# Conservation of Momentum

Hi guys would need some help here, not sure if I did it right cause recently I got so lost/confused in the class, just out of no where. I always understood most of the stuff and the thing that irritates me is that it doesnt seem hard at all. So here goes.

Conservation Of Momentum

1) Two figure skaters are facing each other, with their palms together.Both skaters are intially at rest on a frictionless surface. The 1st skater has a mass of 40kg and the 2nd skater has a mass of 60kg. 1st skater pushes off of skater ( 2nd one ) in such a way that skater #1 moves left and skater #2 moves right. After the puss off skater #2 moves with velocity of 6m/s. Determine the direction and volecity of skater #1 after the push.

Now this is as far as I know a recoil problem. Where do I start at ? I came up with an answer of 9 seconds but the way I did it looks way too easy. Could some one help me out with this one ?

Heres another one.

2) A 100kg fullback is running due west with a velocity of 10m/s. At the goal line he collides head-on with a 120kg linebacker, moving due east at 8m/s. Based on the info given, does the fullback drive the linebacker into the end zone or is he pushed backwards by the linebacker ? Explain the reasoning.

I found the forces of both runners.

• Fullback - 1000N
• Linebacker -960N

So the fullback should make it into the end zone, cause he is running with bigger force than the linebacker.

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Hootenanny
Staff Emeritus
Gold Member
1) Two figure skaters are facing each other, with their palms together.Both skaters are intially at rest on a frictionless surface. The 1st skater has a mass of 40kg and the 2nd skater has a mass of 60kg. 1st skater pushes off of skater ( 2nd one ) in such a way that skater #1 moves left and skater #2 moves right. After the puss off skater #2 moves with velocity of 6m/s. Determine the direction and volecity of skater #1 after the push.

Now this is as far as I know a recoil problem. Where do I start at ? I came up with an answer of 9 seconds but the way I did it looks way too easy. Could some one help me out with this one ?
Well nine seconds isn't correct.
2) A 100kg fullback is running due west with a velocity of 10m/s. At the goal line he collides head-on with a 120kg linebacker, moving due east at 8m/s. Based on the info given, does the fullback drive the linebacker into the end zone or is he pushed backwards by the linebacker ? Explain the reasoning.

I found the forces of both runners.

• Fullback - 1000N
• Linebacker -960N

So the fullback should make it into the end zone, cause he is running with bigger force than the linebacker.
No, you didn't calculate the forces, you calculated their momenta. Since the fullback had a greater momentum than the linebacker, when the two collide thier resultant momentum (assuming they stick together) will be in the same direction as the fullback is travelling (by conservation of momentum). Therefore, your conclusion is correct, but your reasoning is wrong.

Thanks for the answer. Now would this drawing work to support my reasoning. I changed the units as you said that I was wrong.

Before collision
960kg m/s ------> <------ 1000kg m/s​

After collision
<------ 40kg m/s​

Well nine seconds isn't correct.
By making "seconds" red do you mean that the "number" I got is right but my units are wrong ? It should be 9m/s.

Last edited:
Hootenanny
Staff Emeritus
Gold Member
Thanks for the answer. Now would this drawing work to support my reasoning. I changed the units as you said that I was wrong.

Before collision
960kg m/s ------> <------ 1000kg m/s​

After collision
<------ 40kg m/s​
Much better
By making "seconds" red do you mean that the "number" I got is right but my units are wrong ? It should be 9m/s.
Looks better, but don't forget to take account of the direction...