Elastic Collisions

Claimed by Graham Ely, Fall 2017

The Main Idea

So what exactly is an elastic collision? I know you’re thinking, “Oh I know all about elasticity,” but LOL this is not bubble gum or rubber bands guys!

First, we need to define a collision. A collision is an event/process in which two objects interact strongly for a short amount of time and in which there was very little interaction before they interacted and after the interaction. An elastic collision is a collision between two or more objects in which there is no loss in kinetic energy before and after the collision. If we assume that the colliding objects are part of the system and that there is no force from the surroundings, the final kinetic energy is still in the same form as it was initially. This is because the surrounding forces are considered to have a negligible impact compared to the forces that occur between the colliding objects. To keep it simple, this means that kinetic energy in= kinetic energy out. Usually, you can determine if a collision is elastic or not by seeing if the objects bounce off one another, unlike inelastic collisions. To find the difference between the two types of collisions, keep in mind that momentum is transferred for both of them so the best way of differentiating would be to look at the transfer of kinetic energy. If the difference of internal and kinetic energy is equal to zero, then it is elastic. Apart from looking to see if the objects bounce off another or not, we can also judge by looking to see if the objects get deformed, are hotter, have more vibration/rotation or are in an excited state after collision. If any of the above happens, the collision is not elastic. Remember how your parents always told you what goes in must come out? They were talking about elastic collisions... probably.

Additionally, elastic collisions are a wonderful representation of the conservation of momentum which states that the momentum of an isolated system is constant. For an isolated system undergoing an elastic collision momentum in = momentum out.

And I love playing pool because every time a ball hits another and they bounce off one another I’m witnessing an elastic collision so basically I’m a physicist in the lab. The image below demonstrates the main idea of an elastic collision. Boing!!

It's important to note that with macroscopic systems there are no perfectly elastic collisions because there's always at least a dab of dissipation (like some thermal energy given off), but most are nearly elastic. The only time there are perfect collisions is on a microscopic level when atomic systems with quantized energy collide, but that's only if there is enough energy available to raise the systems to an excited quantum state-- but quantum energy is really a whole other topic. Let's focus on elastic collisions!

Check out this funny video on elastic collisions also! https://www.youtube.com/watch?v=W9EqU1_DXUw

Now check out this more informative video on elastic collisions: https://www.youtube.com/watch?v=V4vzNk4qppw

A Mathematical Model

Now lets take what we know about elastic collisions and talk about it in mathematical terms. There are three main mathematical concepts surrounding elastic collisions:

1. [math]\displaystyle{ K_f = K_i }[/math]

2. [math]\displaystyle{ \Delta E_{int} = 0 }[/math]

[math]\displaystyle{ \Delta E_{sys} + \Delta E_{surr} = 0 \\ }[/math] [math]\displaystyle{ \Delta E_{surr} = 0 }[/math] , so [math]\displaystyle{ \Delta E_{sys} = 0 }[/math] [math]\displaystyle{ E_{final} = E_{initial} \\ }[/math]

3. [math]\displaystyle{ \vec{p}_f = \vec{p}_i }[/math]

[math]\displaystyle{ \Delta \vec{p}_{sys} + \Delta \vec{p}_{surr} = 0 \\ }[/math] [math]\displaystyle{ \Delta \vec{p}_{surr} = 0 }[/math] , so [math]\displaystyle{ \Delta \vec{p}_{sys} = 0 }[/math] [math]\displaystyle{ \vec{p}_{final} = \vec{p}_{initial} \\ }[/math]

Example Problem

Cart 1, moving in the positive x direction, collides with cart 2 moving in the negative x direction. Both carts have identical masses and the collisions is (nearly) elastic, as it would be if the carts interacted magnetically or repelled each other through soft springs. What are the final momenta of the two carts?

1. After choosing a correct system, surroundings, and initial and final states, we can apply the momentum principle mentioned above to get a relationship between the initial and final momentums of the two carts. Usually, when we want to consider the system, we will consider the two colliding objects as the system and the rest as the surroundings. The initial state would be before the collision, and the final state would be after.

Momentum Principle: [math]\displaystyle{ \vec{p}_{final} = \vec{p}_{initial} + \vec{F}_{net}\Delta t }[/math]

And it turns into: [math]\displaystyle{ \vec{p}_{1f} + \vec{p}_{2f} = \vec{p}_{1i} + \vec{p}_{1f} }[/math] since during the collision, the [math]\displaystyle{ F_{net} }[/math] is negligible.

2. Next, by applying the energy principle we can gain knowledge about the final and initial kinetic energies. By acknowledging the fact that the change in internal energy is 0 and the fact the reaction happens so quickly that not work or heat transfer is done by the surroundings, the equation can be simplified to include just initial and final kinetic energies.

The Energy Principle: [math]\displaystyle{ \Delta E_{sys} = W + Q }[/math]

We then get rid of the work, heat transfer and internal energies:

[math]\displaystyle{ K_{1f} + \Delta E_{1int} + K_{2f} + \Delta E_{2int} = K_{1i} + K_{2i} + W + Q \\ }[/math] [math]\displaystyle{ \Delta E_{1int} = \Delta E_{2int} = W = Q = 0 \\ }[/math] [math]\displaystyle{ K_{1f} + K_{2f} = K_{1i} + K_{2i} }[/math]

The reason the internal energies are directly crossed out is because we can put them to one side since [math]\displaystyle{ E_{final} = E_{initial} }[/math] and therefore [math]\displaystyle{ E_{final} - E_{initial} = 0 }[/math].

3. Once we have simplified the momentum and energy principles, one can use the relationship between kinetic energy and momentum mentioned above to get a relationship involving both energy and momentum. Once this is obtained, the equation can be shifted around to get both the final momentums.

Kinetic Energy Definition: [math]\displaystyle{ K = \frac{p^2}{2m} }[/math]

Finished Result: [math]\displaystyle{ \frac{p^2_{1f}}{2m} + \frac{p^2_{2f}}{2m} = \frac{p^2_{1i}}{2m} + \frac{p^2_{2i}}{2m} }[/math]

A Computational Model

The trinket model linked demonstrates an elastic collision between two spheres.

Elastic Collision Glowscript Model

The link below demonstrates a basic car crash elastic collision using MATLAB. Take note of the changes in displacement and the time.

Elastic Collision MATLAB model

Examples

Be sure to show all steps in your solution and include diagrams whenever possible. For a lot of the complicated problems, starting with a diagram and writing down the given information and plugging that into either the momentum or energy principle can help you move forward in the problem. It would be better to also write the main principles at the side to remind yourself of how you found it. If you still get stuck, try finding similar examples to that problem and look at the solutions to get on the right track.

Simple

Let’s start with a simple example like the ones you don't see on the test.

Jay and Sarah are best friends. Since they’re best friends they both weigh 55 kg. Jay hadn’t seen Sarah in a long time so when she saw her she ran to her with a velocity of <4,0,0> m/s. Instead of a hug, they were both too excited and collided and bounced back off of each other, and Sarah flew back with a velocity of <7,0,0> m/s. What was Jay’s final velocity?

In a lot of simple elastic collision problems, the momentum principle is all you need to solve them. Most problems will the initial velocities, masses, and one final velocity and you will be asked to solve for a either a final velocity or final momentum. By setting up an equation like the one in this problem, one can easily handle this type of problem.

Middling

When far apart, the momentum of a proton is <5.2 ✕ 10−21, 0, 0> kg · m/s as it approaches another proton that is initially at rest. The two protons repel each other electrically, but they are not close enough to touch. When they are far apart again later, one of the protons now has a velocity of <1.75, .82, 0> m/s. At that instant, what is the momentum of the other proton? HINT: Mass of proton is 1.7 *10^-21 kg.

A lot of people freak out when they see problems involving atomic particles and electrical forces, but the concept is the same no matter the scale. For this problem we are given masses, velocities, and momentum, but the equation p = mv allows us to easily handle these different values. Once again drawing a diagram helps to understand what is actually happening in the problem, making it easier to put the values in the correct places and solve.

Difficult

Okay, let's get a little bit trickier here...

There is a 400 kg train traveling at 55 m/s that collides, elastically of course, with a random 2 kg trashcan that's stationary on the tracks. So afterwards, what are the speeds of both the train and the trashcan after the collision?

This was a difficult problem because of the assumptions we had to make in order to solve it. Because the collision was elastic we were able to disregard the change in internal energy in the energy principle, and since it happened so quickly the work done by the track and heat transfer between the surroundings was negligible. It is tough to tell what assumptions you can make and which ones you can't in problems like these, which is what can make them difficult. On most exams or quizzes the collision problems will be inelastic simply because they are harder, but it is important to understand the fundementals of elastic collisions and how to solve problems involving them.

Connectedness

So how are collisions connected to the real world? Collisions are all around us!

One main example is cars! Lots of car companies will purposely test and wreck cars to test collisions! Data is then sent to places like the Insurance Institute for Highway Safety where we can learn about car agility and more.

Another example of collisions in real life is billiards. This is one of the most accurate real life examples of elastic collisions. One ball hits another ball at rest, and if done right the first ball stops and transfers nearly all its kinetic energy into kinetic energy in the second ball. We consider this a head on collision of equal masses.

A third example is hitting a baseball. Hitting a ball off the end or to close the hands of a bat will cause some vibrational energy from the collision, but if the ball catches the sweet spot the collision is very elastic, and you don't feel any vibration when you strike the ball.

Elastic collisions also happen between particles. The Rutherford Scattering experiment mentioned below is a good example.

History

Collisions are certainly not a knew concept in the world. Ever since the beginning or time things have been colliding and reacting in different ways. It was experiments done by scientists like Newton or Rutherford that started to characterize collisions into categories and apply fundamental principles of physics to the reactions. What we observed above was the Newtonian way.

The history of collisions originates from the Rutherford Scattering experiment. It consisted of shooting alpha particles through a thin gold foil. As Rutherford studied the scattering of alpha particles through metal foils, he first noticed a collision with a single massive positive particle. Although the alpha particles did not hit the nucleus of the gold atoms, they did interact with each other and therefore can be considered as a collision. Since the interaction did not excite the gold atoms, fortunately enough, it was an elastic collision. This lead to the conclusion that the positive charge of the mass was concentrated in the center, the atomic nucleus! The plum pudding model (where the positive and negative charges were stuck within the atom like plums in a pudding), that had been around was disproved. When further research was done, they measured the angle of the 'scattering' or the particles shot through a tin gold foil. Had the collisions been inelastic, the particles would not have been able to bounce back.