# Kinetic energy along with conservation of momentum

1. Nov 14, 2012

### Lolagoeslala

1. The problem statement, all variables and given/known data
a hockey player of mass 80 kg was skating at a velocity of 7.5 m/s [E 20 S] when he accidently hit a linesman who was just standing on the ice. The mass of the linesmass is 90 kg and his velocity after the collision was 3 m/s [E 30 N] find.
a) the velocity of the hockey player after the collision and
b) the kinetic energy lost during the collision as a percentage.

3. The attempt at a solution
given:
m1= 80 kg
m2 = 90 kg
v1 = 7.5 m/s [ E 20 S]
v1 = ?
v2= 0m/s
v2 = 3m/s [ E 30 N]

X component
m1v1 = m1v1 + m2m2
(80kg)(7.047694656m/s[E]) = (80kg)(v1) + (90kg)(2.598076211[E])
563.8155725 kgm/s [E] - 233.826859 kgm/s[E] = (80kg)(v1)
329.9887135 [E] / 80kg = v1
4.124858919 m/s [E] = v1

y components
m1v1 = m1v1 + m2v2
(80kg)(2.565151075m/s) = (80kg)(v1) + (90kg)(1.5m/s[N])
205.212086kgm/s-135m/s[N] = (80kg)(v1)
70.212086kgm/s/80kg = v1
4.252651075m/s = v1

4.124858919 m/s [E] + 4.252651075m/s = v1
5.924 m/s [S 44° E] = v1

b) Ektot = 1/2m1v1^2
Ektot= 1/2(80kg)(7.5m/s)^2
Ektot=2250 J

Ektot = 1/2m1v1^2 + 1/2m2v2
Ektot = 1/2(80kg)(5.924m/s)^2 + 1/2(90kg)(3m/s)^2
Ektot =1403.75104 J + 405 kg J
Ektot = 1808.75104 J

Ektot - Wdef = Ektot`
2250 J - Wdef = 1808.75104 J
Wdef = 441.24896 J

The Wdef is the energy lost during the collision

(441.248896 J / 2250 J) x 100%
19.6 %

2. Nov 14, 2012

### SammyS

Staff Emeritus

This looks like a repeat of your earlier thread.

https://www.physicsforums.com/showthread.php?t=651165&highlight=hockey+player

Perhaps just a new attempt at a solution on your part.

3. Nov 14, 2012

### Lolagoeslala

4. Nov 14, 2012

### micromass

Staff Emeritus
Please don't create multiple threads on the same topic.

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