I'm stcuk, sort of, not really, I need some resurance is all

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In summary, the conversation discusses finding the amount of kinetic energy dissipated when a 250kg dog jumps into an empty boat and causes it to move at a speed of 1.75m/s away from the shore. The equation KE=1/2*m*v^2 is used and the attempt at a solution involves finding the kinetic energy before and after the jump, as well as considering the conservation of momentum. The final answer is found to be 2031.25.
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Homework Statement



Ive got another question that I think I might have failed at.

A dog weighing 250 kg jumps into an empty boat by the dock at a speed of 5m/s causing the boat to move away form the shore at 1.75m/s. Find the amount of kinetic energy dissipated.

Homework Equations



KE=1/2*m*v^2

The Attempt at a Solution



Before

KE=1/2*m*v^2
KE=(.5)(250kg)(5.0m/s)^2
KE(1)=3125

and after

KE=1/2*m*v^2
KE=(.5)(250)(1.75m/s)^2
KE(2)=382.813

KE1-KE2=2742.19

KE dissipated = 2742

So am I right?
 
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  • #2
Welcome to PF!

cooper43 said:
A dog weighing 250 kg jumps into an empty boat by the dock at a speed of 5m/s causing the boat to move away form the shore at 1.75m/s. Find the amount of kinetic energy dissipated.

Hi cooper43! Welcome to PF! :smile:

You need to include the KE of the boat, so put the mass of the boat = M, and use conservation of momentum. :wink:
 
  • #3
Is conservation of momentum P=mv?

And hat do you mean Mass of the boat. Should I leave it as M or do I replace it with the mass of the dog
 
  • #4
cooper43 said:
Is conservation of momentum P=mv?

Nooo … conservation of momentum is mvi + MVi = mvf + MVf

ie total momentum before = total momentum after …

it is always true for a collision. :smile:
And what do you mean Mass of the boat …

Now you're just being ridiculous.

Stop asking questions and start answering the problem.
 
  • #5
tiny-tim said:
Nooo … conservation of momentum is mvi + MVi = mvf + MVf

ie total momentum before = total momentum after …

it is always true for a collision. :smile:


Now you're just being ridiculous.

Stop asking questions and start answering the problem.

LMAO! Well said!

As Tiny-Tim said, Final Momentum = Initial Momentum

Therefore,

mv = mv and you can work out your mass from there if necessary...
 
  • #6
Okay, so i did the m1v1i + M2V2i = mvf + MVf
and M2=464.286

The KE of the dog is 3125

but I am not sure how to gt the other KE

should it be the dog+boat and the final velocity?
 
  • #7
yes, the final value of the kinetic energy is found by considering the mass of the boat and the dog and the final velocity. They act as a single object (theoretically)
 
  • #8
So its KE1=3125 and KE2=1093.75

what do i subtract from what?

wouldnt it normaly be KE1-KE2,,,but it would end up negative
 
  • #9
cooper43 said:
So its KE1=3125 and KE2=1093.75

what do i subtract from what?

wouldnt it normaly be KE1-KE2,,,but it would end up negative

never mind, that made me look like a dumb ***

I figured it out

The amount of kinetic energy dissipated is KE=2031.25
 

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