homework Help: Physics

Posted by Jon on Monday, November 9, 2009 at 11:07pm.

A 60.0 kg girl stands up on a stationary floating raft and decides to go into shore. She dives off the 180 kg floating raft with a velocity of 4.0 m/s [W]. Ignore the substantial friction real objects in water experience.

a) What is the momentum of the girl as she is diving?
b) What is the momentum of raft as the girl is diving?
c) What is the final speed of the raft just after the girl dives?

a) * Let [W] be +ive.
p=mv
=(60.0kg)(4.0m/s)
=240 kg/(m/s)
Correct?

b) a little confused about this one, is it p=mv, and then =(180kg)(4.0m/s)=720kg/(m/s). this doesn't make sense because the raft doesn't have a velocity of 4m/s. (and it does say the raft is stationary- is it possible that the momentum=0?)i was thinking about calculating impulse, but i don't have the information. please help with this one.

c) for this one, i'm going to use p=p' (initial momentum=final momentum), and then work it out to be mv1 + mv2 = mv1' + mv2' where 1 is the girl, and two is the raft.. sub in what i know, and solve for v2'. in this case, would i use v1= 240 kg/(m/s) from part a, (ifso, what would i put for v1'), or would i put 0 for V1, and 240 for v1'? Thennnn, i would have to deal with the raft which i hope i get figured out from part b !

Thanks in advance!

Physics - Dr. Loveday, Thursday, November 12, 2009 at 8:50am
For the the first part you have it right. For the second part, the momentum equals zero because the momentum before she just is zero. After she jumps, the make the total momentum zero, the momentum of the raft and the momentum of the girl must be equal in size but opposite in direction.

For the third part is the same. 0= (Mass of girl)*(velocity of girl) + (Mass of raft)*(velocity of raft)

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