5.7 Health and safety issues related to collisions involving road vehicle users

(motor bikes, cars, lorries, buses, cyclists etc.)

Introduction

Large decelerations (rapid slowing down) of objects (cars crashing or people falling and hitting the ground) involve substantial forces and can obviously cause injury.

Why? Large decelerations require a large resistive force. Recall Newton's 2nd law of motion equation ...

F = ma, to create a large acceleration a, you need a relatively big force F, irrespective of the mass m,

also, the bigger the mass m, the larger the force F needed for a given deceleration.

In principle, the force experienced by an object can be reduced by reducing the deceleration ('slower' slowing down).

Recall: acceleration = change in speed / time taken, a = ∆v/∆t, increase ∆t to decrease a

In terms of momentum, you are trying to change the momentum over as longer time as possible, to minimise the force involved.

In the next section we apply these ideas to design safety features that increase collision times - the time from the initial impact of an object with an obstacle to the object coming to a halt (∆t in terms of above equations) i.e. to reduce the rate of deceleration.

You need to know about things like air bags and safety belts in cars, crumple zones at the front and rear of cars, safety helmets for cycling.

Its all about absorbing and transferring kinetic energy safely if a collision impact occurs.

Applying the physics of forces to safety design

In a collision between a road vehicle and a stationary object the normal contact forces between them will cause work to be done.

The collision will cause energy to be transferred from the vehicles kinetic energy store to several other energy stores.

The thermal energy (impact friction) and elastic potential energy ('crunching' effect) stores of the two objects will increase and some of the kinetic energy will end up as sound.

When everything has 'settled down' after the crash, theoretically, all the kinetic energy store of the moving vehicle eventually ends up increasing the thermal energy store of the environment.

You can build safety features into the design of road vehicles, and, where appropriate, safety clothing.

What you are trying to in most cases is to slow down the deceleration - increasing the collision time or absorbing the kinetic energy of any rapid deceleration and in doing so minimise the force a person's body experiences. A rapid impact produces an extreme deceleration - much more so than even emergency braking.

Its all about minimising injury to people in a rapid change of motion situation.

In terms of physics, its all about absorbing impact energy and increasing the deceleration time - minimising the a in F = ma!

From Newton's 2nd law of motion: F = ma, so for a given mass m, if you can make a the deceleration smaller, the decelerating force F is also reduced and minimises body impact and injury.

Wearing a seat belt reduces the force of impact of the deceleration.

On collision or in emergency braking, the seat belt stretches a little, increasing your deceleration time and decreasing the force your body experience against the seat belt. The rate of change of momentum is reduced (F = ∆mv/∆t)

Fast acting air bags, cushion your body from a violent impact, they also increase deceleration times and reduce the force your body experiences. Again, the rate of change of momentum is reduced (F = ∆mv/∆t)

The air bags rapidly expand and are then compressed as a car occupant crashes into it.

The compression takes a longer time than if you crashed into the dashboard of a crushed car, or even if you were compressed excessively confined in your seat belt.

A car body can have crumple zones, built into the design of the car body, in both the front and back to absorb the kinetic energy of any high impact. This increases the deceleration time, thereby decreasing the force your body experiences.

The photographs of a (faked) moderately violent crash of the car into a brick wall gives you an idea of what a 'crumple zone' is all about.

You would see similar damage to the rear of your car (2nd crumple zone) if someone runs into the back of you.

Cycle helmets and crash helmets

Helmets worn by cyclists or motor bike riders (motorcyclists) have an inner lining of foam (or other energy absorbing material) to cushion the head on impact.

The foam increases the time before your head stops moving due to the impact.

The smaller deceleration over a greater period of time, decreases the force of impact your head experiences.

BIKE HELMETS Everything is designed with safety (and comfort) in mind. The main safety features of the motor cycle crash helmet are the hard protective outer shell and the 'soft' impact energy absorbing liner. The comfort fitting padding 'foam' will absorb kinetic energy on impact. Image from the CALIFORNIA MOTORCYCLIST SAFETY PROGRAM and supported by the California Highway Patrol The scheme advises motorcycle riders with helmets that do not match all the safety design features illustrated, should change helmet!

When out walking, came across a motorcycle couple who kindly allowed me to take photographs.

Both survived a serious accident, but once the crash helmet has been in an impact situation, it must be replaced. You can clearly see all the features described in the diagram above.

So, teenage motorcyclists, buy the safest helmet, it might cost more, but without the best helmet, it might cost you even more.

Research is always going on to develop new materials to increase the performance of safety features, whether it be car bodies or helmets.

The same ideas apply to safety in play areas for children and safety in sports like gymnastics

Playground equipment is sited on safety mats that absorb the force of impact when a child falls on them.

They be made of rubber or foam materials.

The idea of this 'cushioned' play area flooring is to increase the impact time by using material that compresses under impact, which cannot happen with a hard surface.

If gymnasts need to make a landing from a piece of apparatus they need to land on a cushioned surface to lessen the impact force the legs experience and avoid injury.

The safety mats are particularly needed when learning new routines where errors and accidents are more likely to happen.

In both competition and practice, the use of mats is now mandatory on most events and gymnasts may use an additional landing mat, without deduction as long as they land within a specified distance.

Even footballers wear humble shin pads to protect their legs from hard tackles!

The thick layer of material absorbs the energy of impact from the 'tacklers' leg or boot, increasing the impact time and reducing the impact force.

There is now growing concern about the impact affects on the brain of repeated heading a football.

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INDEX of physics notes on reaction times, stopping distances of road vehicles, Newton's 2nd Law, braking friction force, KE calculations