Safety features of a Car and the

Physics behind it

Loo Li YangJonathan KooBao Jianda

Seat Belts

• PURPOSE: The seatbelt holds the passenger in place, preventing the passenger from flying forward as the car stops abruptly in the case of a collision.

A Real Life Scenario…

• When a car stops suddenly due to a collision with another object, the car's acceleration decreases very quickly in a short period of time. This is called deceleration. Newton's Law of Inertia explains how this happens.

Law of Inertia

• “An object in motion continues in motion with the same speed and direction unless acted upon by an unbalanced force.”

So what happens?

• As the car collides with another object, the other object provides the force which changes the speed and direction. The speed decelerates quickly due to the impact.

• When all this happens the passenger is not being acted upon by a force to slow them down.

• As the person continues in their same direction and speed ( forward and the same speed that the car was going) the seatbelt catches them, holding them back from flying through the air.

What if there is no seat belt?

• Even if one is not wearing a seatbelt, a force will still have to act on the person in order to slow them down. This force will come from the dashboard or windshield as the person crashes into it, causing a lot of damage to themselves.

Breakdown of seat belt

• A seatbelt has two parts. The first part rests over the passengers pelvis and the second part rests over the shoulder and across the chest. When the car stops abruptly the seatbelt applies the stopping force across a large section of the body so the damage is reduced.

Specifications

• Seatbelts are made of flexible materials which have more give then a dashboard or windshield would have.

• Seatbelts are designed so that the machinery behind them tightens up the belt to hold the passenger in place when the car decelerates quickly

Airbag

• The purpose of an airbag is to help the passenger in the car reduce their speed in  collision without getting injured.

• An airbag provides a force over time. This is known as impulse. The more time the force has to act on the passenger to slow them down, the less damage caused to the passenger.

How it works

• About 15 to 20 milliseconds after the collision occurs the crash sensors decide whether or not the collision is serious enough to inflate the airbag (usually 6 - 10 km/h).

• If the crash sensors decide to inflate the airbag it will be deflated at about 25 milliseconds after the crash.

• It takes about 20 milliseconds to inflate the airbag for the person to land into.

• Around 60 milliseconds the person has made contact with the airbag and the airbag now starts to deflate.

• The passenger continues to be acted on by the airbag as it is in the deflation process which takes about 35 to 40 milliseconds.

Crumple Zones

• The front and back ends of the car have now been made into crumple zones.

• The idea is to increase the amount of time that the force during the collision is acting on the car.

How it helps

• The reason that the time must increase is so the force can act over a longer period of time reducing the force when it gets the centre of the car where the driver and passengers are.

• Crumple zones help prevent some damage and injuries that could occur otherwise.

Tires

• The tread on a tire provides traction with the road surface.

• Without tread, a car could spin out of control, skid on roads, roll over and even hydroplane.

The physics of tires - FRICTION

• Friction is the force between two objects as they move over one another such as a car's tire and the surface which it is travelling on.

• During rainy or snowy days, the rain and snow reduces the coefficient of kinetic friction, thus increasing the chance of spinning out of control.

Headrests

• Headrests are found in every type of car to prevent neck injuries that occur during an automobile collision. The way the neck becomes injured is due to physics.

Real life situation• Imagine a person that is standing still getting

pushed forward by another person. As the person pushing goes towards the second person they are going at a certain speed while the second person is not moving.

• As the person pushes the second person they seem to remain in almost the same position, taking only a step forward but their head goes backward and then forward. They may complain about their neck being sore for a while.

Brakes

• Brakes use physics to perform their job in a car and are very important in a collision because in order to stop in time the brakes must be in good working order.

• There are a few aspects of physics that brakes deal with.

• One is force which is shown through a lever in the pedal. As your foot pushes down on the pedal providing a force the lever multiples this force.

Hydraulic Brakes

• Brakes also use hydraulics which involve physics because of the forces involved. A hydraulic system is similar to a lever system when physics is involved. In the hydraulic system the force is multiplied as is done in the lever system.

Multiplying forces

• In order for forces to be multiplied the sizes of the pistons used must be different. If you apply a force to a smaller diameter piston and it connects to a larger diameter piston the force will be multiplied by the difference in area between the two pistons