# Momentum and Collisions, and Cars

## Momentum and Collisions

• Momentum, kgm/s = mass, kg x velocity m/s     p = m x v
• Momentum is a property of moving objects
• the greater the mass of an object, the greater its velocity, the more momentum the object has
• momentum is a vector quantity, it has size and direction
• Momentum before = momentum after
• in a closed system (no external forces act), the total momentum before an event is the same after as well = Conservation of Momentum
•  Forces cause changes in momentum:
• when a force acts on an object, it causes a change in momentum
• a bigger force = faster change of momentum and greater accelertation
• if someone's momentum changed very quickly, the forces on the body are big and more likelty to cause injury.
• --- cars are designed with safety features that slow people down over a longer time when they have a crash
• the longer it takes for a change in momentum, the smaller the force.
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## Car Design and Safety

• brakes work against the Kinetic Energy of a car
• applying the brakes, means work is done
• the brakes reduce kinetic energy of the car by transferring it into heat energy (and sound)
• new regenerative braking systems make use of energy instead of converting in all to heat whilst braking.
• Regenerative brakes use the system that drives the vehicle doing the majority of braking
• rather than convertin gthe kinetic energy into heat energy, the brakes put the vehicle's motor into reverse. as the motor runs back, the wheels are slowed down.
• the motor acts as an electric generator, converting kinetic energy into electrical energy that is stored as chemical energy in the vehicles battery. = adv of regenerative brakes, they store energy rather than wasting it.
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## Car Design and Safety

• cars are designed to convert Kinetic energy safely in a crash
• is a car crashes it slows down very quickly meaning a lot of kinetic energy is converted into other forms of energy in a short amount of time = dangerous for people inside
• in a crash, there is a big change in momentum over a short time, the people inside feel huge foces
• cars are desgined to convert kinetic energy of the car and its passengers in a way safest for the people. They increase the time over which momentum changes happen, this lessens the forces on passengers.
• Crumple zones: at front and back of car crumple up on impact, the cars kinetic energy is converted into other forms of energy by the car body which changes shape. Crumple zones increase the impact time decreasing the force produced by change in momentum.
• Side impact bars: strong, metal tubes in door panels - help direct the kinetic energy of the crash away from people to other areas of the car
• Seat belts: stretch, increase time taken for wearer to stop. - reduced forces acting in the chest. some kinetic energy of wearer is absorbed by seat belt stretching.
• Air bags: slow you down more gradually and prevent you from hitting hard surfaces inside.
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## Car Design and Safety

• cars have different power ratings
• the sixze and design of car engines determine how powerful they are
• the more powerful the engine, the more energy it transfers from its fuel every second, the faster its top speed can be
• eg... the power output of a typical small car would be smaller than a sports car
• cars are designed to be aerodynamic, shaped in a way air flows very easily and smoothly past it, minimising the air resistance
• cars reach top speed when the resistive force = the driving force provided by the engine. with less air resistance to overcome, the car reaches higher speeds. aerodynamic cars have higher top speeds.
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