Power

Power

Power is the rate of doing work i.e. how much per seconed
1. Power is not the same thing as force or energy
Power = work done (or energy transferred) / time taken
1. P= E/t
Power is measured in watts (J/s)
1. One watt = 1 joule of energy transferred per seconed
Calculating power output
1. Timed run upstairs
  1. The energy transferred is the potential energy you gain ( = mgh) Power = mgh/t
2. Timed Accerleration
  1. The energy transferred is the kinetic energy you gain (=1/2mv squared) Power = 1/2mv squared/t
Momentum and Collisions
1. Momentum = Mass x Velocity
  1. P = m x v
2. Momentum is a property of moving objects
3. The greater the mass the greater its velocity and the more momentum it has
4. In a closed system the total momentum before an event (e.g. a collision) is the same as after the event. This is called conservation of Momentum
5. Forces cause changes in Momentum
  1. A larger force means a faster change of momentum (greater acceleration)
  2. If someones momentum changes very quickly (car crash) the forces on the body will be very large and more likely to cause ingury
  3. Thats why cars have 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
Car design and safety
1. Brakes do work against the kinetic energy of the car
  1. When you apply the brakes to slow down a car work is done
    1. The brakes reduce the kinetic energy of the car by transferring it to heat and sound energy.
    2. Regenerative brakes use the system that drives the vehicle to do the majority of the braking. Rather than converting the kinetic energy of the vehicle into heat energy the brakes put the vehicles motor into reverse. With the motor running backwards the wheels are slowed. At the same time the motor acts as an electric generator converting kinetic energy into electrical energy that is stored as chemical energy in the vehicles battery. This is the advantage of regenerative brakes they store energy of braking rather than wasting it
2. Cars are designed to convert kinetic energy safely in a crash
  1. If the car crashes it will slow down very quickly
  2. Crumple zones at the front and back of the car crumple up on impact
    1. The cars kinetic energy is converted into other forms of energy by the car body as it changes shape
    2. Side impact bars are strong metal tubes fitted into car door panels. They help direct the kinetic energy of the crash away from the passengers to other areas of the car such as crumple zones
    3. Seat belts stretch slightly increasing the time taken for the wearer to stop. This reduces the forces acting in the chest. Some of the kinetic energy of the wearer is absorbed by the seat belt stretching
    4. Air bags also slow you down more gradually and prevent you from hitting hard surfaces inside the car
3. Cars have different power ratings
  1. The size and design of cars determine how powerful they are
    1. The more powerful an engine the more energy it transfers from its fuel so the faster it will be
  2. Cars are also designed to be aerodynamic this means they are streamlined minimising air resistance
    1. Cars reach their top speed when the resistance force equals the driving force provided by the engine

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	Created by sian.allison almost 12 years ago