I'm stcuk, sort of, not really, I need some resurance is all (1 Viewer)

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1. The problem statement, all variables and given/known data

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.

2. Relevant equations

KE=1/2*m*v^2

3. 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?
 

tiny-tim

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Welcome to PF!

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:
 
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
 

tiny-tim

Science Advisor
Homework Helper
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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.
 
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...
 
Okay, so i did the m1v1i + M2V2i = mvf + MVf
and M2=464.286

The KE of the dog is 3125

but im not sure how to gt the other KE

should it be the dog+boat and the final velocity?
 
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)
 
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
 
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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