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Elastic and Inelastic Collisions is a part of CBSE Class 11 Physics Unit 4 Chapter 6 Work Energy and Power. As per the syllabus, Unit IV along with Unit V and VI carry a total weightage in Class 11 Examinations. Students are also recommended to go through CBSE Class 12 Physics Syllabus.
Read Unit IV Chapter 6 Important Questions
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Understanding Collisions
Two bodies collide with each other when they come in contact violently for a short span of time. The collision of two vehicles in an accident, a falling object landing on a surface, or collision of balls while playing billiards are some examples of collision.
Types of Collisions
The line along which the internal forces act during a collision is called the line of collision. There are various types of collisions on different bases- Line of collision and change in kinetic energy.
Types of Collisions with Respect to the Line of Collision
There are two types of such collisions, head-on and oblique.
Head-on Collision or One-Dimensional Collision
When two bodies having their velocities along the line of collision collide with each other, the situation is called head-on collision. Or when the lines joining their center on masses is parallel to the direction of their velocities.
Oblique Collision or Two-Dimensional Collision
In the case of oblique collision, one of the two bodies has its velocity at an angle with the line of collision. Or when the line joining their centre of masses is not parallel to the direction of their velocities.
Types of Collisions on the Basis of Change in Kinetic Energy
The three types of collisions which depend on the change in kinetic energy are elastic collision, inelastic collision, and perfectly inelastic collision.
Elastic Collision
In an elastic collision, the kinetic energy of the system will remain constant which means there will be no change in the kinetic energy of the system before and after the collision. It also follows the conservation of momentum.
Consider two bodies with masses m1 and m2, and velocities u1, u2 before the collision, and v1, v2 after the collision.
The conservation of momentum before and after a collision can be expressed as
m1u1 + m2u2 = m1v1 + m2v2.
Similarly, the conservation of kinetic energy before and after the collision can be expressed as
To find the values, these equations can be solved as
And if both the masses are the same,
v1 = u2
v2 = u1
Examples of Elastic Collision
- Balls hitting each other while playing billiards.
- A ball thrown and bouncing to the same height it was thrown from, is an example of elastic collision as there is no net change in the kinetic energy.
- Collision of atoms is also an elastic collision.
Inelastic Collision
In an inelastic collision, the kinetic energy of the system is not conserved, unlike inelastic collision. The kinetic energy is lost as it gets dissipated in other forms of energy like heat, sound, etc, or is absorbed by the body. But they follow the conservation of momentum, like an elastic collision.
When two bodies collide, the final velocity of the body in an inelastic collision is
V=(m1v1+m2v2)/(m1+m2)
Where V is the final velocity of the body.
Examples of Inelastic Collision
- When the ball is dropped on the ground, and it fails to reach the height it was dropped from.
- The accident of two vehicles is an inelastic collision.
- If clay is thrown at a wall, it sticks to it.
Difference between Elastic and Inelastic Collision
| Elastic Collision | Inelastic Collision |
| The net kinetic energy of the system is conserved | The net kinetic energy of the system is not conserved |
| Energy does not get converted into other forms | The kinetic energy gets converted into other forms of energy like heat, etc |
| It is rarely possible, as practically there is always a change in energy | This is the collision that happens naturally in real life. |
| Are possible with subatomic particles | Possible in most of the bodies and particles |
| Two electrically charged particles colliding with each other without generating heat | Collision of two vehicles |
Perfectly Inelastic Collision
A collision in which the two bodies move together after the collision, after the loss of the maximum amount of kinetic energy, is called a perfectly or completely inelastic motion. The kinetic energy is lost by bonding the two bodies together.
Note: The momentum of the bodies is conserved only when there is no friction on the surface or air resistance.
For two bodies a and b,
Sample Questions
Ques. What is collision?
Ans. Collision is when two bodies violently come in contact with each other violently for a short span of time.
Ques. What are the types of the collision?
Ans. There are three collisions on the basis of conservation of kinetic energy-Elastic, Inelastic, and perfectly inelastic.
Ques. Why is There A Loss Of Kinetic Energy in Inelastic Collisions?
Ans. The conservation of the momentum of the system is possible in an inelastic collision. Kinetic energy conservation failed in this collision. This happens because the kinetic energy is transferred into some other form of energy.
Ques. Which type of collision can we see in real life?
Ans. We see inelastic experiences mostly in real life in the form of road accidents, playing games like billiards, falling of objects onto surfaces, etc.
Ques. Where can we find collisions in everyday life?
Ans. Games like carrom, billiards, cricket, etc are some simple examples of collisions.
Ques. The Crash Of Two Cars Is Elastic or Not. Explain?
Ans. Conservation of kinetic energy is not maintained. Kinetic energy is transformed into sound energy, heat energy, and deformation of the objects. An elastic collision happens when two objects collide and bounce back to its initial place. So, the collision of two cars is not elastic rather, inelastic.
Ques. Do elastic collisions show conservation of momentum?
Ans. Yes, elastic as well as inelastic collisions follow the conservation of momentum.
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