# Kinetic Energy problem

1. Nov 9, 2009

### HeavyGuy

1. The problem statement, all variables and given/known data
An arrow slows down from 43 m/s to 28 m/s as it passes through an apple. If the 493-g apple was originally at rest and sped up to 0.44 m/s, what is the mass of the arrow

Vapple,i = 0 m/s
Vapple,f = 0.44 m/s
Varrow,i = 43 m/s
Varrow,f = 28 m/s
m,apple = o.493 kg
m,arrow = ?

2. Relevant equations

im guessing we only need Eki = Ekf which is 1/2mvi(ar)^2 + 1/2mvi(ap)^2 + 1/2mvf(ar)^2 + 1/2mvf(ap)^2

3. The attempt at a solution

i plugged it all into the equation so it looks like this near the end

924.5m = 0.0477224 + 392m

i then subtracted the 392 from 924.5 and divided that number by the long decimal to solve for m, but answer was totally off. can anyone tell me where i went wrong? thanks

2. Nov 9, 2009

### indietro

i think i would approach this questions as a conservation of momentum problem rather than

3. Nov 9, 2009

### indietro

I think i would approach this problem as a conservation of momentum rather than kinetic energy.
Pi = Pf
marvi ar + m apvi ap = marvf ar + m apvf ap

so you know the mass of the apple, both initial and final velocities of the arrow as well as both initial and final velocities of the apple!

Hope that helps!

4. Nov 9, 2009

### HeavyGuy

thanks a lot! i was wondering what i was doing wrong lol