Collisions (AQA A Level Physics)

• In both collisions and explosions, momentum is always conserved
• However, kinetic energy might not always be
• A collision (or explosion) is:
  • elastic if the kinetic energy is conserved
  • inelastic if the kinetic energy is not conserved

• Collisions are when objects strike against each other
  • Elastic collisions are commonly those where the objects colliding do not stick together and then move in opposite directions
  • Inelastic collisions are commonly those where the 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 passenger is in contact with the object 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, the safety features are designed to absorb the energy from the impact 
  • This increases the time taken for the change in momentum of the passenger to occur
  • The increased time reduces the force exerted on the passenger and therefore reduces the risk of injury

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 contact time over which the passenger's momentum reaches zero and, therefore, reduce the force exerted on them during a collision

• Airbags
  • These are deployed from the dashboard and steering wheel (and in newer cars, from the doors) when a collision occurs
  • They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
  • They increase the contact time over which the passenger changes momentum, thereby reducing the force exerted on them

• 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
  • Crumple zones increase the time over which the vehicle's momentum reaches zero, reducing the force on the passengers

• The reduced force due to the increase in contact time 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