Work Done Work is done (or energy is used up), when a force moves an object. d F The amount of work done depends on the distance the object is moved. The work done (E W ) is given by the formula:

x ÷ EWEW F d QuantityUnit Work Done ( E W ) Force ( F ) Distance ( d ) Joules ( J ) Newtons ( N ) Metres ( m ) Example 1 Calculate the work done when a force of 12 N moves a suitcase a distance of 20 m.

Example 2 A gardener does 12 kJ of work by pushing a wheelbarrow a distance of 150 m. Calculate the force exerted on the wheelbarrow by the gardener.

Yellow Book Work Done – Page Q

Potential Energy When an object is situated above ground level, it is said to have potential energy (E P ), sometimes known as gravitational potential energy. m h ground level

x ÷ The potential energy stored in a mass m which is a height h metres above ground level is given by: EPEP m g h QuantityUnit Potential Energy ( E P ) Mass ( m ) Gravitational Field Strength ( g ) Height ( h ) Joules ( J ) Kilograms ( kg ) Newtons per Kilogram ( N kg -1 ) Metres ( m )

Example 1 A 3 kg box is on a shelf 4 m above the ground. Calculate the potential energy stored in the box.

Example 2 A 2 kg flowerpot sits on a windowsill. The flowerpot has 50 J of energy. Calculate the height of the windowsill above the ground.

Yellow Book Potential Energy – Page 71 Q87 (a) (b) (c), 89, 92, 93, 95, 96

My Potential Energy Here is how to calculate my potential energy as I run up a set of stairs. h m Results My mass, The amount of energy supplied by me is J. The energy used up by me as I run up the stairs is J. Calculate your potential energy using:

My Power Measuring my power when I climb up stairs. I can calculate the power I develop if I know the time it takes to get up the stairs. Calculation The energy supplied by me as I climbed up the stairs was J. (from previous work) stopwatch

Yellow Book Energy and Power – Page 73 Q109, 110, 111, 112

Kinetic Energy All moving objects have kinetic energy (E K ). The formula for kinetic energy is x ÷ EKEK m v2v2 QuantityUnit Kinetic Energy ( E K ) Mass ( m ) Speed ( v ) Joules ( J ) Kilograms ( kg ) Metres per Second ( ms -1 )

Example 1 A car has a mass of 500 kg and a speed of 15 ms -1. Calculate the kinetic energy of the car.

Example 2 An 8 kg object has 36 J of kinetic energy. Calculate the speed of the object.

Yellow Book Kinetic Energy – Page 72 Q99, 102, 103, 104

Potential to Kinetic Potential energy is always turned into kinetic energy (e.g when a high object falls to the ground). We can use the relationship to calculate the speed reached by a falling object. all kinetic all potential all kinetic

Example 1 A 3 kg object is on a window ledge 45 m above the ground. It then falls off the ledge to the ground below. 45 m m = 3 kg (a)Calculate the potential energy at the start.

(b)State how much kinetic energy the object has, just as it reaches the ground. (c)Calculate the speed of the object at the ground.

Q1.A 7 kg flowerpot sits on a window ledge 25 m above the ground. (a)Calculate the potential energy of the flowerpot on the window ledge. (b)The flowerpot falls off the ledge. State the kinetic energy as the flowerpot hits the ground. (c)Calculate the speed it hits the ground at. 1,750 J 22.4 ms -1 Q2.An object of mass 5 kg falls off a shelf. It hits the ground with a speed of 12 ms -1. (a)Calculate the kinetic energy of the object just as it hits the ground. (b)Calculate the height of the shelf. 360 J 7.2 m

Combining Potential and Kinetic Don’t need mass to calculate speed.

Yellow Book Potential and Kinetic Energy – Page 74 Q115, 116, 119, 120