• In both collisions and explosions, momentum is always conserved
• However,?kinetic energy?might not always be
• A collision (or explosion) is either:
  • Elastic?– if the kinetic energy?is?conserved
  • Inelastic?– if the kinetic energy is?not?conserved

   

• Collisions are when objects striking against each other
  • Elastic?collisions are commonly those where objects colliding do not stick together and then?move in opposite directions
  • Inelastic?collision are commonly those where objects collide and?stick together?after the collision

   

Elastic collisions are where two objects move in opposite directions. Inelastic collisions are where two objects stick together

• An explosion is commonly to do with?recoil
  • For example, a gun recoiling after shooting a bullet or an unstable nucleus emitting an alpha particle and a daughter nucleus

   

• To find out whether a collision is elastic or inelastic,?compare the kinetic energy before and after the collision
• The equation for kinetic energy is:

• The force of an impact in a vehicle collision can be?decreased?by?increasing?the contact time over which the collision occurs
  • The contact time is the time in which the vehicle or the passenger is in contact with what it has collided with

   

• Vehicles have safety features such as?crumple zones,?seat belts?and?airbags?to account for this
  • For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place
  • This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest
  • The increased time reduces the force and risk of injury on a passenger

   

The seat belt, airbag and crumple zones help reduce the risk of injury on a passenger

Designing Safety Features

• Vehicle safety features are designed to absorb energy upon an impact by changing?shape
• Seat belts
  • These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
  • They are designed to stretch slightly to increase the time for the passenger's momentum to reach zero and reduce the force on them in a collision

   

• Airbags
  • These are deployed at the front on the dashboard and steering wheel when a collision occurs
  • They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact

   

• Crumple zones
  • These are designed into the exterior of vehicles
  • They are at the front and back and are designed to crush or crumple in a controlled way in a collision
  • This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
  • The crumple zones increase the time over which the vehicle comes to rest,?lowering the impact force?on the passengers

   

• The effect of the increase in time and force can be shown on a force-time graph
  • For the same change in momentum, which depends on the mass and speed of a vehicle, the increase in contact time will result in a?decrease?in the maximum force exerted on the vehicle and passenger
  • This is demonstrated by a lower peak and wider base on a force-time graph

   

The increase in contact time Δt decreases the force for the same impulse