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Physics in action: impulse

A very important application of impulse is improving safety and reducing injuries. In many cases, an object needs to be brought to rest from a certain initial velocity. This means there is a certain specified change in momentum. If the time during which the momentum changes can be increased then the force that must be applied will be less and so it will cause less damage. This is the principle behind arrestor beds for trucks, airbags, and bending your knees when you jump off a chair and land on the ground.

Air-bags in motor vehicles

Air bags are used in motor vehicles because they are able to reduce the effect of the force experienced by a person during an accident. Air bags extend the time required to stop the momentum of the driver and passenger. During a collision, the motion of the driver and passenger carries them towards the windshield. If they are stopped by a collision with the windshield, it would result in a large force exerted over a short time in order to bring them to a stop. If instead of hitting the windshield, the driver and passenger hit an air bag, then the time of the impact is increased. Increasing the time of the impact results in a decrease in the force.

Padding as protection during sports

The same principle explains why wicket keepers in cricket use padded gloves or why there are padded mats in gymnastics. In cricket, when the wicket keeper catches the ball, the padding is slightly compressible, thus reducing the effect of the force on the wicket keepers 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 trucks reaches an arrestor bed 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. High speed films of the collisions between bats/rackets and balls have shown 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. This means that a hitter can cause the ball to leave the racket/bat faster by following through. 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.

Which situation is more dangerous for the occupants of the cars? When cars bounce off each other, or rebound, there is a larger change in momentum and therefore a larger impulse. A larger impulse 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.

Egg throw

This activity demonstrates the effect of impulse and how it is used to improve safety. Have two learners hold up a bed sheet or large piece of fabric. Then toss an egg at the sheet. The egg should not break, because the collision between the egg and the bed sheet lasts over an extended period of time since the bed sheet has some give in it. By increasing the time of the collision, the force of the impact is minimized. Take care to aim at the sheet, because if you miss the sheet, you will definitely break the egg and have to clean up the mess!

Exercise

  1. A canon, mass 500 kg, fires a shell, mass 1 kg, horizontally to the right at 500 m · s - 1 . What is the magnitude and direction of the initial recoil velocity of the canon?
  2. A trolley of mass 1 kg is moving with a speed of 3 m · s - 1 . A block of wood, mass 0,5 kg, is dropped vertically into the trolley. Immediately after the collision, the speed of the trolley and block is 2 m · s - 1 . By way of calculation, show whether momentum is conserved in the collision.
  3. A 7200 kg empty railway truck is stationary. A fertilizer firm loads 10800 kg fertilizer into the truck. A second, identical, empty truck is moving at 10 m · s - 1 when it collides with the loaded truck.
    1. If the empty truck stops completely immediately after the collision, use a conservation law to calculate the velocity of the loaded truck immediately after the collision.
    2. Calculate the distance that the loaded truck moves after collision, if a constant frictional force of 24 kN acts on the truck.
  4. A child drops a squash ball of mass 0,05 kg. The ball strikes the ground with a velocity of 4 m · s - 1 and rebounds with a velocity of 3 m · s - 1 . Does the law of conservation of momentum apply to this situation? Explain.
  5. A bullet of mass 50 g travelling horizontally at 600 m · s - 1 strikes a stationary wooden block of mass 2 kg resting on a smooth horizontal surface. The bullet gets stuck in the block.
    1. Name and state the principle which can be applied to find the speed of the block-and-bullet system after the bullet entered the block.
    2. Calculate the speed of the bullet-and-block system immediately after impact.
    3. If the time of impact was 5 x 10 - 4 seconds, calculate the force that the bullet exerts on the block during impact.

Questions & Answers

what is a nuclei
Acacia Reply
it's the core off an atom.
Emmanuel
Define the term electromagnetism
Bokiyana Reply
Definition of ohm's law
Bokiyana
The current flowing through a conductor is directly proportional to the potential difference across the end of the conductor.
Lettie
how do i calculate ohms law
Tiisetso Reply
How can i resolve vectors into components
Nhlalala Reply
Fx=fcos Fy=fsin
Vutomi
how many Atoms Do exist?
Jhon Reply
bullet fired from the gun
Prem Reply
explain why HCL is a Bronstead lowry acid . use an equation of HCL in water to explain
Mihlali Reply
what is electron. ( you should not allowed to tell it properties in it defination.)
vigyan Reply
a charged particle.
Siyabonga
investigative question
Obakeng Reply
how do you create a bomb
Nasty Reply
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
if you want to create bomb use this chemical substance oxidised, sulphuric acid,tobacco,gas and flammable substance , mix all this substance together and put in strong mental bottle like gas bottle put in . you must close the bottle very parked and carry that bottle carefully and shake in 5 minutes
Aaron
after shake put this bottle were the temperature is 100 degrees Celsius after moment that bottle will be high explosion like bomb
Aaron
what is mass
asmerom Reply
is object amount matter contain by object
Aaron
what is hypothesis
Aaron
hypothesis is a tentative explanation that does not have facts and it can be proven to be wrong or right
MPHO
wath is physical science
Siphelo Reply
what is physics
Dineo Reply
the branch of science concerned with the nature and properties of matter and energy
Thabisto
use a vector scale diagram and represent the two displacements(5m east and 7m west)
itumeleng Reply
what os covalent?
Shellah Reply

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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