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# Physics - Momentum Question Watch

1. Hi everyone,

I've been unable to do a question on momentum.

A girl is travelling on her skateboard at 4.0 m/s. She throws a 10kg shot put forwards at 8.0 m/s. Her mass was, combined with that of the skateboard, is 50kg. How fast is she travelling after throwing the shot put?

Thanks for any help in advance.
2. (Original post by SquareBlock)
Hi everyone,

I've been unable to do a question on momentum.

A girl is travelling on her skateboard at 4.0 m/s. She throws a 10kg shot put forwards at 8.0 m/s. Her was, combined with that of the skateboard, is 50kg. How fast is she travelling after throwing the shot put?

Thanks for any help in advance.
Is 50kg including the shot put?
3. I've worked it out but I'm certain it's wrong lol
4. (Original post by SquareBlock)
Hi everyone,

I've been unable to do a question on momentum.

A girl is travelling on her skateboard at 4.0 m/s. She throws a 10kg shot put forwards at 8.0 m/s. Her was, combined with that of the skateboard, is 50kg. How fast is she travelling after throwing the shot put?

Thanks for any help in advance.
5. I've worked it out now and I got 3m/s.

I set my work up as follows:

initial mass= 50kg
initial velocity=4.0m/s

final mass= 10kg (shotput) and 40kg (girl)
final velocity= 8.0m/s (shotput) and unknown v (girl)

(initial momentum)=(final momentum)
(50 x 4) = (10 x 8) + (40 x v)
200= 80 + 50v
40v= 120
v=120/40
v=3m/s
6. (Original post by CorpusLuteum)
Yeah, the answer is 3.2 m/s. Was still unable to work it out knowing the answer.
7. (Original post by champ_mc99)
Is 50kg including the shot put?
I don't think so. Just the girl and the skateboard.
8. (Original post by SquareBlock)
I don't think so. Just the girl and the skateboard.
Oh right. I included 50kg as the mass of the girl and shot-put combined.
9. Initial momentum = Final momentum
(50 +10) x 4 = (10 x 8) + (50 x v)

Solve for v
10. (Original post by violavenisis)
I've worked it out now and I got 3m/s.

I set my work up as follows:

initial mass= 60kg
initial velocity=4.0m/s

final mass= 10kg (shotput) and 50kg (girl)
final velocity= 8.0m/s (shotput) and unknown v (girl)

(initial momentum)=(final momentum)
(60 x 4) = (10 x 8) + (50 x v)
240= 80 + 50v
50v= 160
v=160/50
v=3.2m/s
I've changed around the values in the quote. This is the right aswer.
11. (Original post by lyamlim97)
Initial momentum = Final momentum
(50 +10) x 4 = (10 x 8) + (50 x v)

Solve for v
Thanks! I got it
12. Okay, basically:

First of all you must work out the initial momentum of the girl on the skate board (with the shot put in her hand):

1) Add all the weight of the girl on the skateboard and the shot put together to get an overall weight
50kg + 10kg = 60kg.

2) The equation for momentum is
momentum=mass x velocity
Therefore the girl's initial momentum (with the shotput in her hand still) =
60kg x 4 m/s = 240 kgm/s

3) Now because the girl is throwing the shot put away from her initial stance on the skateboard you must take away the momentum of the shot put from the initial momentum.
BUT FIRST find the momentum of the shot put which is step 4.

4) 10kg x 8m/s = 80kgm/s

5) Now do (initial momentum - momentum of shot put = momentum of the girl after she has thrown the shot put)
So : 240kgm/s - 80kgm/s = 160kgm/s

6) now the question is asking for the velocity of the girl after throwing the shot put. Use the momentum equation:
momentum=mass x velocity
Re-arrange it to make VELOCITY the subject:
Velocity = Momentum divided by mass.

7) Put in the mass of the girl without a shotput and work out the velocity:
160kgm/s divided by 50kg = 3.2m/s
13. Oh ****. Well there you go, a million explanations.

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