WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.372 Unit S P 8: Energy , forces doing work. 00:00:10.033 --> 00:00:15.548 When a force moves an object through a distance in its direction, it transfers energy mechanically. 00:00:15.698 --> 00:00:19.900 This energy transfer is called work done and is measured in joules. 00:00:22.567 --> 00:00:30.758 For a constant force acting along the movement, work done equals force times distance moved in the direction of the force, E equals F times D. 00:00:30.908 --> 00:00:36.165 Use force in newtons and distance in metres; 1 joule equals 1 newton metres. 00:00:36.315 --> 00:00:39.726 Both force and displacement are along the same direction. 00:00:44.400 --> 00:00:50.319 Pushing a crate with 20 newtons through 3 metres in the force direction transfers 60 joules. 00:00:50.469 --> 00:00:54.029 Do not multiply by distance in an unrelated direction. 00:00:56.700 --> 00:01:05.487 If an object does not move, no mechanical work is done on it by the supporting force, even though a person's muscles may still transfer energy internally. 00:01:08.167 --> 00:01:12.013 A force at right angles to an object’s movement does no work on it. 00:01:12.163 --> 00:01:19.336 For example, the inward force on an object moving in a circle at constant speed changes its direction but not its speed. 00:01:22.000 --> 00:01:27.727 Work done against friction transfers energy to thermal stores of the surfaces and surroundings. 00:01:27.877 --> 00:01:31.034 This can be useful in brakes but unwanted in machinery. 00:01:33.700 --> 00:01:38.414 Measure force using a newton meter and displacement using a ruler or tape. 00:01:38.564 --> 00:01:43.779 A constant-force experiment needs the force direction aligned with the measured movement. 00:01:46.433 --> 00:01:54.409 Energy can be stored kinetically, thermally, chemically, gravitationally, elastically, magnetically, electrostatically or nuclearly. 00:01:54.559 --> 00:02:00.049 Transfer pathways include mechanical work, electrical transfer, heating and radiation. 00:02:02.700 --> 00:02:06.595 Energy is conserved: it cannot be created or destroyed. 00:02:06.745 --> 00:02:12.962 In a closed system, transfers move energy between stores or places, but the total stays the same. 00:02:15.633 --> 00:02:23.151 A motor lifting a load transfers energy electrically to the motor and mechanically to the load’s gravitational potential store. 00:02:23.301 --> 00:02:27.803 Some energy spreads to thermal stores in the surroundings (is dissipated). 00:02:27.953 --> 00:02:33.169 Useful gravitational energy plus unwanted transfers equals input energy. 00:02:37.833 --> 00:02:44.413 When a falling object speeds up, its gravitational potential store decreases while its kinetic store increases. 00:02:44.563 --> 00:02:50.259 Drag adds thermal transfers, so the kinetic gain may be smaller than the gravitational decrease. 00:02:52.933 --> 00:03:03.022 Dissipation spreads energy into less useful thermal stores; sound can carry energy away. “Wasted” describes usefulness for the task, not failure of conservation. 00:03:05.700 --> 00:03:11.105 Energy-flow diagrams identify input, useful output and unwanted transfers. 00:03:11.255 --> 00:03:15.417 For quantitative diagrams, the total output must equal input. 00:03:18.100 --> 00:03:23.549 Change in gravitational potential energy equals M times G times delta H. 00:03:23.699 --> 00:03:32.838 Use mass in kilograms, gravitational field strength in newtons per kilogram and vertical height change in metres to calculate energy in joules. 00:03:35.500 --> 00:03:42.187 A 5 kilograms mass raised 2 metres where G equals 10 newtons per kilogram gains 100 joules. 00:03:42.337 --> 00:03:46.890 The change depends on vertical rise, not the length of an inclined path. 00:03:49.567 --> 00:03:56.605 Kinetic energy equals one half times M times V squared, using mass in kilograms and speed in metres per second. 00:03:56.755 --> 00:04:00.982 Doubling mass doubles kinetic energy; doubling speed quadruples it. 00:04:03.633 --> 00:04:11.845 A 2 kilograms object at 4 metres per second has kinetic energy one half times 2 times 16 equals 16 joules. 00:04:11.995 --> 00:04:17.987 To find speed, rearrange V equals the square root of the quantity two times E divided by M. 00:04:20.667 --> 00:04:28.430 If all lost G P E becomes kinetic energy, M times G times delta H equals one half times M times V squared. 00:04:28.580 --> 00:04:33.763 The mass cancels; ignoring drag, the speed gained from a height depends on G and height. 00:04:33.913 --> 00:04:36.781 Conservation connects the two energy stores. 00:04:41.433 --> 00:04:45.057 To stop an object, its kinetic energy must be transferred. 00:04:45.207 --> 00:04:54.498 For a constant braking-force magnitude, F times D equals one half times M times V squared; larger speed increases required stopping distance strongly. 00:04:57.167 --> 00:05:07.203 For a load raised at constant speed, lifting force equals its weight and work done equals weight times vertical distance equals M times G times delta H. 00:05:07.353 --> 00:05:11.313 Acceleration or other losses may change input requirements. 00:05:13.967 --> 00:05:19.150 Power is the rate of energy transfer or work done: P equals E divided by T. 00:05:19.300 --> 00:05:23.525 Power is measured in watts; 1 watt equals 1 joule per second. 00:05:26.200 --> 00:05:31.361 A machine doing 600 joules of work in 3 seconds has power 200 watts. 00:05:31.511 --> 00:05:36.299 Two machines can do equal work but have different power if they take different times. 00:05:38.967 --> 00:05:43.464 Rearrange to E equals P times T and T equals E divided by P. 00:05:43.614 --> 00:05:48.190 Convert minutes to seconds and kilowatts to watts before using S I units. 00:05:50.867 --> 00:06:01.273 To investigate power while climbing stairs, measure mass, vertical height gained and time; calculate useful power equals M times G times delta H, divided by T. 00:06:01.423 --> 00:06:04.556 Use safe walking or running conditions and repeat. 00:06:04.706 --> 00:06:07.879 Use vertical height, not the sloping staircase length. 00:06:12.533 --> 00:06:17.695 To investigate lifting power, time raising a known weight through a measured vertical height. 00:06:17.845 --> 00:06:21.404 Control the load and height when comparing machines or people. 00:06:24.067 --> 00:06:31.264 Measured useful lifting power may be smaller than total input power because energy is also transferred to surroundings. 00:06:31.414 --> 00:06:35.209 State whether a calculation uses input or useful output. 00:06:37.867 --> 00:06:42.626 Efficiency equals useful energy output divided by total energy input. 00:06:42.776 --> 00:06:49.377 The same ratio applies to useful power divided by input power when comparing the same operating interval. 00:06:52.033 --> 00:06:58.282 Efficiency is a fraction from 0 to 1 or a percentage from 0 percent to 100 percent. 00:06:58.432 --> 00:07:04.639 A 60 percent efficient motor gives 60 joules useful output for every 100 joules input. 00:07:04.789 --> 00:07:09.270 Both examples have efficiency 60 percent; energy totals balance. 00:07:13.933 --> 00:07:17.546 Useful output equals efficiency fraction times input. 00:07:17.696 --> 00:07:26.208 Total input equals useful output divided by efficiency fraction; use 0.6 rather than 60 in these equations for 60 percent. 00:07:28.867 --> 00:07:31.586 Lubrication can reduce frictional heating. 00:07:31.736 --> 00:07:38.007 Insulation reduces unwanted heating of the surroundings; the improvement depends on the intended task. 00:07:40.667 --> 00:07:45.179 For a kettle, heating the water is useful, while heating the room is unwanted. 00:07:45.329 --> 00:07:48.970 For a lamp, light is useful while most heating is unwanted. 00:07:49.120 --> 00:07:52.232 Identify the task before classifying transfers. 00:07:54.900 --> 00:07:57.683 That completes Energy — forces doing work. 00:07:57.833 --> 00:08:01.179 Revisit the notes and test yourself on the revision website.