Kinetic energy transferred in elastic collision of 7.0g and 19.0g coins

[mortho]

Homework Statement

A 7.0 g coin moving to the right at 22.0 cm/s makes an elastic head-on collision with a 19.0 g coin that is initially at rest. After the collision, the 7.0 g coin moves to the left at 12.5 cm/s.
Find the amount of kinetic energy transferred to the 19.0 g coin.

Homework Equations

m1v1+m2v2
1/2mv2

The Attempt at a Solution

so i found the final velocity of the 19.0 g coin and it was 12.9 cm/s to the right and i used the kinetic energy formula and set everything equal to each other and then subtracted and got 660 J but it's wrong. This was just the difference so I'm guessing that's why its wrong. But i don't know how to find just the energy transfered. Is that the same and difference? Thanks

 

[mortho]

Does anyone know if I'm just supposed to subtract or what? thanks :)

 

[Feldoh]

Since the question asks about energy transferred, I take it that it's an elastic collision in which case energy is conserved (Ki=Kf)

The 19g coin initially has no kinetic energy so whatever it's kinetic energy is after the collision is the amount transferred.

Other then that watch your units.

 

[mortho]

so is it going to be only the 19.0 g coin itself or 1/2mv2+1/2mv2 of the 7 g and 19 g coin? in which case for both combined is 2128 J or by itself 1581 J

 

[Feldoh]

Just the 19g coin, the question only asks for the energy from the 19g coin. J=Nm are you sure your units are right? It looks a bit high.

 

[mortho]

Yes my assignment is already due but i figured what i had done wrong. My units weren't correct. i was supposed to convert the cm/s but the problem hadn't stated that so i might have a chance. Thanks for everyone's help! :)

 

[kbaumen]

The problem doesn't have to state that. m/s is the standard unit in SI system, so if you use any other unit for speed in these formulas, the result will be incorrect.