Hard Physics Question

[macca69831]

Just got given this physics question, its pretty difficult, ive completed it, and just wanted anyone else to have a shot.

'Two polar bears, stationary on skates on ice, are facing one another. George, of mass 800kg, throws a 100kg medicine ball to Doug. Doug has a mass of 500kg.
As a result of throwing the ball, George moves backwards with a speed of 2ms^-1 while the ball's speed relative to George is 6ms^-1.
Doug Catches the ball and throws it back, at the same speed relative to the ice, to George.
Calculate Georges speed, relative to Doug, after George has caught the ball.'
Show all working and correct physics formula

[QuantumJG]

Ok this question is trickier than I thought!

[/0]

800kg polar bears should not be ice-skating

[macca69831]

Ok this question is trickier than I thought!

It is pretty tricky

800kg polar bears should not be ice-skating

Sooooo true

[Cthulhu]

an 800kg polar bear is an obese polar bear!

[/0]

I get 28/45 is that what you got?

[kamil9876]

'Two polar bears, stationary on skates on ice, are facing one another. George, of mass 800kg, throws a 100kg medicine ball to Doug. Doug has a mass of 500kg.
As a result of throwing the ball, George moves backwards with a speed of 2ms-1 while the ball's speed relative to George is 6ms-1.

Does this statement make sense? We have a system with 0 momentum initially. We then have a system with a total momentum of 100*4-2*800=-1200kg.m/s^2. (Doug is still at rest and was initially at rest). (I also assumed that it meant 2m/s backwards in the initial reference frame where they were all stationary).

[QuantumJG]

'Two polar bears, stationary on skates on ice, are facing one another. George, of mass 800kg, throws a 100kg medicine ball to Doug. Doug has a mass of 500kg.
As a result of throwing the ball, George moves backwards with a speed of 2ms-1 while the ball's speed relative to George is 6ms-1.

Does this statement make sense? We have a system with 0 momentum initially. We then have a system with a total momentum of 100*4-2*800=-1200kg.m/s^2. (Doug is still at rest and was initially at rest). (I also assumed that it meant 2m/s backwards in the initial reference frame where they were all stationary).

Yeah this my dilemma.

Ok so relative to the ice we have:

With the ball throw:

m_Georgeu_George + m_Ballu_Ball = m_Georgev_George + m_Ballv_Ball

Implying 800*0 + 100*0 = -800*2 + 100*4 = -1200 =/= 0

So I'm confused! Shouldn't the ball and George have the same momentum (the magnitude)?   

[Mao]

I assume it's a typo and the speed of the ball is 16 m/s?

[QuantumJG]

an 800kg polar bear is an obese polar bear!

Lol! According to wikapedia they should have a mass of 680kg at the most. I also don't get how a 800kg bear can throw a ball 1/8th it's mass at 4m/s and have a recoil speed of 2m/s.

I get 28/45 is that what you got?

How did you solve this?

[QuantumJG]

After having very little confidence in my working I got 10.4m/s. :S

[kamil9876]

I assume it's a typo and the speed of the ball is 16 m/s?

Or the bear moves slower(hence QuantumJG's amazement of the speed).

[/0]

I was assuming that there wasn't a typo (which still doesn't explain why momentum wasn't conserved)
The ball is moving at 4m/s relative to the ground.

Doug catching:



Doug throwing:



George catching:



So we have the speed as (before I accidentally added vectorially)

[Cthulhu]

Thats what I did but I thought I was on the wrong track so didn't publish.

[QuantumJG]

I was assuming that there wasn't a typo (which still doesn't explain why momentum wasn't conserved)
The ball is moving at 4m/s relative to the ground.

Doug catching:



Doug throwing:



George catching:



So we have the speed as (before I accidentally added vectorially)

Ok so you worked backwards!

I didn't consider that.