Edited by mdumenci3 (Fall 2017)

This entry covers collisions, the processes in which large interactions are
seen constrained in a short time period, with little interaction preceding and
succeeding the same period.

==Idea==
Collisions aren't special case events. They are commonly seen everywhere in our
daily lives: hopping a basketball, dropping our overly expensive phones and
car crashes being a few examples. As with everything we've learned so far,
collisions behave according to the fundamental principles. We're analyzing
them separately, because the nature of collisions allows us to make assumptions
that help us reason about these processes, and solve for unknowns.

==Assumptions==
===Conserved momentum and energy===
If we take the colliding objects as the system, what assumption can we make
about the total momentum and energy of this system?

In our definition of a collision, we've said that interactions preceding
and succeeding the process are tiny compared to the ones we see during the
process. The forces that contribute to this sudden spike of interactions is
internal to the system: they are between the colliding objects. Since we take
them as the system, we can make the assumption that the outside forces that act on the
system are negligible.

By this assumption, the change in the total momentum and energy of the system
(of both the colliding objects) is negligible.

For <math>\Delta p = F * \Delta t</math>, <math>F</math> is negligible, thus
<math>\Delta p</math> is negligible.

For <math>\Delta E = W_{surr} + Q</math>, <math>W_{surr} = F_{surr} * \Delta x</math>
and <math>Q = m * C * \Delta t</math>. <math>F_{surr}</math> and <math>\Delta t</math>
are negligible. Thus <math>\Delta E</math> is negligible.

Therefore, we can assume that the total momentum and energy of the system
(of both the colliding objects) don't change during the collision.

===Elasticity/Inelasticity and Kinetic Energy===
We can further characterize collisions by looking at the energy interactions
within the systems of collision.

For a brief overview, please watch this simulation on collisions: [https://www.youtube.com/watch?v=Xe2r6wey26E]

In [[Elastic Collisions]], the total kinetic energy is conserved. None of the
kinetic energy of the system is transformed from or into <math>E_{internal}</math>,
such as thermal energy and spring potential energy.

An example for such a process is the Newton's Cradle, where the kinetic energy
of the colliding ball gets transformed into the kinetic energy of the last
ball in the cradle. The total kinetic energy is conserved.

In [[Inelastic Collisions]], the total kinetic energy is not conserved. Some/all
of the kinetic energy of the system can be transformed from or into
<math>E_{internal}</math>.

An example for such a process is a car crash, where the kinetic energy of the
colliding car gets transformed into thermal energy and internal energy contained
in the crumbled, tense parts of the car chassis.

([[Inelastic Collisions]] where there's maximum kinetic energy dissipation
are called [[Maximally Inelastic Collision]]. This doesn't necessarily mean
that ''all'' of the kinetic energy gets transformed into <math>E_{internal}</math>,
since the total momentum is conserved. Collisions where the objects stick together are
[[Maximally Inelastic Collision]].)

==Mathematical model==
By our assumptions, <math>p_f = p_i</math> and <math>E_f = E_i</math>
where subscripts <math>f</math> and <math>i</math> mean final and initial,
respectively.

In an elastic collision, <math>K_{total, f} = K_{total, i}</math>.

In an inelastic collision, this equation doesn't hold, since
<math>\Delta E = \Delta K + \Delta E_{internal} = 0</math>, and
<math>\Delta E_{internal}</math> might be nonzero.

==Computational model==
In this model, two objects of different masses are moving towards each other
with equal magnitude, but opposite direction momenta.

The objects collide, and after the collision, it is observed that the objects
are embedded within each other with final velocity zero.

This is an example of a [[Maximally Inelastic Collision]]: none of the kinetic
energy is conserved.

[https://trinket.io/glowscript/e600a7595e Computational Model]

Note: The behavior of the colliding objects are hardcoded in the model to
embed (vs. bouncing, etc.) This is an assumption we make about the
materials/internal structure of the balls. Correctly modeling deforming
objects without simplifying assumptions is a very complex task.

==Examples==

===Question 1===
A <math>0.5 kg</math> soccer ball is moving with a speed of <math>5 m/s</math>
directly towards a <math>0.7 kg</math> basket ball, which is at rest.

The two balls collide and stick together. What will be their final speed?

[[File:Collision-example-1-mert.png|500px]]

===Answer 1===
Let <math>m_1 = 0.5 kg</math>, <math>v_1 = 5 m/s</math>,
<math>m_2 = 0.7 kg</math>, <math>v_2 = 0 m/s</math>, where
subscripts <math>1</math> and <math>2</math> signify objects 1 and 2.

We're going to compute <math>v_f</math> for the collided mass.

We know that momentum is conserved. <math>p_f = p_i \implies p_f = p_1 + p_2
\implies p_f = (m_1 * v_1) + (m_2 * v_2)</math>.

We know that the objects stick together, hence their mass <math>m_1 + m_2</math>.
Then <math>p_f = (m_1 + m_2) * v_f \implies (m_1 + m_2) * v_f
= (m_1 * v_1) + (m_2 * v_2)</math>.

If we solve for <math>v_f</math>, we get

<math>
\begin{aligned}
v_f &= \frac{(m_1 * v_1) + (m_2 * v_2)}{m_1 + m_2} \\
&= \frac{(0.5 * 5) +(0.7 * 0)}{0.5 + 0.7} \\
&= 2.083 m/s
\end{aligned}
</math>

This is a [[Maximally Inelastic Collision]], as the collided objects stuck
together. <math>K_{tot, f} < K_{tot, i}</math>.

-----

===Question 2===
A <math>10 kg</math> asteroid with a velocity of <math><10, 0, 0> m/s</math>
crashes into the Earth in the Sahara Desert and bounces back into the space with
the same speed and opposite direction.

Nobody is hurt, but some say that they felt the Earth recoil during
this unusual collision. Find the recoil velocity of the earth to see if it was
significant enough for people to have felt it. (Keep in mind that the human
sensory facilities aren't very precise.)

[[File:Collision-example-2-mert.png|500px]]

===Answer 2===
Let <math>m_a = 10 kg</math>, <math>m_e = 6 * 10^{24} kg</math>,
<math>v_{a, i} = <10, 0, 0> m/s</math>, <math>v_{a, f} = <-10, 0, 0> m/s</math>,
and <math>v_{e, i} = <0, 0, 0> m/s</math>, where subscripts <math>a</math>
and <math>e</math> signify the asteroid and the earth respectively.

We're going to compute <math>v_{e, f}</math>

We know that momentum is conserved.

<math>
\begin{aligned}
&p_f = p_i \\
\implies &p_{a, f} + p_{e, f} = p_{a, i} + p_{e, i} \\
\implies &(m_a * v_{a, f}) + (m_e * v_{e, f})
= (m_a * v_{a, i}) + (m_e * v_{e, i})
\end{aligned}
</math>

Solving for <math>v_{e, f}</math>, we get

<math>
\begin{aligned}
v_{e, f}
&= \frac{(m_a * v_{a, i}) + (m_e * v_{e, i}) - (m_a * v_{a, f})}{m_e} \\
&= \frac{(10 * <10, 0, 0>) + (6 * 10^{24} * <0, 0, 0>)
- (10 * <-10, 0, 0>)}{6 * 10^{24}} \\
&= \frac{<100, 0, 0> + <100, 0, 0>}{6 * 10^{24}} \\
&= <3.33 * 10^{-23}, 0, 0> m/s
\end{aligned}
</math>

The recoil velocity of the earth is too small for anyone to feel it.

-----

===Question 3===
Two rocks in space are approaching a third, stationary rock. Rock 1 has a mass
of <math>30 kg</math> and moves with a velocity <math><13, -10, -2> m/s</math>.
Rock 2 has a mass of <math>60 kg</math>, and moves with a velocity
<math><-3, 23, 3> m/s</math>. Rock 3 has a mass of <math>35 kg</math>
and is stationary. The rocks collide at Rock 3's location.

[[File:Collision-example-3a-mert.png|500px]]

After the collision, it is observed that Rock 1 has embedded
into Rock 3, which is now moving with a new velocity of <math><3, 5, 2> m/s</math>,
and that a chunk of mass <math>15 kg</math> has broken off of the combined rock.

Rock 2, in the meantime, has a new velocity of <math><-2, 12, 2> m/s</math>.

[[File:Collision-example-3b-mert.png|500px]]

What is the change in the internal energy <math>\Delta E_{internal}</math> in
the system as a result of this three-way collision?

===Answer 3===
Due to the complexity of this question, we are going to use a computer to
solve this problem. Conceptually, everything remains the same, but instead of
computing things by hand, we're getting help from a computer. Feel free to
solve it by hand if you want to!

# Define the initial masses/velocities of the rocks.
mass_1_i = 30
mass_2_i = 60
mass_3_i = 35

v_1_i = vector(13, -10, -2)
v_2_i = vector(-3, 23, 3)
v_3_i = vector(0, 0, 0)

# Calculate the initial total momentum.
total_p_i = (mass_1_i * v_1_i) + (mass_2_i * v_2_i) + (mass_3_i * v_3_i)

# Calculate the masses/velocities of the rocks after the collision.
mass_chunk = 15

mass_2_f = 60
mass_3_f = mass_1_i + mass_3_i - mass_chunk

v_2_f = vector(-2, 12, 2)
v_3_f = vector(3, 5, 2)

# Calculate the velocity of the chunk. Remember that the momentum is conserved.
v_chunk = (total_p_i - ((v_2_f * mass_2_f) + (v_3_f * mass_3_f))) / mass_chunk

# Calculate the initial total kinetic energy.
def kinetic_energy(mass, velocity):
return 1/2 * mass * mag(velocity) ** 2

k_tot_i = kinetic_energy(mass_1_i, v_1_i) \
+ kinetic_energy(mass_2_i, v_2_i) \
+ kinetic_energy(mass_3_i, v_3_i)

# Calculate the final total kinetic energy.
k_tot_f = kinetic_energy(mass_2_f, v_2_f) \
+ kinetic_energy(mass_3_f, v_3_f) \
+ kinetic_energy(mass_chunk, v_chunk)

# Calculate the change in internal energy.
d_e_internal = k_tot_f - k_tot_i

This Python snippet, when evaluated, will show that
<math>\Delta E_{internal} = 5175 J</math>.

==Connectedness==
As we mentioned in the introduction to this article, collisions are everywhere
in our daily lives. Some examples are a bat colliding with a baseball in your
neighbor's backyard, two cars crashing in the highway in front of you, and so
on.

Depending on the nature of the collision we can usually guess whether it's
an [[Elastic Collisions]] or an [[Inelastic Collisions]], and reason about it.
(If it's a car crash, things crumble, so there's increase in thermal energy, etc.)

Another very interesting application of collisions is in the field of astronomy.
The universe is ever-changing and we never know what might come our way.
A large meteor could be flying at the Earth with a very high speed and we could
use our knowledge of collisions to find out how the Earth will move after the
collision, what the temperature change of the area would be, and how the rest
of the Earth would be effected by the collision. By using our collision skills,
we could evacuate anyone who is prone to harm during the collision.

Since collisions are ubiquitous processes, knowledge about them is instrumental
for any engineer who deals with the physical world.

==History==
We've mentioned that collisions are not special case events in Physics--
the fundamental principles apply fully for them, and we reason about them
using these principles. Then it won't surprise us that the history of collisions
is tangled with the history of the fundamental principles.

In 1661, Christiaan Huygens, a Dutch physicist, concluded that when a moving
body struck a stationary object of equal mass, the initial moving body would
lose all of its momentum while the second object would pick up the same amount
of velocity that the moving body had before the collision. He also stated that
the total "quantity of motion" should be the same before and after the
collision. This was the first thought of the conservation of momentum.

In 1687, Sir Isaac Newton went a little further and stated in his third law of
motion that the forces of action and reaction between two bodies are opposite
and equal. Because of this law; we know that the total momentum of a closed system is constant.
Every force in the system has a reciprocal pair with an opposing direction, which
implies that their momenta cancel out, keeping the total momentum constant.

In Ernest Rutherford's famous Gold Foil Experiment of 1899, he found out that the plum
pudding model of the atom was inaccurate, and that there was a concentrated
core at the center of the atom (nucleus) and a lot of empty space around.
He didn't have access to the sophisticated tools/knowledge that we have now to
assist in his discovery: he did this through investigating atomic collisions
between alpha particles and gold foil. He shot alpha particles to a piece of
thin gold foil, and observed that some of the particles were scattered
through angles larger than 90 degrees during the collisions. Using collision
principles, he was then able to reason that the atom had most of its mass
concentrated in a dense core, rather than spread out evenly throughout the atom
as the plum pudding model suggested.

==See also==
[[Maximally Inelastic Collision]], [[Inelastic Collisions]],
[[Elastic Collisions]], [[Momentum Principle]], [[Energy Principle]],
[[Kinetic Energy]], [[Net Force]].

==Further reading==
Chabay, Ruth W., and Bruce A. Sherwood. Matter and Interactions. 4th ed. Vol. 1.
Hoboken, NJ: Wiley, 2015. Print. Matter and Interactions.

Ehrhardt, H., and L. A. Morgan. Electron Collisions with Molecules, Clusters,
and Surfaces. New York: Plenum, 1994. Print.

==References==
Chabay, Ruth W., and Bruce A. Sherwood. Matter and Interactions. 4th ed. Vol. 1.
Hoboken, NJ: Wiley, 2015. Print. Matter and Interactions.

"The Gold Foil Experiment." The Gold Foil Experiment. N.p., n.d. Web. 17 Apr.
2016.