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5.11.1 - Elastic and Inelastic Collisions

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Principle of Conservation of Momentum

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Teacher
Teacher

Today, we're going to discuss the conservation of momentum in collisions. Can anyone tell me what momentum is?

Student 1
Student 1

Is momentum the 'mass in motion' or something like that?

Teacher
Teacher

Exactly! Momentum is the product of an object's mass and velocity. So in a collision, the total momentum before the collision equals the total momentum after the collision. Can anyone give me an example of this?

Student 2
Student 2

Like when two cars collide? Their overall momentum before they crash is the same as after the crash, right?

Teacher
Teacher

Correct! And we can math that out with the equation P_initial = P_final. Remember, this is true for both elastic and inelastic collisions.

Student 3
Student 3

What happens during the collision?

Teacher
Teacher

Great question! During collisions, forces act on the two objects that cause changes in their momentum. These are known as impulsive forces.

Student 4
Student 4

Why do different collisions lead to different outcomes?

Teacher
Teacher

Excellent point. The outcome depends on how kinetic energy is treated during the collision. We'll delve more into that shortly. For now, let's summarize: Momentum is conserved in collisions, and the forces are equal and opposite.

Types of Collisions: Elastic and Inelastic

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Teacher
Teacher

Now, let's explore types of collisions. We have elastic collisions where kinetic energy is conserved. Can anyone explain what happens after such collisions?

Student 1
Student 1

I think in elastic collisions, both objects bounce off each other without losing any energy.

Teacher
Teacher

Exactly! Picture it like two perfect bouncy balls. Now, in contrast, what occurs in an inelastic collision?

Student 2
Student 2

In an inelastic collision, some energy is lost, right? The objects might get deformed.

Teacher
Teacher

Correct. Energy could turn into heat or sound. If they stick together, it’s called a completely inelastic collision. Can you name a real-world example of that?

Student 3
Student 3

A car crash where the cars crumple and stick together!

Teacher
Teacher

Exactly! They share their momentum but lose kinetic energy in the process. So to summarize: Inelastic collisions lose kinetic energy, while elastic collisions conserve it.

Effects of Deformation in Collisions

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Teacher
Teacher

Let's dive deeper into what happens during a collision. What do you think about the energy transformation?

Student 2
Student 2

Is it like energy turning into heat or sound when two things collide?

Teacher
Teacher

Exactly! In inelastic collisions, part of the kinetic energy transforms into other forms of energy. That's a crucial point when analyzing real-world collisions.

Student 4
Student 4

But how do scientists know if a collision is elastic or inelastic?

Teacher
Teacher

Good question! They determine this by measuring the kinetic energy before and after the collision. If it remains the same, it’s elastic; if not, it’s inelastic.

Student 1
Student 1

So, when we see dents on collision, it indicates inelastic?

Teacher
Teacher

Yes! Dents indicate energy was used to deform the materials, a key characteristic of inelastic collisions. Let's recap: deformations typically lead to energy losses in non-elastic collisions while perfectly elastic ones conserve energy.

Introduction & Overview

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Quick Overview

This section explores the concepts of elastic and inelastic collisions, highlighting the conservation of momentum and the transformation of kinetic energy during these events.

Standard

This section discusses the principles of conservation of linear momentum in collisions, distinguishing between elastic and inelastic collisions, where kinetic energy is conserved in elastic collisions but not in inelastic ones. It also explains the role of impulse forces and the effects of collision deformation, including sound and heat loss.

Detailed

5.11.1: Elastic and Inelastic Collisions

In any type of collision, the crucial principle of conservation of linear momentum holds, stating that the total momentum before a collision equals the total momentum after the collision. This can be mathematically expressed through the impulse-momentum theorem, which shows that the changes in momentum (9; b4p_1 ext{ and } 9; b4p_2 ext{ for objects 1 and 2}) are equal and opposite due to action-reaction forces described by Newton’s Third Law.

While momentum is conserved in all collisions, kinetic energy is not necessarily conserved. This leads to the distinction between elastic and inelastic collisions. An elastic collision occurs when two colliding bodies bounce off each other without any kinetic energy loss—imagine particles acting like a perfect spring. In contrast, during a completely inelastic collision, the colliding objects might stick together post-collision, sharing energy but losing some to deformation, sound, and heat. The more common scenario—inelastic collision—includes partial energy loss, where objects deform but also separate after impact.

Understanding these collision types and their implications is crucial for analyzing various physical systems.

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Audio Book

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Conservation of Momentum in Collisions

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In all collisions the total linear momentum is conserved; the initial momentum of the system is equal to the final momentum of the system. One can argue this as follows. When two objects collide, the mutual impulsive forces acting over the collision time ∆t cause a change in their respective momenta : ∆p1 = F12 ∆t ∆p2 = F21 ∆t where F12 is the force exerted on the first particle by the second particle. F21 is likewise the force exerted on the second particle by the first particle. Now from Newton’s third law, F12 = − F21. This implies ∆p1 + ∆p2 = 0. The above conclusion is true even though the forces vary in a complex fashion during the collision time ∆t. Since the third law is true at every instant, the total impulse on the first object is equal and opposite to that on the second.

Detailed Explanation

The principle of conservation of momentum states that the total momentum of two objects before they collide will equal their total momentum after the collision. This is due to Newton's third law, which asserts that for every action (force on one object), there is an equal and opposite reaction (force on the other object). When two objects collide, the forces they exert on each other during the collision cause changes in their momentum. These changes in momentum are dependent on the force exerted and the time duration of the collision. Because the forces exerted by the two objects on each other are equal and opposite, the total momentum before the collision is conserved after the collision.

Examples & Analogies

Imagine two ice skaters pushing off each other on an ice rink. If one skater has a larger mass, when they push off, the lighter skater will move away faster while the heavier skater will move slower. Despite the difference in their velocities, the total momentum (mass times velocity) for the two skaters before and after the push remains equal. This reflects the conservation of momentum.

Kinetic Energy in Collisions

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On the other hand, the total kinetic energy of the system is not necessarily conserved. The impact and deformation during collision may generate heat and sound. Part of the initial kinetic energy is transformed into other forms of energy. A useful way to visualise the deformation during collision is in terms of a ‘compressed spring’. If the ‘spring’ connecting the two masses regains its original shape without loss in energy, then the initial kinetic energy is equal to the final kinetic energy but the kinetic energy during the collision time ∆t is not constant. Such a collision is called an elastic collision. On the other hand the deformation may not be relieved and the two bodies could move together after the collision. A collision in which the two particles move together after the collision is called a completely inelastic collision. The intermediate case where the deformation is partly relieved and some of the initial kinetic energy is lost is more common and is appropriately called an inelastic collision.

Detailed Explanation

In physics, we categorize collisions based on whether or not kinetic energy is conserved. In elastic collisions, both momentum and kinetic energy are conserved. This means that the objects collide and bounce off without losing any speed—like two ideal billiard balls. In inelastic collisions, momentum is conserved, but kinetic energy is not. Some of the energy is transformed into other forms of energy, such as heat or sound, or is used to deform the objects. In completely inelastic collisions, the objects stick together after colliding, which results in the maximum amount of kinetic energy being transformed.

Examples & Analogies

Think about a car crash. Two cars collide; if they bounce off each other without any damage, that would represent an elastic collision—though such collisions are rare in the real world. More often, cars crumple upon impact, absorbing energy and losing some of their kinetic energy in the process. This is akin to an inelastic collision, where they may still move apart but not retain the same kinetic energy they had prior to the collision.

Definitions & Key Concepts

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Key Concepts

  • Conservation of Momentum: The total momentum before a collision equals the total momentum after the collision.

  • Elastic Collision: A collision where kinetic energy is conserved.

  • Inelastic Collision: A collision where some kinetic energy is transformed into other forms of energy.

  • Impulse: The change in momentum resulting from a force applied over time.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • When a rubber ball bounces back after impacting the ground with no loss of speed, it demonstrates an elastic collision.

  • In a car crash where vehicles crumple and stick together, the collision is inelastic as kinetic energy is lost.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Momentum stays the same, in collisions it won’t wane; Elastic springs retain the game, Inelastic brings some pain.

📖 Fascinating Stories

  • Imagine two energetic kids at a playground, bouncing with joy. They share the swing perfectly—just like two elastic balls that bounce back. When some kids collide in the mud, they stick—like inelastic collisions, losing some playfulness as they get messy.

🧠 Other Memory Gems

  • Remember 'MEP' for collisions: 'Momentum' is conserved, 'Elastic' keeps energy, 'Partially' lost in inelastic.

🎯 Super Acronyms

For remembering types of collisions, use 'EPI'

  • E: for Elastic
  • P: for Partially inelastic
  • I: for Inelastic.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Momentum

    Definition:

    The product of mass and velocity of an object, representing the quantity of motion it possesses.

  • Term: Elastic Collision

    Definition:

    A collision in which both momentum and kinetic energy are conserved.

  • Term: Inelastic Collision

    Definition:

    A collision in which momentum is conserved, but kinetic energy is not.

  • Term: Completely Inelastic Collision

    Definition:

    A type of inelastic collision where the colliding bodies stick together after impact.

  • Term: Impulse

    Definition:

    The change in momentum of an object when a force is applied over time.