Question

A 1 kg mass moving to the right at a speed of 10m/s collides head-on in a perfectly inelastic collision with a stationary 2kg mass. How much KE is lost during the collision?

First I found V_f to be 3.3m/s

then I found KE_i=1/2m_1v_1i^2 +1/2 m2V_2i^2
=1/2(1)(10)^2 +1/2(2)(0)^2
=50J

then KE_f=
=1/2(1)(3.3)^2 +1/2(2)(3.3)^2
=16.335J

then change in KE =16.335-50
=-33.665 J
is this correct? And is the answer negative because this is how much KE was lost?

Answer 1

If they stuck together, it is correct. Perfectly inelastic is without meaning, inelastic would have been enough, however, it should have stated the masses were stuck.
KE lost is 33.7J. I would say KE lost is 33.7J, and forget the negative sign.