Difference between Elastic Collision and Inelastic Collision

Quantity	Elastic Collision	Inelastic Collision
Momentum	Conserved	Conserved
Kinetic Energy	Conserved	Not conserved
Total Energy	Conserved (always)	Conserved (always)
Coefficient of Restitution (e)	[e = 1]	[0 ≤ e < 1]

Property	Elastic Collision	Inelastic Collision
Definition	Both momentum & kinetic energy conserved	Only momentum conserved
Coefficient of Restitution	[e = 1]	[0 ≤ e < 1]
Kinetic Energy	Same before and after collision	Partly converted into heat, sound, deformation
Deformation	Temporary	Permanent
Example	Billiard balls, gas molecules	Car crash, clay balls, football hitting wall
Velocity Relation	[ v_2 – v_1 = u_1 – u_2 ]	[ v_2 – v_1 = e(u_1 – u_2) ]
Energy Loss	No loss	Partial loss
Nature	Ideal, rare in real life	Common in real situations
Post-Collision Behavior	Bodies bounce apart	Bodies may move together
Mathematical Indicator	[ e = 1 ]	[ e < 1 ]

Let initial total kinetic energy = [ E_i ] and final = [ E_f ].

In elastic collisions, [ E_i = E_f ].
In inelastic collisions, [ E_i > E_f ], and the loss [ (E_i – E_f) ] goes into:
- Internal deformation
- Heat and sound energy
- Vibration of molecules

Elastic Collisions	Inelastic Collisions
Study of gas molecules in kinetic theory	Design of crumple zones in vehicles
Rutherford scattering experiments	Ballistics and momentum transfer
Newton’s cradle toy	Sports impacts (e.g., cricket ball hitting bat)

1. Is momentum conserved in all types of collisions?

Yes, momentum is always conserved in all collisions, provided no external force acts on the system.

2. Why is kinetic energy not conserved in inelastic collisions?

Because some kinetic energy converts into heat, sound, or deformation energy.

3. Can an inelastic collision ever become elastic?

No, because energy loss is unavoidable once deformation occurs.

4. What does perfectly inelastic mean?

It means the colliding bodies stick together and move with a common velocity after collision.

5. In which type of collision is coefficient of restitution less than 1?

In inelastic collisions.

6. Can energy be created or destroyed during a collision?

No, total energy is conserved. Only its form changes.

7. Why are molecular collisions nearly elastic?

Because internal deformation is negligible at that microscopic scale.

8. What happens to velocity directions after a perfectly elastic collision?

The bodies exchange their velocities if their masses are equal.

9. Is sound energy in collisions part of kinetic energy?

No, it’s a form of vibrational energy — converted from lost kinetic energy.

10. Why are most real-world collisions inelastic?

Because perfect elasticity is impossible due to inevitable heat and deformation losses.

1. If kinetic energy is not conserved, does it mean momentum isn’t conserved?

No, momentum conservation is independent of energy conservation; it always holds true.

2. Can a ball bouncing on the floor be considered elastic?

Not perfectly — some energy is lost, so it’s partially elastic (e.g., e ≈ 0.8 for rubber).

3. Is the coefficient of restitution zero for all inelastic collisions?

No, it’s zero only for perfectly inelastic collisions; for others, [0 < e < 1].

4. Can an elastic collision occur in real life?

Almost never perfectly, but some collisions (like atomic or molecular) are nearly elastic.

5. Does “energy lost” in an inelastic collision disappear?

No, it transforms into heat, sound, or deformation — total energy remains constant.