Unit 3.

The Effect of Natural Forces on your Vehicle

 

A.  Natural forces affecting driving, general

Natural forces, general, introduction

There are 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. The laws of nature and physics are present at all times and must be kept in mind while operating a vehicle. If you try to break them, you will end up losing control of your vehicle and perhaps cause an accident that could have been prevented.

Natural forces, general, accidents caused by misjudging

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.

B.  The Force of Gravity

Gravity, introduction

Gravity is the force that pulls all objects towards the center of the earth. Gravity also affects your speed of travel when going uphill and downhill.

Traction is the result of friction between the road and your tires caused by the weight of the car pushing the tires to the road due to gravity. Traction is necessary for you to steer your car.

Gravity, driving uphill and downhill

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 smoothly apply your breaks to slow to a safe speed and control your vehicle.

Gravity, parking on an incline

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 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 this is not a curb, you should turn your wheels toward the side of the road.

C. The law of Inertia

Inertia, introduction

Objects that are moving tend to continue moving and objects that are not moving tend to 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.

Inertia, effect on driving

While driving, inertia 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 rest of your body is pushed forward by the seat. This causes whiplash. Using your headrest mitigates injuries due to whiplash.

Inertia, counteracting with seatbelts

Wearing a seatbelt while driving will stop the forward momentum of your body if you have a collision. It will prevent you from hitting the windshield, steering wheel, and other parts of the interior of your vehicle.

D.  Potential energy

Potential energy, introduction

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 does not have this potential energy.

Potential energy, effect on driving and parking

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.

E.  Kinetic energy

Kinetic energy, introduction

Kinetic energy is the energy a body possesses because it is in motion. For example, the potential energy of 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 potential or kinetic energy.

Kinetic energy, effect on driving

As you increase your driving speed, both your body and your vehicle acquire kinetic energy which eventually must be:

The kinetic energy of your body while it is in motion, of loose objects in the car, and of the car itself, all increase with weight and the square of your speed so that:

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

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 still converted into heat through friction, but not in your brakes (ouch!).

F.  Momentum

Momentum, introduction

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.

Momentum, effect on driving

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

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.

G.  The force of friction

Friction, introduction

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:

Friction occurs:

Friction, effect on stopping

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 aides in stopping the vehicle by producing more friction between the tires and the road surface.

The kinetic energy of your moving vehicle is converted into heat during braking because of the friction of your brakes against your wheels and the friction of your tires against the road.

Friction, effect of roads and tires

Worn or over-inflated tires will reduce the friction between your tires and the road surface, reducing the traction needed to:

Road surface changes due to:

Above listed road and weather conditions could result in you losing control of your vehicle.

Friction, effects on braking

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) when your brakes are locked as they are in a locked-wheel skid. If the components of your brakes become too hot, they cannot release any more heat and will fail to absorb any more kinetic energy (such as when traveling down a steep hill and using your brakes a lot).

The purpose of anti-lock brakes is to prevent your wheels from locking during a quick stop so that your brakes can continue to absorb energy and you can maintain traction.

The loss of traction in a stop or turn can be avoided by:

 

Friction, effects on vehicle components

The friction that the clutch creates when released can:

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:

Improper lubrication of your engine and transmission will cause excessive friction which will result in mechanical failure. You could be left stranded or lose control of your vehicle as a result. (See Unit 6 for more detail.)

H. Centrifugal force

Centrifugal force, introduction

Centrifugal force is the tendency for objects to be pulled outward when rotating around a center.

Centrifugal force, effect on turns

In a turn, your car is subject to centrifugal force which is pulling your car away from the direction you want to turn and into a straight line. Traction is necessary to keep from losing control in a turn.

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:

Centrifugal force can be partly overcome by leaning toward the inside of the turn when driving a motorcycle or riding a bicycle.

I.  Forces in a crash

Forces in a crash, introduction

In a crash:

Forces in a crash, head-on collisions

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 abutment, 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. In some cases the smaller and lighter vehicle may be crushed as in:

The impact of a train against a vehicle can be compared to the impact of a vehicle on an aluminum can.

Forces in a crash, reducing them

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:

The purpose of barrels filled with sand in front of an abutment (such as a support for a highway over pass) is to reduce the forces on your vehicle by allowing your vehicle to travel farther than if it hit the abutment directly.

Modern vehicles have a number of features that reduce the forces on your body during a crash by absorbing energy and increasing the distance over which the impact occurs.  These include:

If the force of impact is excessive, such as during a head on collision with another vehicle, the above safety features may still not be enough to prevent serious injury or death. Your seatbelt is the best protection against injury if you have in an accident.

 

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