Dissipation of mechàanical energy in collision of sandwiches

[emeraldempres]

On a greasy, essentially frictionless lunch counter, a submarine sandwich of mass 0.500 kg, moving with speed 3.50 m/s to the left, collides with a grilled cheese sandwich of mass 0.200 kg moving with speed 2.00 m/s to the right.

How much mechanical energy dissipates in the collision?

I calculated the final velocity (-1.93 m/s) but I don not know where to go after that

 

[calef]

In this collision, think about what's being conserved and what isn't.

Momentum is always conserved. That leaves just one thing that isn't.

 

[Doc Al]

Compare the initial total KE of both sandwiches to the final KE.

 

[emeraldempres]

i think that the kinetic energy is what is not being conserved because it is being transferred to puck b.

the initial ke of a is
a: ke= .5(.5)(-3.5^2)= 3.0625
b: ke= .5 (.2)(2^2)= 1

and the final ke is ke= .5(.5+.2)(-1.93^2)= 1.304

I had all these and subtract them from eacj other and got -2.76

 

[calef]

Pretty sure that's right, although the negative sign is unnecessary. As long as you understand that your answer is the change in kinetic energy, that is, the kinetic energy of your system went down by that much, you're good.

 

[emeraldempres]

I just tried that one and my homework says that -2.76 and 2.76 are not right. Anything else i can do?

 

[Doc Al]

the initial ke of a is
a: ke= .5(.5)(-3.5^2)= 3.0625
b: ke= .5 (.2)(2^2)= 1

Redo that second calculation.

 

[emeraldempres]

thanks. i went back through my math and found an error, the answer is -2.16 J