WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.520 Unit C P 7: Energy , forces doing work. 00:00:10.200 --> 00:00:15.715 When a force moves an object through a distance in its direction, it transfers energy mechanically. 00:00:15.865 --> 00:00:20.067 This energy transfer is called work done and is measured in joules. 00:00:22.733 --> 00:00:30.925 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. 00:00:31.075 --> 00:00:36.332 Use force in newtons and distance in metres; 1 joule equals 1 newton metres. 00:00:36.482 --> 00:00:39.893 Both force and displacement are along the same direction. 00:00:44.567 --> 00:00:50.485 Pushing a crate with 20 newtons through 3 metres in the force direction transfers 60 joules. 00:00:50.635 --> 00:00:54.195 Do not multiply by distance in an unrelated direction. 00:00:56.867 --> 00:01:05.654 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. 00:01:08.333 --> 00:01:12.179 A force at right angles to an object’s movement does no work on it. 00:01:12.329 --> 00:01:19.503 For example, the inward force on an object moving in a circle at constant speed changes its direction but not its speed. 00:01:22.167 --> 00:01:27.893 Work done against friction transfers energy to thermal stores of the surfaces and surroundings. 00:01:28.043 --> 00:01:31.201 This can be useful in brakes but unwanted in machinery. 00:01:33.867 --> 00:01:38.581 Measure force using a newton meter and displacement using a ruler or tape. 00:01:38.731 --> 00:01:43.945 A constant-force experiment needs the force direction aligned with the measured movement. 00:01:46.600 --> 00:01:54.575 Energy can be stored kinetically, thermally, chemically, gravitationally, elastically, magnetically, electrostatically or nuclearly. 00:01:54.725 --> 00:02:00.216 Transfer pathways include mechanical work, electrical transfer, heating and radiation. 00:02:02.867 --> 00:02:06.761 Energy is conserved: it cannot be created or destroyed. 00:02:06.911 --> 00:02:13.129 In a closed system, transfers move energy between stores or places, but the total stays the same. 00:02:15.800 --> 00:02:23.317 A motor lifting a load transfers energy electrically to the motor and mechanically to the load’s gravitational potential store. 00:02:23.467 --> 00:02:27.969 Some energy spreads to thermal stores in the surroundings (is dissipated). 00:02:28.119 --> 00:02:33.335 Useful gravitational energy plus unwanted transfers equals input energy. 00:02:38.000 --> 00:02:44.580 When a falling object speeds up, its gravitational potential store decreases while its kinetic store increases. 00:02:44.730 --> 00:02:50.425 Drag adds thermal transfers, so the kinetic gain may be smaller than the gravitational decrease. 00:02:53.100 --> 00:03:03.189 Dissipation spreads energy into less useful thermal stores; sound can carry energy away. “Wasted” describes usefulness for the task, not failure of conservation. 00:03:05.867 --> 00:03:11.272 Energy-flow diagrams identify input, useful output and unwanted transfers. 00:03:11.422 --> 00:03:15.584 For quantitative diagrams, the total output must equal input. 00:03:18.267 --> 00:03:23.716 Change in gravitational potential energy equals M times G times delta H. 00:03:23.866 --> 00:03:33.005 Use mass in kilograms, gravitational field strength in newtons per kilogram and vertical height change in metres to calculate energy in joules. 00:03:35.667 --> 00:03:42.353 A 5 kilograms mass raised 2 metres where G equals 10 newtons per kilogram gains 100 joules. 00:03:42.503 --> 00:03:47.057 The change depends on vertical rise, not the length of an inclined path. 00:03:49.733 --> 00:03:56.771 Kinetic energy equals one half times M times V squared, using mass in kilograms and speed in metres per second. 00:03:56.921 --> 00:04:01.149 Doubling mass doubles kinetic energy; doubling speed quadruples it. 00:04:03.800 --> 00:04:12.011 A 2 kilograms object at 4 metres per second has kinetic energy one half times 2 times 16 equals 16 joules. 00:04:12.161 --> 00:04:18.154 To find speed, rearrange V equals the square root of the quantity two times E divided by M. 00:04:20.833 --> 00:04:28.597 If all lost G P E becomes kinetic energy, M times G times delta H equals one half times M times V squared. 00:04:28.747 --> 00:04:33.930 The mass cancels; ignoring drag, the speed gained from a height depends on G and height. 00:04:34.080 --> 00:04:36.947 Conservation connects the two energy stores. 00:04:41.600 --> 00:04:45.224 To stop an object, its kinetic energy must be transferred. 00:04:45.374 --> 00:04:54.665 For a constant braking-force magnitude, F times D equals one half times M times V squared; larger speed increases required stopping distance strongly. 00:04:57.333 --> 00:05:07.369 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. 00:05:07.519 --> 00:05:11.479 Acceleration or other losses may change input requirements. 00:05:14.133 --> 00:05:19.317 Power is the rate of energy transfer or work done: P equals E divided by T. 00:05:19.467 --> 00:05:23.691 Power is measured in watts; 1 watt equals 1 joule per second. 00:05:26.367 --> 00:05:31.527 A machine doing 600 joules of work in 3 seconds has power 200 watts. 00:05:31.677 --> 00:05:36.465 Two machines can do equal work but have different power if they take different times. 00:05:39.133 --> 00:05:43.631 Rearrange to E equals P times T and T equals E divided by P. 00:05:43.781 --> 00:05:48.357 Convert minutes to seconds and kilowatts to watts before using S I units. 00:05:51.033 --> 00:06:01.439 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. 00:06:01.589 --> 00:06:04.723 Use safe walking or running conditions and repeat. 00:06:04.873 --> 00:06:08.046 Use vertical height, not the sloping staircase length. 00:06:12.700 --> 00:06:17.862 To investigate lifting power, time raising a known weight through a measured vertical height. 00:06:18.012 --> 00:06:21.571 Control the load and height when comparing machines or people. 00:06:24.233 --> 00:06:31.431 Measured useful lifting power may be smaller than total input power because energy is also transferred to surroundings. 00:06:31.581 --> 00:06:35.375 State whether a calculation uses input or useful output. 00:06:38.033 --> 00:06:42.793 Efficiency equals useful energy output divided by total energy input. 00:06:42.943 --> 00:06:49.544 The same ratio applies to useful power divided by input power when comparing the same operating interval. 00:06:52.200 --> 00:06:58.449 Efficiency is a fraction from 0 to 1 or a percentage from 0 percent to 100 percent. 00:06:58.599 --> 00:07:04.806 A 60 percent efficient motor gives 60 joules useful output for every 100 joules input. 00:07:04.956 --> 00:07:09.437 Both examples have efficiency 60 percent; energy totals balance. 00:07:14.100 --> 00:07:17.713 Useful output equals efficiency fraction times input. 00:07:17.863 --> 00:07:26.375 Total input equals useful output divided by efficiency fraction; use 0.6 rather than 60 in these equations for 60 percent. 00:07:29.033 --> 00:07:31.753 Lubrication can reduce frictional heating. 00:07:31.903 --> 00:07:38.174 Insulation reduces unwanted heating of the surroundings; the improvement depends on the intended task. 00:07:40.833 --> 00:07:45.345 For a kettle, heating the water is useful, while heating the room is unwanted. 00:07:45.495 --> 00:07:49.137 For a lamp, light is useful while most heating is unwanted. 00:07:49.287 --> 00:07:52.399 Identify the task before classifying transfers. 00:07:55.067 --> 00:07:57.850 That completes Energy — forces doing work. 00:07:58.000 --> 00:08:01.345 Revisit the notes and test yourself on the revision website.