What is Force?*FORCE IS A PUSH OR PULL* FORCE IS MEASURED INNEWTONS (N)
WorkDefinition:Work done by a constant force on an object is given bythe product of the force and the distance moved by theobject in the direction of the force. (TB) F F d Work = Force x distance moved W=Fxd
NO WORK DONE IF1. the body does not move.2. the force is perpendicular to the direction of movement.
Principle of Conservation of Energy• It states that energy cannot be created or destroyed in any process.• It can be converted from one form to another or transferred from one body to another, but the total amount remains constant. (TB)
Worked Example 4A bricklayer lifts 50 bricks each weighing 15 Nthrough a vertical height of 1.2 m in 1 min andplaces them at rest on a wall. Calculate(a) the work done(b) the average power needed.
Energy • Energy is the capacity to do work. • The SI unit for energy is joule (J). • Different forms of EnergyKinetic Potential Heat Sound ElectricalEnergy Energy Energy Energy Energy Nuclear Chemical Gravitational Elastic Energy Energy
Energy Transformation1. Fossil fuel is used to generate electricity in a power station. burn Turbine Generator Chemical Heat Kinetic Electrical energy in fuel energy energy of turbine energy2. Stored water in a dam is used to generate electricity in a hydroelectric power station. Gravitational Kinetic Kinetic Electrical Potential energy of water energy of turbine energy of water energy
Gravitational Potential Energy (GPE)• GPE is the energy possessed by a body due to its height above the ground.• Formula: G.P.E. = mgh (from conversion of WD to GPE)• where m= mass of body (in kg) g = gravitational field strength (in N/kg)• h = vertical height above ground (in m)
Worked Example 1A brick of mass 2 kg is at a height of 3 mabove the ground. What is the gravitationalpotential energy of the brick?[Take g as 10 N/kg ]
Kinetic Energy (KE)• Kinetic energy is the energy possessed by a body due to its motion.• Formula: K.E. = ½ m v2 where m = mass (in kg) v = speed (in m/s)
Rearranging the KE equationWhat is the rearranged version of this equation forcalculating speed? √ v = 2KE m
Worked Example 2A bullet of mass 200 g is travelling at aspeed of 60 m/s. What is the kinetic energyof the bullet?
Dangerous speeding?Use the KE = ½mv2 equation to fill in the kinetic energyvalues in the table below for two cars each travelling attwo different speeds. 1,000 kg 2,000 kg 20 mph KE = 40 kJ KE = 80 kJ 40 mph KE = 160 kJ KE = 320 kJWhat factor – mass or speed – has the greatest effect onthe kinetic energy of a moving object?
Too much kinetic energyDoubling the mass of a moving object doubles its kineticenergy, but doubling the speed quadruples its kineticenergy.If the speed of a car is slightly above the speed limit, itskinetic energy is much greater than it would be at thespeed limit. This means that: It is more difficult to stop the car and there is more chance of an accident.
GPE and KEWhen a body falls from a height, its GPE is changed intoKE during the fall. GPEAs the body gains speed, more and moreof its GPE is changed into KE.Just before the body strikes the ground, all GPE+its GPE is changed into KE. K.E.The total amount of energy at thebeginning and the end is the same. K.E.
Worked Example 3A ball of mass 2 kg is released from rest at the top of abuilding of height 12 m.(a) What is the speed of the ball when it reaches the ground?(b) What happens to its kinetic energy after it has struck the ground?
Worked Example 5Solution(a) By the principle of conservation of energy, K.E. = GPE ½ m v2 = mgh v = 2gh = 2x10x12 = 15.5 m/s(b) The kinetic energy is changed into elastic PE and heat and sound.
Note• Whenever there is friction or impact:• heat (mostly) and sound are produced.