Thermodynamics and Inelastic Collisions: Conservation of Momentum
Introduction
Welcome to the wild and wacky world of inelastic collisions, where things get sticky and kinetic energy takes a nosedive faster than you can say "conservation of momentum"! 🌟 In this guide, we're going to break down the essentials of inelastic collisions—where to find them, how they work, and why kinetic energy always seems to wander off like a cat with a mind of its own. 🐱
What Are Inelastic Collisions?
Picture this: you're walking through your school hallway, and you bump into a friend. Instead of bouncing off and continuing your merry way, you both get tangled up, awkwardly stuck together until you both stumble to the nearest wall. That's basically what happens in an inelastic collision—where two objects collide and stick together, resulting in a big ol' loss of kinetic energy. 👫
In a perfect collision—an elastic one—both momentum and kinetic energy are conserved. It's like a flawless dance where everyone knows their steps. But in an inelastic collision, while momentum sticks around like that one friend who doesn't know when to leave, kinetic energy becomes the party pooper that leaves early, transforming into other forms of energy like heat, sound, or Barney Stinsonlevels of "legendary." 🌡️🎶
Fundamental Concept: Conservation of Momentum
Momentum is like the VIP guest list of the physics world—it must be conserved. Whether it's an elastic or inelastic collision, the total momentum before the collision is equal to the total momentum after the collision. It’s why we can say, "Momentum is king." 👑
Mathematically, if two objects with masses m₁ and m₂ collide with initial velocities v₁ and v₂:
Initial Momentum: [ p_\text{initial} = m₁v₁ + m₂v₂ ]
Final Momentum (for Inelastic Collision where they stick together): [ p_\text{final} = (m₁ + m₂) v_f ]
Since momentum is conserved: [ m₁v₁ + m₂v₂ = (m₁ + m₂) v_f ]
The Mathematical Dance: Example Problem
Imagine two shopping carts, one loaded with textbooks (mass = 5 kg) and the other with a bunch of snacks (mass = 2 kg). The textbook cart (5 kg) zooms down the aisle at 3 m/s toward the stationary snack cart. They collide and stick together in an epic display of inelasticity.

Is this collision elastic or inelastic?
 Definitely inelastic. After all, they stick together, like peanut butter and jelly.

What is the common final velocity of the carts after the collision?
[ \begin{aligned} & \text{Initial Momentum:} & & p_\text{initial} = 5 , \text{kg} \cdot 3 , \text{m/s} + 2 , \text{kg} \cdot 0 , \text{m/s} = 15 , \text{kg} \cdot \text{m/s} \ & \text{Final Momentum:} & & p_\text{final} = (5 , \text{kg} + 2 , \text{kg}) \cdot v_f = 7 , \text{kg} \cdot v_f \ & & & 15 , \text{kg} \cdot \text{m/s} = 7 , \text{kg} \cdot v_f \ & & & v_f = \frac{15 , \text{kg} \cdot \text{m/s}}{7 , \text{kg}} \approx 2.14 , \text{m/s} \end{aligned} ]
 What is the initial kinetic energy of the 5 kg cart?
[ KE_\text{initial} = \frac{1}{2} m v^2 = \frac{1}{2} \cdot 5 , \text{kg} \cdot (3 , \text{m/s})^2 = 22.5 , \text{J} ]
 What is the final kinetic energy of the 5 kg cart?
Since they stick together, treat it as one mass of 7 kg:
[ KE_\text{final} = \frac{1}{2} \cdot 7 , \text{kg} \cdot (2.14 , \text{m/s})^2 \approx 16 , \text{J} ]
 What is the change in kinetic energy of the 5 kg cart during the collision?
[ \Delta KE = KE_\text{final}  KE_\text{initial} = 16 , \text{J}  22.5 , \text{J} = 6.5 , \text{J} ]
BOOM! Energy conservation just left the chat.
Molecular Level and Collisions
On a molecular level, collisions are often not purely elastic. Think of them as awkward handshakes instead of perfect highfives. The tiny particles jiggle and squish, converting a bit of that kinetic energy into internal energy, heat, or even wild quantum samba moves. 💃
Key Differences: Inelastic vs. Elastic
Let’s break it down:
 Elastic Collisions:
 Think of protons at a subatomic trampoline park—bouncing off each other with joy, conserving both momentum and kinetic energy.
 Inelastic Collisions:
 Imagine two lumps of dough. Colliding, they merge into one dough blob, conserving momentum but spreading kinetic energy into heat, sound, and maybe a messy countertop.
Key Vocabulary to Review
 Completely Inelastic Collision: Two objects collide, stick together, and become besties for life—also known as moving as a single unit.
 Inelastic Collisions: Collisions where kinetic energy isn't conserved. Energy transforms into heat, sound, or deformation—think of a car crash, but with fewer insurance claims.
Go forth, young physicists, and conquer those inelastic collisions! May your experiments be sticky, your momentum conserved, and your kinetic energy—well, may it find its way back eventually. 🚀