The Top Ten Car Rules of Physics

Physics Rules of The Road

Everything about cars involves several aspects of physics. 

1.First we must talk about some safety guidelines. When a person is driving along in a car, they must wear a seat belt! This is because when a person is in a car, the car and the person are moving forward together. When the car comes to a stop the person will not stop and continue to move. We know this because of Newton's First Law which states that an object in motion or an object at rest will remain doing so unless acted upon by an outside force. In other words, objects like to be lazy and continue in the motion they are already in (inertia). So when the car stops and the person keeps moving, they very easily fly forward. However, seat belts prevent this from happening. Seat belts are able to stop the person when the car stops. This also goes for unfortunate accidents such as leaving your cup of Joe on the roof of your car. When this sugared and sweetened cup is at rest on top of the car and the car takes off, the coffee will fall. Because Newton's first law tells us that an object in motion 
or an object at rest will remain doing so unless acted upon by an outside force, we know that the while the car starts to move, the coffee will remain motionless, causing an lack of caffeine to start your morning. 

2. So how do airbags keep you from getting hurt?
Lets first establish that Momentum is defined as the product if the mass of an object and its velocity.
P= momentum
m= mass
v= velocity
P=mv
The change in momentum is formatted as Δp= pfinal-pinitial 
-A moving object can have a large momentum if either its mass or its velocity are large. 
-In order to have momentum you must be in motion.
Impulse is a force exerted for a certain amount of time. 
Therefor, Impulse is force x time interval.
And impulse is then defined as Ft.
Impulse changes momentum- the greater the impulse exerted on something, the greater will be the change in momentum. 
Impulse= change in momentum
Ft= Δ(mv)
J= FΔt

3. The law of conservation of energy is the law that states  "The total amount of energy in a system remains constant (is conserved), although energy within the system can be changed from one form to another or transferred from one object to another, energy cannot be created or destroyed. It can only be transformed." Before the car starts to move, there is potential energy, once it does, it becomes kinetic energy (energy of movement).
It also means that work in= work out.
Work= force x distance
Machines are able to decrease the amount of force needed for work to happen. (pulleys,ramps)
P=mv
ΔP= pfinal-pinitial
ΔP is the same regardless if you stop quickly or slowly. 
You will go from moving to not moving not moving no matter how you stop. Therefor, the change in momentum is the same no matter how you stop. J= ΔP 
J= FΔt 
J remains the same.
J is the same whether or not there is an impulse.
The airbag increases the time of the impulse and increase the distance between your body and the steering wheel. Since work stays the same, the force must compensate and decrease. Without an airbag, work is equal to a large force and a small distance. 
With an airbag, work is equal to a small force and a large distance. Less force = less injury!


4. Newton's second law states that acceleration is directly proportional to force and inversely proportional to mass.
 a=F/m 
 Acceleration is the change in velocity over a certain time interval
(a= change in velocity/ time interval)
(a=Δ v/t)
Constant acceleration is when you gain the same amount of speed (m/s^2) per second. 
You can find how fast a car is going at a constant acceleration by using this formula: v=at
And you can find the distance of a car moving at a constant acceleration by this formula: d=1/2at^2

If a train runs into a parked car, they both will experience the same force. Neither one exerts a larger force on the other. However, they will experience different accelerations. The one with the smaller mass (parked car), will experience a greater acceleration than the train. This is because mass and acceleration are inversely proportional to acceleration. 


5. So let's explore this concept. When a car hits another car they will experience the same amount of force. We know this from Newton's third law. 
Newton's Third Law states that every action has and equal and opposite reaction. Action reaction pairs are two equal and opposite forces of which include equal sizes and opposite directions. They are represented through vectors. 

For example, if you are holding an apple, the action reaction pair would be: 
-hand pushes apple up; apple pushes hand down. 
-Horse pulls buggy left;Buggy pulls horse right.
-Car hits truck; truck hits car.

A action reaction pair would not be:
-A girl pushing a present up; the earth pulling the present down.
Although they are equal and opposite, they are not a pair. A action reaction pair requires two components, where as this has more than two. 
So another rule of the road is to watch for buggy's! You know, those people on holiday that want nothing less than to be pulled in a carriage by some horses and tour your city! You do not want to crash into them because that would be just bad! 
It looks a bit like this-

 So how does this happen? How is the horse able to pull the buggy if they exert the same amount of force?

 It is important to note that the strength of the pull does not matter. The horse will pull the buggy with the same force the buggy pulls the horse with. We know this from Newton's third law that states that every action has and equal and opposite reaction. The reason the horse is able to pull the buggy is because the horse is pushing the ground harder than the buggy does. The horse and the buggy will move in the direction of the horse. 

Here are the action reaction pairs:

-Horse pulls buggy right; buggy pulls horse left

-Buggy pushes ground forward; the ground pushes the buggy backward

-Horse pushes ground left; ground pushes horse right

6. Lets talk about race cars!

I have to establish the difference between speed and velocity. 

-Velocity requires direction while speed does not require direction.

So this is a simple concept but an important one! On a racetrack you can have a constant speed but not a constant velocity. This is because the race track is circular and constantly changing direction. However, you can still go just as fast on the curves if you wanted to.  

7. Tangential speed is the speed of an object moving along a circular path (radial distance x rotational spin) or (v~rw)

Rotational speed (or angular speed) involves the amount of rotations per unit of time. 

On the race track we see several cars. However take a look at the innermost orange one and the outermost green one. If these cars were to stay side by side the whole race, they will go around the race track the same amount of times, so they have the same rotational speed. However, the outermost green race car will have to have a greater tangential velocity.This is because he located further from the axis of rotation and will have to cover more distance than the car on the inside of the track in the same amount of time. Thus, he will have to go faster and have a higher tangential velocity. 

The size of your wheels can also affect the wheel's rotational velocity. Big wheels= large rotational velocity

small wheels= small rotational velocity.

The bigger the wheels the bigger are, the more mass it has and then the more RI it has. 
The smaller the wheels are, the less mass there is and less RI. 
Tangential velocity is the linear speed of an object moving along a circular path. 
You want a large tangential velocity, which requires wheels moving at a fast pace.

8. Force of Friction

So, acceleration occurs anytime a net force (total force) is on an object. 

Equilibrium occurs when an object is at rest or moving at a constant velocity.  In other words, anytime the net force (total force added together) is at zero. 

Force is a push or pull (measured in Newtons(N). 

Anytime the net force is opposite of the v direction, the object will slow down. 

So when you press the breaks of your car, the Fnet is opposite the v direction and it will bring your car to a stop. 

9. So how does the car stay on the ground and not fly up into the stars and moon? GRAVITY! The gravitational force is proportional to mass (F~m) and inversely proportional to distance (F~1/a). If you're driving on a mountain top, you will experience more gravity because gravitational force is measured from the middle of the planet. 

The Universal Gravitational Law is F=Gm1m2/d². This problem essentially demonstrates the relationship between force and distance².

This way the car will stay on the earth. 

10. Lat but not least, tiny cars! 

Here is a picture of my pal, Henry, and I with our tiny mouse trap car, Thomas! Thomas was a speed demon and demonstrated many aspects of physics. We originally designed him in a fashionable triangle shape with two records and a CD. But after watching some tragic mousetrap car failures, we decided this was a bad idea. After reviewing Newton's second law, we knew that the mass had to be smaller if we really wanted Thomas to accelerate and go fast! We also experimented with other laws, and equations, and all sorts of scientific things. Now, if you want to put your knowledge of cars and physics to the test, make a mouse trap car!