Have you ever wondered how a seat belt or an airbag can save a person's life during a collision? What do you think is the physics behind it? Can we actually save lives using the principles of physics? The short answer is yes. In the following article, we will go through the safety systems used in vehicles, and other fields that reduce the impact of a force and explain their working. At the end of this article, you will be able to understand why these systems are important and how it uses the laws of physics to save lives.

Safety first meaning

The importance of safety is paramount in our everyday lives. As we study the different forces and energies, we must also identify the potential hazards and safety measures that we need to use while working around them. These safety measures can be followed in all aspects of your life. From turning on a light switch to driving a car. Following these measures does not just promote your safety, but it can also ensure the safety of the people around you. First, let's understand the physics behind safety.

Importance of safety first in physics

Before we understand the working of these safety devices. Let us look at what happens when moving objects collide. Every moving object with a mass has momentum associated with it.

The equation for momentum is

\[p=m\times v\]

or in words

\[\text{momentum}=\text{mass}\times \text{velocity}\]

Momentum is a vector quantity, i.e., it has both magnitude and direction.

Fig. 1. The momentum reduces after the collision due to the drop in velocity, The sum of their momentum will be the same before and after the collision.

Look at the above image, when travelling at the same speed the black block has a much higher momentum than the blue one. This is because the momentum depends on two things, the mass of the moving body and its velocity. This means that, during a collision, there is a rapid change in momentum. This happens due to the rapid decrease in the velocity of the blocks. But when two objects collide, their total momentum will remain the same before and after the collision. This happens due to a principle called conservation of momentum. This applies to all collisions unless there is any other external force acting on the bodies after the collision. What's interesting, is that the equation for force can also be written as the rate of change of momentum, as shown below.

The equation for force is

\[F=ma\]

Acceleration can also be written as

\[a=\dfrac{v-u}{\Delta t}\]

Substitute the equation for acceleration in the equation for force, and we get the following

\[F=m\dfrac{v-u}{\Delta t}\]

Re-arrange the equation

\[F=\dfrac{mv-mu}{\Delta t}=\dfrac{\Delta p}{\Delta t}\]

Force can also be defined as the rate of change in momentum. As you can see, the force is inversely proportional to time. This means that the force during a collision can be reduced by increasing the time over which the collision occurs. The basic principle behind most safety devices is to increase the time over which the force transmits between the two bodies in a collision. This period is called the contact time. Most devices that we will be talking, about use this principle to reduce the risk of injury.

Safety first tools

Modern automobiles have been safer than ever. Earlier vehicles were rigid and difficult to control, making crashes a common sight. Modern vehicles have several systems that reduce the risk of serious injuries to passengers during the event of a collision. Let us look at a few of these devices that depend on the basic principles of physics.

Crumple Zone

Have you noticed how much more modern vehicles deform to a far greater extent as compared to vehicles that were built 50 years ago? No, this is not due to modern vehicles being built poorly. What if we told you that this was done deliberately to ensure the safety of the passengers?

Fig. 2. Crumple zone for rear and front collisions, during a collision these zones absorb the bulk of the impact reducing the force transmitted to the passengers, Car Insurance Calculator online

The crumple zone is located at the exterior of the vehicle and is positioned at the front and rear of a vehicle. It is designed to deform severely during a crash. As the crumple zone deforms, it absorbs a lot of the forces during the collision. Crumpling also increases the time to bring the vehicle to a stop as it continues to deform, thus reducing the magnitude of the impact force on the passengers. The change in momentum takes longer if a body is deformable. If the body was rigid, the change in momentum would be instant and the passengers would have to bear a much higher force. Some devices work using the same principle in the interior of the vehicles.

Seat Belt

Fig. 3. Seatbelts and airbags work together for safety by increasing the time taken for an impact to occur and hence reducing the force on the passengers, University of Central Florida

Seat belts also work similarly. During a collision, a seat belt stretches slightly instead of rigidly holding the passenger. This reduces the force exerted on the body of the passenger by increasing the time over which the passenger's momentum changes.

Airbags

Airbags work with seat belts to protect the passengers in the interior of the vehicle. An airbag inflates in 0.03 seconds during a crash. They reduce the force of impact between the driver/ passengers with the interior of the vehicles. The cushion of the air will deflate as soon as the driver hits the airbag this increases the time of contact between the passengers and the interior of the vehicle thus reducing the extent of the injury.

Now that you understand how they work to save lives during the event of a collision, always remember to wear your seat belts. Many devices work on similar principles, such as deformable bumpers that reduce the impact on a pedestrian, curtain airbags, etc. Another popular safety device that is used in parks and recreational areas is a crash mat.

Crash Mats

Crash mats also work using the same principles of increasing the time over which the momentum changes. Crash mats are used in gymnasiums and indoor rock climbing centres to reduce the impact when a person falls on the ground. They're made of soft sponge-like material to reduce the risk of injuries.

Fig. 4. The Crash mats used in gymnasiums absorb the force and increase the time over which the weight of the person falling is distributed to the ground, Hope-education

When a person falls on a crash mat, the soft cushion absorbs the force and is in contact with the body for a longer time than a rigid surface. The momentum of the person changes over a longer time, exerting a smaller force on the body.

Electrical safety first

Electricity has the potential to be dangerous. Insulation, earthing, fuses, and circuit breakers all help to keep us safe from electrocution. Electricity is a helpful energy source, but it can also be quite hazardous. We can be electrocuted in a variety of ways, including the following:

• Coming in contact with exposed live wires.

• Turning on switches with wet hands.

• Incorrect earthing of electrical devices.

• Insert metal conductors into live sockets.

Let's look at a few electrical safety features used in our daily lives.

Safety plug

The safety plug that you see in every device has three pins on it. The top pin grounds the device. The pin on the right that you see is the live wire. It's also attached to the fuse, which is the white cylindrical component. The blue wire on the left is the neutral wire, which is used to complete the circuit. The body of the plug is of rubber or plastic, as they're good insulators of electricity.

Fig. 5. The yellow wire (ground) is used to protect the device and the user from unexpected shocks, The live wire is attached to the fuse for an added layer of protection and the blue wire completed the circuit, Wikimedia Commons CC-BY-SA 4.0

Fuse

You can see the electric fuse used in the above image of the plug. A fuse protects electrical devices from sudden surges in electricity. If a failure in an appliance causes too much current to flow, the fuse will blow and the circuit will be broken. This safeguards the wires and the appliance. The fuse has a wire that melts easily when the current is too high. The wire heats up until it melts, and the circuit is broken.

Fig. 6. A 10 A fuse will break the circuit when the magnitude of current flowing through it increases beyond 10 A. Fuses in plugs are made in standard ratings, most common are 3 A, 5 A and 13 A, Wikimedia Commons CC-BY-SA 3.0

A fuse with a slightly higher rating than the device must be used to protect it from surges. Let's look at the general rules and regulations mandated by the government for safety.

Safety first: rules and regulations

Legislation is used by the government to regulate company behaviour and prevent people from working in dangerous environments. There are rules in place to protect both consumers and employees.

The employees should be provided with the following by the employer:

• Appropriate training to perform the job safely.

• Safety equipment is needed to conduct operations safely.

• Clean and free-of-cost drinking water.

• Access to clean and safe washrooms/toilets.

• First aid kit and trained personnel to administer first aid.

The employees are expected to:

• Complete the training provided by their employer diligently.

• Use the safety equipment provided in an intended way.

• Report any risks to their manager.

This brings us to the end of the article, let's look at what we've learned so far.