Is a collision elastic if momentum is conserved but kinetic energy isn't?

[new324]

Im preparing for an Elastic Collision lab today and was wondering is someone would check my prelab questions.

A 5 kg object traveling at 4 m/s collides head on with a 10 kg object moving toward it with a speed of 3 m/s. The 10 kg object stops dead after the collision.

a) What is the final speed of the 5 kg object?

I used conservation of momentum.

p1i + p2i = p1f + p2f

20 - 30= p1f + 0; p1f= -10; v1f= -2 m/s

b) Is the collision elastic? This is where i question myself. Since the lab is for an elastic collision I would think it would be elastic. However, when I plug these values into their KE values it seems otherwise.

KE1i + KE2i = KE1f + KE2f

(.5 * 5 * 16) - (.5 * 10 *9) = (.5 * 5 * 4) + 0
Since 40 - 45 = 5; and KE1f = 10; It is inelastic. Right?

 

[Matty9879]

10 is your final speed not Ke for inelastic collisions use only conservation of momentum

 

[neutrino]

b) Is the collision elastic? This is where i question myself. Since the lab is for an elastic collision I would think it would be elastic. However, when I plug these values into their KE values it seems otherwise.

KE1i + KE2i = KE1f + KE2f

(.5 * 5 * 16) - (.5 * 10 *9) = (.5 * 5 * 4) + 0
Since 40 - 45 = 5; and KE1f = 10; It is inelastic. Right?

It is inelastic, but there is no negative sign when you calculate the kinetic energies. Therefore the initial kinetic energy of the system is 85 J.