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- Thread starter chel
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help.. please.. :(

- #3

Doc Al

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Show what you've done so far and where you are stuck.

Hint: What's conserved?

Hint: What's conserved?

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- #5

Doc Al

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Try considering the snowball and man as a single system.

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(0.5kg) * (20m/s)= (70kg)* (?)

im not really sure if im going on the right track i don't know much about physics..

i feel so helpless by not knowing these things.. :(

- #7

Cyosis

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It's close, but not correct. Use conservation of momentum. The system here is the man and the snowball. What is the momentum before the collision? What is the momentum after the collision? To elaborate a bit further. Before the impact the snow ball has a certain speed and mass and so does the man, his speed is 0 since he's at rest. The question states that after the collision the snowball sticks to the man. Can you do anything with this information?

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uhmm, the momentum before and after the collision was zero?

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Doc Al

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Another hint: This is an example of a totally inelastic collision. This short reference might help: https://www.physicsforums.com/showpost.php?p=2157983&postcount=5"

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uhmm, the momentum before and after the collision is zero??

- #11

Doc Al

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No. To find the total momentumuhmm, the momentum before and after the collision was zero?

- #12

Cyosis

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In the system of man and snowball total momentum is conserved. The total momentum before the collision is not zero, thus it cannot be zero after the collision. Could you write down the equations for momentum for the system before the collision? p=...?uhmm, the momentum before and after the collision was zero?

Edit: Since I didn't post any new information with this post I will expand a bit upon it.

Before the collision:

What is the mass of the snowball?

What is the speed of the snowball?

What is he mass of the man?

What is the speed of the man?

After the collision:

What is the mass of the man and the snowball that is stuck to him?

What is the speed of the combined object that is man and snowball?

Now what is the definition of momentum? Plug in the numbers and solve.

Answer all these questions if you haven't found the solution yet.

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can i use this formula?

(m)*(20m/s)+(m)(0m/s)=(2m)(velocity after the collision)?

(m)*(20m/s)+(m)(0m/s)=(2m)(velocity after the collision)?

- #14

Cyosis

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Can you tell us what conservation of momentum means and do you know how you write momentum down in equation form?

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(0.5kg)*(20m/s)+(70kg)(0m/s)=(0.5kg+70kg)(Vafter)?

10kg.m/s=(70.5kg)(Vafter)

Vafter=10kg.m/s / 70.5kg

= 0.14m/s

- #16

Cyosis

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You got it!

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- #18

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hai,, guys thanks for the effort in helping me solving this problem..

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you mean to say tama ako? i mean i got it the correct answer?????

- #20

Cyosis

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Tama ako? Anyway yes you found the correct method and answer to your problem.

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- #22

Cyosis

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The correct equation to use was [itex]m_{sb} v_{sb}+m_{man}v_{man}=(m_{sb}+m_{man}) v_{final}[/itex]. The interesting part is [itex](m_{sb}+m_{man}) =70.5kg[/itex].

The equation you used the first time was [itex]m_{sb} v_{sb}+m_{man}v_{man}=m_{man} v_{final}[/itex]. Here you are ignoring that when a snowball is stuck to a man his total mass is increased. Because the snowball is so light both equations gave the same answer with a 2-decimal accuracy. Try to recalculate it by saying that the mass of the snowball is 100kg, keep the rest of the numbers the same. You will see that your answer is very different.

More intuitively: If I throw a snowball at you, you won't move much because its mass is low. If I were to throw a wrecking ball at you, you would move quite a bit since its mass is high compared to yours, assuming both snowball and wrecking ball hit you at the same speed.

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tama ako means im correct... bye.. thanks :zzz:Tama ako? Anyway yes you found the correct method and answer to your problem.

- #24

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oh! i see,, i'll keep that in mind... well thanks anyway,, i still have 10 more questions to answer tomorrow,, thank you,, nyters and byers

The correct equation to use was [itex]m_{sb} v_{sb}+m_{man}v_{man}=(m_{sb}+m_{man}) v_{final}[/itex]. The interesting part is [itex](m_{sb}+m_{man}) =70.5kg[/itex].

The equation you used the first time was [itex]m_{sb} v_{sb}+m_{man}v_{man}=m_{man} v_{final}[/itex]. Here you are ignoring that when a snowball is stuck to a man his total mass is increased. Because the snowball is so light both equations gave the same answer with a 2-decimal accuracy. Try to recalculate it by saying that the mass of the snowball is 100kg, keep the rest of the numbers the same. You will see that your answer is very different.

More intuitively: If I throw a snowball at you, you won't move much because its mass is low. If I were to throw a wrecking ball at you, you would move quite a bit since its mass is high compared to yours, assuming both snowball and wrecking ball hit you at the same speed.

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