I am new to physics and I hope you can help me solve this problem.

When two objects collide with each other and stick, there is an internal force, so linear momentum is conserved.

But when I calculate the initial and final kinetic energies of the system, it shows me that energy is lost when the collision occurs. This shows me that it is an inelastic collision. But how is the energy lost without the kinetic energy being transmitted into any other force.

I thought that it is due to the friction between the two objects, but my thinking should be wrong because in this stage, friction is not a course to decrease the kinetic energy of the system.

So, can you tell me what happened to the decreased energy of the system? In which energy type was it transmitted? I hope for a brief explanation.


In case of a perfectly inelastic collision (zero coefficient of restitution), the two bodies stick together. In such a collision, kinetic energy is lost by bonding the two bodies together. This bonding energy usually results in a maximum kinetic energy loss of the system.

As you've rightly said, only momentum is conserved in inelastic collision. The loss of energy is due to the sticking of the bodies and the forming of bonding energy due to the sticking.


In an inelastic collision the kinetic energy of the system is not conserved. The energy that you find missing has been dissipated as other forms of energy. This can be heat, vibrational energy transferred to atoms of the colliding particles, energy lost to deform the atomic structure etc.

I imagined that it is due to the friction between two objects, but my thought should be wrong because in this stage friction is not a course to decrease the kinetic energy of the system

I don't think you are wrong. Friction can decrease the kinetic energy, say if it resulted in heat generation during the collision.

  • $\begingroup$ So, think that there is a object of 4Kg with 0 velocity, and 2Kg object with 6ms-1 velocity. After the collision they don' t stick each other, but get the same velocity. There is no stick between each other so we can imaginate, it is a elastic collision such like other elastic collisions without sticking. Calculations shows both objects have 2 ms-1 but in this s stage also energy is lost. When we are not sticking them why we can't same it for a normal elastic collision? $\endgroup$ – Osal Selaka Oct 22 '17 at 5:28
  • $\begingroup$ Inelastic collision does not mean that the objects always stick together.( I think that's where your confusion lies). Objects sticking together is just one of the cases - called perfectly inelastic collision. The final velocities of colliding objects depend on their mass and initial velocities as you can obtain by applying conservation of momentum. As a thumb rule - inelastic collision means that kinetic energy is not conserved. $\endgroup$ – gan Oct 22 '17 at 5:47

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