High School Physics

Applications of Impulse to improve safety – physics notes

Improving safety and reducing injuries is a critical application of impulse. In many cases, an object must be brought to a complete stop from a given initial velocity. This indicates that there is a specific change in momentum. If the time taken for momentum to change is increased, the force that must be applied is reduced, and thus the damage is reduced. This is the idea behind truck arrestor beds, airbags, and bending your knees when you jump from a chair and land on the ground.

We can use the formula of impulse to understand this easily. The impulse formula is as follows; F Δt = Δp where F is the force, and Δt is the time duration when the momentum change happens by Δp. So, you can see for a given Δp (change in momentum), if Δt is increased, force F decreases.

Here we will discuss a few applications of impulse where the goal is to improve safety.

Air-Bags in Motor Vehicles

Airbags are used in automobiles because they can reduce the force experienced by a person during an accident. Airbags lengthen the time required to stop the driver and passenger’s momentum.

During a collision, the motion of the driver and passenger propels them towards the windshield, requiring a large force to be applied in a short period of time to stop their momentum.

If the driver and passenger collide with an airbag instead of the windshield, the time of impact is increased. The force decreases as the duration of the impact increases.

Padding as Protection During Sports

The same principle explains why wicket keepers in cricket wear padded gloves and why gymnastics use padded mats. When the wicket keeper catches the ball in cricket, the padding is slightly compressible, thus reducing the effect of the force on the wicket keeper’s hands. Similarly, if a gymnast falls, the padding compresses and reduces the effect of the force on the gymnast’s body.

Arrestor Beds for Trucks

An arrestor bed is a patch of ground that is softer than the road. Trucks use these when they have to make an emergency stop. When a truck reaches an arrestor bed and makes an emergency stop then the time interval over which the momentum is changed is increased. This decreases the force and causes the truck to slow down.

Follow-Through in Sports

In sports where rackets and bats are used, like tennis, cricket, squash, badminton, and baseball, the hitter is often encouraged to follow through when striking the ball.

It is seen that following through increases the time over which the collision between the racket/bat and ball occurs. This increase in the time of the collision causes an increase in the velocity change of the ball, without applying additional force.

This means that a hitter can cause the ball to leave the racket/bat faster by following through, without applying additional force. In these sports, returning the ball with a higher velocity often increases the chances of success.

Crumple Zones in Cars

Another safety application of trying to reduce the force experienced is in crumple zones in cars.

When two cars have a collision, two things can happen:
(1) the cars bounce off each other, or (2) the cars crumple together.

The situation is more dangerous for the occupants of the cars when cars bounce off each other.

When cars bounce off each other, or rebound, there is a larger change in momentum and therefore a larger impulse. A larger impulse in a very short span of time means that a greater force is experienced by the occupants of the cars.

When cars crumple together, there is a smaller change in momentum and therefore a smaller impulse. The smaller impulse means that the occupants of the cars experience a smaller force.

Car manufacturers use this idea and design crumple zones into cars, such that the car has a greater chance of crumpling than rebounding in a collision. Also, when the car crumples, the change in the car’s momentum happens over a longer time. Both these effects result in a smaller force on the occupants of the car, thereby increasing their chances of survival.

Related Post: You can refer to a related post as well to know more about force, momentum & car safety and the post is here: force, momentum, and car safety.