## Physics question on kinetic energy?

Question

Particle A of mass 500g is moving with a constant velocity of 12m s^-1 towards particle B of mass 1.5kg which is initially at rest. After collision, particle A comes to a rest and particle B moves with a new velocity of 4 m s^-1.

What is the loss in kinetic energy of the system?

How do I find the loss in the kinetic energy of the system, what does that mean?

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General Physics
6 years
1 Answer
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## Answer ( 1 )

Hi A.

In a collision you have, the only thing that remains constant is the total momentum before and after the collision. Since you are asked to determined the change of kinetic energy, let assume beforehand we are in the presence of an inelastic collision, where the internal forces do work so there is a change in the kinetic energy:

ΔEc = Wint

ΔEc = Ecf – Eci

Lets evaluate the energies before (Eci) and after (Ecf) the collision:

ΔEc = [0 + (1/2)*mB*vB²] – [(1/2)*mA*vA² + 0]

ΔEc = (1/2)*[(1.5 kg)*(4 m/s)² – (0.5 kg)*(12 m/s)²] = -24 kg*m²/s² = -24 J

The negative sign just tells you that you are losing energy in the collision.

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Source(s): I’m a Chemist with 4 years of teaching experience