2.3 physics

• Work done


  When a force acts on an object causing it to move, energy is transferred i.e. energy is converted from one form to another (usually chemical energy to kinetic or potential energy)
• The total amount of energy remains constant during energy transfer (as it must do according to the law of conservation of energy)
• Gravitational potential energy (GPE)


  An elevated object has more energy and can lose or gain energy as it changes elevation. GPE depends on the height and the mass of the object.
• Kinetic energy (KE)


  Objects can have kinetic energy when moving with a velocity. The faster it is moving, the greater its kinetic energy. KE also depends on the mass of the object.
• Elastic potential energy


  Some objects may store elastic potential energy as a result of deformation. A prime example is when an elastic band, a catapult or a spring is stretched.
• Hooke's Law


  The change in length or extension of a spring depends on the force applied and a value known as the spring constant (k). This spring constant is specific to each spring and is a measure of its stiffness.
• Force-extension graph


  • Shows the spring constant as the gradient, so the relative stiffnesses of different springs can be assessed. A steeper gradient means a stiffer spring, so it extends less when a force is applied.
• The force-extension graph assumes the springs are undergoing elastic deformation, so will return to their original length when the force is removed. Therefore, all lines should pass through the origin.
• If too great a force is applied, the spring will pass its elastic limit and the gradient will no longer be a straight line. The spring no longer obeys Hooke's Law at this stage and can now be said to be plastic, undergoing plastic deformation if further force is added.
• Energy efficiency


  Vehicles are very inefficient. In order for a vehicle to move and hence do useful work, it must firstly convert the chemical energy in fuel to mechanical energy, which drives wheel axles. Petrol engines typically have a <38% conversion rate. As a result >62% of the chemical energy in the fuel is lost to non-useful energy forms, such as heat.
• Frictional forces between the tyres and the road surface and the wheels and axles further reduce efficiency, since they convert some of the mechanical energy generated by the engine into heat. Mechanical friction within the engine, as fuel and air are pumped in and out, also partially explains the energy conversion rate of petrol engines.
• Drag forces must also be overcome in order to do useful work. As a result, more energy is lost to non-useful forms. This results in a further reduction in vehicle efficiency.
• Improving energy efficiency


  The amount of energy lost in non-useful forms can be reduced by making the vehicle more energy efficient. This can be done by improving engine efficiency and/or reducing the effect of frictional and drag forces.
• Engines


  • Car engines come in different sizes, providing different amounts of power. A 3 litre engine provides more power than a 1.2 litre engine. More powerful engines tend to run faster, allowing the car to travel faster. However, these engines require more fuel to run and are often less energy efficient.
• Larger engines use up more fuel, are more expensive to run and are worse for the environment.
• Aerodynamics


  Vehicles can be made to be streamlined, so that air travels more smoothly over or around them, reducing air resistance and drag forces. This means less power is required from the engine to overcome air resistance. Therefore less fuel is used. As a result the vehicle is more efficient.
• Friction


  Friction increases energy loss through production of non-useful energy such as heat. This reduces the efficiency of the vehicle as well as potentially causing dangerous overheating.
• Rolling resistance


  A measure of the force necessary to move the tyre forward, which is directly proportional to the weight of the load supported by the vehicle's tyres. It is influenced by the friction between the wheels and the road and that between the wheels and axles.
• New technologies have been developed to reduce rolling resistance, including using newly developed materials in tyres and new tyre tread designs to improve the traction between the tyre and the ground.
• Idling and inertial losses

  Another significant energy loss comes when a vehicle idles at traffic lights or in traffic. New vehicle technologies have been developed to try and reduce these losses by turning the engine off when the vehicle comes to a stop. It then restarts automatically when the accelerator is pressed again.
• The repeated acceleration and deceleration, made necessary by traffic lights, requires more energy than travelling at an approximately constant velocity. Although arguably unavoidable, since traffic lights are perhaps a necessary safety measure, this inertial energy expenditure means that more fuel is burnt making travel less energy efficient.