Forces doing work and their effects

• Energy is transferred whenever things happen
• Electricity transfers energy to an electric light bulb, which then transfers the energy to the surroundings by light and heating
• Energy can also be transferred when a force makes something move
• Work done


  The energy transferred by a force
• Calculating work done


  Work done = force x distance moved in the direction of the force
• Work done = F x d
• The force must be in the direction of the movement
• Power


  The rate at which energy is transferred
• Power is also the rate of doing work
• Power is measured in watts (W). 1 watt means 1 joule of work done per second
• If two ospreys lift the same sized fish 2 metres in the air, they will both have done the same amount of work. But if one lifts its fish in a shorter time, that bird has produced a greater power
• Calculating power


  Power (W) = work done (J) / time taken (s)
• Power = E/t
• Some adverts still quote the power of vehicles in horsepower. This term was first used in the 1700s to compare the power of steam engines with the power of a horse. 1 horsepower = 746 watts
• An electric motor can transfer energy to a system by heating. This involves the word friction.
• Objects can interact (affect each other) by exerting forces on each other
• If the objects are touching, then the forces between them are contact forces
• When you stand on the floor, there is an upwards force from the floor on you called the normal contact force
• The narrowboat in photo A needs an engine to keep it moving because water resistance (a form of friction) slows it down
• The force from the engine and water resistance are both contact forces
• The narrowboat is floating because of another contact force, called upthrust, from the water
• The upthrust is balanced by a non-contact force called gravity. Gravity does not need to touch the boat to give it weight
• The upthrust and the weight are both acting on the same object
• Gravity is a force that occurs between any two objects that have mass
• The Moon stays in orbit around the Earth because the two bodies are attracting each other
• The force from the Moon on the Earth is the same size as the force from the Earth on the Moon, but in the opposite direction
• The gravitational forces between two objects with mass can be represented as vectors (arrows that show both direction and magnitude)
• These two forces are action-reaction forces (pairs of forces acting on different objects, in opposite directions)
• The mass of the Moon is 7.3 x 10^22 kg, the mass of the Earth is 6.0 x 10^24 kg
• Force is a vector quantity because it has direction as well as a size (magnitude)
• Mass is a scalar quantity because it has a magnitude but no direction
• The space around an object where it can affect other objects is called a force field
• The Moon and the Earth affect each other because the Moon is within the Earth's gravitational field and vice versa
• Other forces that affect objects they are not in direct contact with are magnetism and static electricity
• A magnet can attract objects made from magnetic materials including iron, nickel and cobalt
• A magnet can attract or repel another magnet
• The space around a magnet where it can affect other materials is called the magnetic field
• An object charged with static electricity has an electric field (electrostatic field) around it
• The electric field can affect objects within it
• Two objects with the same charge that are close to each other produce a pair of forces that are equal in size and acting in opposite directions