All drivers are subject to the natural laws and laws of physics that affect any moving, or stationary, object. There are many natural forces acting on your vehicle, such as:
These forces affect how your vehicle handles, regardless of how well your vehicle is designed or how skilled you are at handling it.
Understanding these forces will help you control your vehicle during turns, stops, and everyday driving conditions. Knowing how they affect driving your vehicle may also help you react appropriately to an emergency situation or avoid a collision.
Otherwise, you may lose control of your vehicle and could cause an accident that could have been prevented.
By misjudging natural forces, you can be pulled out of a curve and lose control. You may lose traction on wet pavement and be unable to stop or be traveling too fast to stop quickly in heavy traffic and cause an accident.
Gravity is the force that pulls all objects towards the center of the earth. Gravity also affects your speed of travel when you're going uphill and downhill because of the change in gravitational pull as you move towards, or away from, the center of the earth.
Traction is the result of friction between the road and your tires caused by the weight (gravity effect) of the car pushing the tires to the road due to gravity. Traction is necessary for you to steer your car.
When you are driving uphill, the force of gravity is working against you to slow you down and you may need to accelerate or change to a lower gear to maintain your speed.
When you drive downhill, the reverse is true. Gravity will cause you to go faster and increase your stopping distance. You may need to shift to a lower gear or apply your brakes to slow to a safe speed and control your vehicle.
When you leave a vehicle parked on an incline, gravity works to pull your vehicle downhill.
To keep your vehicle from rolling away, you should leave your vehicle in a low gear or in "park" if it has an automatic transmission. You should always engage your parking brake and you may even need to block your wheels by placing an object in front or behind of the tires.
Just in case your parking brake fails, you should always turn the front wheels of your vehicle so that it will not roll into the traffic lane. The way you should turn your wheels depends on whether you are parked facing uphill or downhill and whether or not there is a curb.
If you are parking facing downhill, you should always turn your front wheels towards the curb or side of the road.
If you are parking facing uphill and there is a curb, you should turn your front wheels towards the middle of the road and allow the vehicle to roll back against the curb. If you are parking facing uphill and there is not a curb, you should turn your wheels toward the side of the road.
Sir Isaac Newton (1643 - 1727) was an English scientist. His discoveries and theories in "nature's laws" lead to Newtonian Physics, and his famous laws, including:
He was also the discoverer of Calculus. Without his laws and discoveries, the automobile might never have been invented!
The Law of Inertia (an object in motion, remains in motion, in a straight line, at a constant speed, until acted upon by an unbalanced force), is what keeps a car going straight down the highway, with minimal steering effort.
The Law of Gravitation (one object attracts another, via a straight line, drawn between the objects' center. The larger the mass the stronger the attraction) is why cars slow down on a hill (moving away from the center of the earth) and why roads are "banked" in curves, to keep the car on the roadway.
Physics is important, because it explains the limits of a vehicle's capabilities, safety, and operation.
Law of Inertia:
An object in motion, will tend to remain in motion, in a straight line and at a constant speed, unless it is acted upon by an outside force.
Objects that are moving will continue moving in a straight line and at a constant speed, while objects that are not moving remain at rest, unless acted upon by some other force. This is called the law of inertia.
For example, vessels in space can move really far without much spent energy because there is very little air acting to slow them down. When you are waiting for a green signal on flat pavement at an intersection, you will not move unless you engage the engine or are otherwise pushed.
While you're driving, momentum keeps your vehicle moving, unless it is acted upon by something, such as your brakes, the road surface, a fixed object (such as a tree), or another vehicle.
Inertia causes your body and loose objects in your car to keep moving forward when your vehicle stops suddenly.
You may be injured because of the inertia and momentum of loose objects in your car that fly through the air during a sudden stop.
When you are hit from behind while stopped, your head tends to stay in place due to inertia while the seat pushes the rest of your body forward. This causes whiplash. Using your head restraint mitigates injuries due to whiplash.
The pictures to the right show common ways that people tend to wear their seat belts. These three are the wrong way, and can cause injuries!
In the left example, the seat belt has been placed behind the shoulder. It now rests on the bottom of the ribs. In a collision, the body would likely bend at the waist lap belt, and all the inertia forces would be focused on these ribs, breaking them or pushing them into the chest cavity where the lungs and other vital organs can be punctured.
In the middle example, the belt is too loose. This will allow the body to move forward, at the speed of the vehicle, and then suddenly being stopped by the belt snapping against the breast bone.
In the right example, the belt is twisted, producing a much smaller contact area withe the breast bone. The forces will be focused on this area and, much like a hammer, exerts a large force on a small area (like the difference between a slap and a punch).
In this picture, the belt is snug, positioned properly, and can diffuse the forces all along its length. While there may be some bruising, the body is forced to stop almost instantaneously, instead of hitting the windshield or dashboard.
Potential energy is the energy that an object possesses because of its position or form. For example, a book on a table has the "potential" energy to fall to the floor, whereas a book that is already on the floor has the potential energy to "fall" towards the center of the earth.
When you are parked on a hill, gravity causes your car to have potential energy. This energy is converted to kinetic energy (motion) if it breaks loose and rolls down the hill. There is also potential energy built up in the components of your car's suspension system that may cause you to swerve when you come out of a turn.
Kinetic energy is the energy a body possesses because it is in motion. For example, the potential energy had by a book on a table is converted to kinetic energy (motion) when it falls. The book lying flat on the floor does not have this same kinetic energy.
As you increase your driving speed, both your body and your vehicle acquire kinetic energy which eventually must be:
Kinetic energy is present in both the vehicle and you. If the vehicle stops suddenly, your kinetic energy will keep you moving forward.
Have you ever played catch with a water balloon? Air bags treat your head as a water balloon, and, by slowly deflating, they "catch" your head, allowing it to move forward with slower speeds, and preventing impact with the steering wheel.
KE = 1/2 MV2 or Kinetic Energy = one-half the Mass times Speed (velocity) squared
The kinetic energy of everything increases with weight (mass) and the square of your speed. This includes:
This means:
Kinetic Energy's Effect on Stopping Distance
The kinetic energy of your moving vehicle determines your ability to stop the car. In addition to the distance traveled due to your reaction time, your stopping distance will be:
Applying the brakes causes friction. Friction produces heat . The friction converts the kinetic energy into static (not-in-motion) energy. The more kinetic energy a vehicle has, the more heat is required to do the conversion.
Disc brakes, as shown in the image, can dissipate the heat quickly (the RED areas), so that more heat can be used to bring the car to a complete stop.
Gravity decreases your kinetic energy when you are driving uphill and increases it when you are driving downhill. Therefore, the force of gravity will make it:
Braking to a stop converts kinetic energy into heat energy in your brakes through friction. If you and your vehicle are involved in a collision, the kinetic energy is also converted into heat through friction.
The force of a moving object is called momentum. The momentum of an object is proportional to its weight and speed. For example, a brick traveling at 10 MPH has more momentum (force) than a chunk of Styrofoam traveling at the same speed. Energy cannot be created nor destroyed; it can only be converted into another form of energy.
When you are driving, both you and your vehicle have acquired momentum which is proportional to the weight of your vehicle and its speed. If you increase your speed from 10 MPH to 20 MPH, you double your car's momentum, and if you increase your speed from 10 MPH to 50 MPH, you increase your car's momentum five times.
When you make a controlled stop, the momentum of your vehicle must be overcome by the:
When you are in a crash, the momentum and kinetic energy of your vehicle and body must be absorbed, which results in heat, the deformation of your vehicle, and possible injury to your body.
Maximum Tread Contact = Maximum Friction
Friction is a force caused by the contact of one surface on another. It results in the resistance of an object moving over a surface. For example, it is easier to move your hand over fine sandpaper than over rough sandpaper because there is less friction caused by the surface of the fine sandpaper.
Friction is:
The friction of your tires can be altered by changes in the road surface. Road surfaces change due to:
This could result in you losing control of your vehicle. Skidding while braking is caused by the friction of your brakes being stronger than the friction force between your tires and the road, which causes you to lose traction.
The kinetic energy of your vehicle cannot be converted into friction in your brakes (heat energy) if your brakes are locked during a skid. Newton's first law of inertia takes over in this case. Since there is more inertia in the moving vehicle, and less friction available during locked-up braking, the vehicle will continue to skid, in a straight line, even if the steering wheels are turned. Modern anti-locking braking systems are designed to prevent brakes from locking up.
Because friction is increased by the weight of your vehicle, a fully-loaded truck has more stopping power than does an empty one because it is heavier. The weight helps to stop the vehicle by producing more friction between the tires and the road surface.
But this is true only if we disregard momentum. Because of the greater momentum of the loaded truck, it will actually have less stopping power.
The friction on your brakes and clutch results in brake and clutch wear. If used improperly, excessive wear to the brake pads and clutch can occur. To avoid this:
Friction can also cause wear, and possible destruction, of other parts of your vehicle, such as bearings and internal engine parts. To lower the friction on these parts, manufacturers use a lubrication system to provide a thin film of oil or grease between the moving parts. This is the same thing that happens when you add oil to a squeaky door hinge.
Centrifugal force is the tendency for objects to be pulled outward when rotating around a center. Physics teachers will often say that centrifugal force isn't actually a force at all. It is actually the inertia of an object going in a straight line, as shown in the image.
When you make a turn, your car is subject to centrifugal force (inertia), which is pulling your car away from the direction you want to turn and into a straight line. In addition, all four wheels are pointed in slightly different directions, causing a "controlled skid" throughout the turn. Traction is necessary to keep from losing control in a turn. Too much speed, or too much braking can cause a loss of the necessary traction.
Have you ever ridden a bicycle or motorcycle into a turn or curve? You must tilt your bike and lean to the inside of the turn in order to accomplish the same thing, because you only have two wheels, thus less friction (traction).
Banked roadways improve your traction in turns. They help in overcoming the centrifugal force that is pulling you away from the direction in which you want to turn.
In order to keep a vehicle in a turn without allowing centrifugal force to pull the car out, you should:
The pictures illustrate what traffic engineers do when designing a banked curve and safe speeds.
In a crash:
The forces that stop your car during a crash will be greatest if you have a head-on collision with another vehicle or large immovable object, such as a bridge, wall, or a tree, because the momentum and kinetic energy of your car must be absorbed almost instantaneously.
If two vehicles are involved in a collision moving at the same rate of speed, the vehicle that weighs less will take the greater impact. The larger and heavier the vehicle, the greater the energy and momentum. The smaller and lighter vehicle will have greater deceleration and may even be pushed in the reverse direction of travel.
You can reduce the forces on you and your car during an unavoidable crash if you are able to redirect your path toward objects that will cause your car to stop over a greater distance, such as:
1. Impact Force is Highest
A head on collision produces the highest impact force. If two identical vehicles are moving at 40 mph and collide, head on, the FOI would be the same as driving into a brick wall at 40 mph (not 80 mph, or adding the speed of both vehicles together, as some might intuitively, but incorrectly, suspect).
2. Impact Force is High
A T-Bone, or side-impact, collision is the next deadliest. Since the vehicles are moving in different directions, some of the FOI is dissipated in forward motion.
3. Impact Force is Medium
A "sideswipe" collision reduces the FOI by spreading it to the sides of the vehicles, as well as opposing directions.
4. Impact Force is Small
A rear-end collision produces a much smaller amount of FOI, since the impact is transferred to the car in front, which then also moves forward.
How fast am I going?
Take your Speed, add 1/2 your Speed. The result is your speed in feet per second.
S + 1/2S = S (feet / second)
How long will it take to stop?
Take your speed, multiply it by 1/10 of your speed and divide the result by two. The result is your stopping distance, after the brakes are applied, in feet.
(S x 1/10S)/2 = SD (feet)
How long will it really take?
Since it takes about one second to recognize a danger and then react to it, add the distance you travel in one second to the stopping distance to find the total distance, in feet, it will take you to stop.
(S + 1/2S) + (S x 1/10 S /2) = Total Stopping Distance (feet)
What is kinetic energy, and what effect does it have on me?
Kinetic energy, or momentum affects all aspects of driving. To stop a moving vehicle, you must dissipate (get rid of) the kinetic energy through the friction of the brakes or by hitting another object (called the force of impact). To steer the car, you must overcome the kinetic energy of the vehicle (it wants to keep going in a straight line) in order to change direction.
KE = 1/2M x V2
Any increase in the mass (weight) of the vehicle will proportionately increase the KE. Doubling the mass will double the KE.
Any increase in the velocity (speed) of the vehicle will increase the KE by the square of the change in speed. Doubling the velocity will quadruple the KE.
By braking and reducing your speed in half, the KE is now only one-fourth (1/4) from what it was at your original speed. The less KE there is, the less damage that occurs to both you and your vehicle in a crash.
What is the coefficient of friction?
This is the amount of traction provided by the "footprint" of the four tires in contact with the road. Four brand new tires, on a perfectly smooth surface, gives a CF of 1.0; on smooth, dry asphalt the CF is 0.9; on wet pavement or melted ice, the CF is only 0.35; frozen asphalt gives a CF of 0.20; and ice has a CF of only 0.05.
You will now answer 5 questions to test what you learned during this lesson. You must answer all questions correctly to receive completion credit for this lesson. You may answer the questions as many times as necessary to get them right.
You should review the lesson material if you don't do well on the quiz.
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California Parent-Teen Training Guide
California Driver Handbook & DMV Publications
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