Collisions (AQA A Level Physics): Revision Note

Elastic & inelastic collisions

• 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

• 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:

$E_k=\frac{1}{2}m{v}^2$

• Where:

  • $E_k$ = kinetic energy (J)

  • $m$ = mass (kg)

  • $v$ = velocity (m s^–1)

Momentum conservation issues

• 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

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