What is the Inelastic Collision?

S4-SA1-0827

Grade Level:

Class 8

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

An inelastic collision is a type of collision where the total kinetic energy of the objects is not conserved (it decreases), but the total momentum is still conserved. In such collisions, some kinetic energy is usually converted into other forms like heat, sound, or deformation.

Simple Example

Quick Example

Imagine two auto-rickshaws bumping into each other on a busy street. If they collide and then stick together or get tangled up, moving as one unit afterwards, that's an inelastic collision. Their initial 'moving energy' (kinetic energy) reduces because some of it went into the sound of the crash and bending metal.

Worked Example

Step-by-Step

Let's say a small toy car (Mass M1 = 0.1 kg) moving at 2 m/s collides with a stationary larger toy truck (Mass M2 = 0.3 kg). After the collision, they stick together and move as one unit.

Step 1: Calculate initial momentum. Initial momentum = M1 * V1 + M2 * V2 = (0.1 kg * 2 m/s) + (0.3 kg * 0 m/s) = 0.2 kg m/s.
---Step 2: Since they stick together, their final mass is M_final = M1 + M2 = 0.1 kg + 0.3 kg = 0.4 kg.
---Step 3: Momentum is conserved. So, Initial Momentum = Final Momentum. Final Momentum = M_final * V_final.
---Step 4: 0.2 kg m/s = 0.4 kg * V_final.
---Step 5: Calculate V_final = 0.2 kg m/s / 0.4 kg = 0.5 m/s.
---Step 6: Now, let's check kinetic energy. Initial Kinetic Energy = 0.5 * M1 * V1^2 + 0.5 * M2 * V2^2 = 0.5 * 0.1 * (2)^2 + 0.5 * 0.3 * (0)^2 = 0.5 * 0.1 * 4 + 0 = 0.2 Joules.
---Step 7: Final Kinetic Energy = 0.5 * M_final * V_final^2 = 0.5 * 0.4 * (0.5)^2 = 0.5 * 0.4 * 0.25 = 0.05 Joules.
---Step 8: Since 0.2 Joules (initial) is greater than 0.05 Joules (final), kinetic energy is NOT conserved. This confirms it's an inelastic collision.

Answer: The final velocity of the combined toys is 0.5 m/s, and kinetic energy is lost, confirming an inelastic collision.

Why It Matters

Understanding inelastic collisions helps engineers design safer cars (EVs) that absorb impact energy in crashes, and helps in robotics for grippers that hold objects securely. It's also crucial in space technology for docking spacecraft, ensuring gentle connections without excessive rebound.

Common Mistakes

MISTAKE: Assuming kinetic energy is always conserved in all types of collisions. | CORRECTION: Kinetic energy is only conserved in 'elastic' collisions. In inelastic collisions, kinetic energy is lost, usually as heat, sound, or deformation.

MISTAKE: Thinking that momentum is not conserved in inelastic collisions. | CORRECTION: Momentum is always conserved in ALL types of collisions, both elastic and inelastic, as long as no external forces act on the system.

MISTAKE: Confusing 'inelastic' with 'objects bounce off each other'. | CORRECTION: In a perfectly inelastic collision, objects stick together and move as one. In other inelastic collisions, they might separate but still lose kinetic energy.

Practice Questions

Try It Yourself

QUESTION: A 0.5 kg cricket ball moving at 10 m/s hits a stationary 1.5 kg wicket. If the ball and wicket move together after impact, what is their combined velocity? | ANSWER: 2.5 m/s

QUESTION: Two carts, one with mass 2 kg moving at 3 m/s and another with mass 4 kg moving at 1 m/s in the same direction, collide and stick together. What is their final velocity? | ANSWER: 1.67 m/s (approx)

QUESTION: A 50g bullet travelling at 200 m/s strikes and embeds itself in a 950g wooden block resting on a frictionless surface. What is the final velocity of the bullet-block system, and how much kinetic energy is lost during the collision? | ANSWER: Final velocity = 10 m/s; Kinetic energy lost = 950 Joules

MCQ

Quick Quiz

Which of the following statements is TRUE for an inelastic collision?

Both momentum and kinetic energy are conserved.

Momentum is conserved, but kinetic energy is not conserved.

Kinetic energy is conserved, but momentum is not conserved.

Neither momentum nor kinetic energy is conserved.

The Correct Answer Is:

In an inelastic collision, the total momentum of the system before and after the collision remains the same (conserved). However, some of the kinetic energy is converted into other forms, so kinetic energy is not conserved.

Real World Connection

In the Real World

When a car undergoes a crash test for safety ratings, the collision is highly inelastic. Engineers at companies like Tata Motors or Mahindra study how much the car deforms and how much energy is absorbed to design safer vehicles. This protects passengers by converting the car's kinetic energy into crumpling, rather than transferring it all to the occupants.

Key Vocabulary

Key Terms

COLLISION: An event where two or more objects exert forces on each other for a short period. | MOMENTUM: A measure of the mass and velocity of an object (mass x velocity). | KINETIC ENERGY: The energy an object possesses due to its motion. | CONSERVED: Remaining constant; not changing.

What's Next

What to Learn Next

Next, you should learn about 'Elastic Collisions' to understand the difference. Knowing both types of collisions will give you a complete picture of how objects interact and transfer energy, which is important for understanding more advanced physics concepts.