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

[cooper43]

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?

 

[tiny-tim]

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!

You need to include the KE of the boat, so put the mass of the boat = M, and use conservation of momentum.

 

[cooper43]

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]

Is conservation of momentum P=mv?

Nooo … conservation of momentum is mv_i + MV_i = mv_f + MV_f …

ie total momentum before = total momentum after …

it is always true for a collision.

And what do you mean Mass of the boat …

Now you're just being ridiculous.

Stop asking questions and start answering the problem.

 

[Miney]

Nooo … conservation of momentum is mv_i + MV_i = mv_f + MV_f …

ie total momentum before = total momentum after …

it is always true for a collision.


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...

 

[cooper43]

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?

 

[dontdisturbmycircles]

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)

 

[cooper43]

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

 

[cooper43]

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