WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:07.257 Unit S P 9: Forces and their effects. 00:00:09.933 --> 00:00:13.121 Objects interact by exerting forces on each other. 00:00:13.271 --> 00:00:17.569 A force is a vector with magnitude and direction, measured in newtons. 00:00:20.233 --> 00:00:23.984 Contact forces require physical contact between objects. 00:00:24.134 --> 00:00:29.807 Examples include normal contact force, friction, tension, drag and upthrust from a fluid. 00:00:32.467 --> 00:00:38.407 The normal contact force is the support force acting at right angles (perpendicular) to a surface. 00:00:38.557 --> 00:00:44.027 It acts upwards on a horizontal table and at right angles to the slope on a sloping surface. 00:00:44.177 --> 00:00:48.816 The perpendicular arrow rises leftwards because the slope rises rightwards. 00:00:53.467 --> 00:00:58.501 Friction opposes relative sliding or the tendency to slide between surfaces. 00:00:58.651 --> 00:01:02.751 It can also provide useful grip, such as between shoes and the ground. 00:01:05.433 --> 00:01:09.755 Tension acts along a taut string or cable, pulling the attached object. 00:01:09.905 --> 00:01:12.741 A string does not push an object along its length. 00:01:15.400 --> 00:01:21.021 Air and water resistance are drag forces associated with relative motion through a fluid. 00:01:21.171 --> 00:01:26.121 Upthrust is an upward force from a fluid and does not require an object to be moving. 00:01:28.800 --> 00:01:35.326 Newton’s third law: two interacting objects exert equal, opposite forces of the same type on each other. 00:01:35.476 --> 00:01:37.503 These act on different objects. 00:01:40.167 --> 00:01:45.010 Earth attracts the Moon and the Moon attracts Earth with equal gravitational forces. 00:01:45.160 --> 00:01:48.339 Their different masses mean their accelerations are different. 00:01:51.000 --> 00:01:53.623 Non-contact forces act at a distance. 00:01:53.773 --> 00:01:59.327 Gravitational, electrostatic and magnetic interactions are represented using fields. 00:02:02.000 --> 00:02:06.502 A field is a region in which an appropriate object experiences a force. 00:02:06.652 --> 00:02:09.807 It is not a material string connecting the objects. 00:02:12.467 --> 00:02:16.447 All masses have gravitational fields and attract other masses. 00:02:16.597 --> 00:02:21.090 Weight is the gravitational force on an object in a gravitational field. 00:02:23.767 --> 00:02:26.655 Charged objects have electrostatic fields. 00:02:26.805 --> 00:02:33.663 Like charges repel and unlike charges attract: two electrons repel; an electron and a proton attract. 00:02:33.813 --> 00:02:36.757 Charge signs determine attraction or repulsion. 00:02:41.433 --> 00:02:49.085 Magnetic fields affect other magnets and magnetic materials such as iron, nickel and cobalt; steel can also be magnetic. 00:02:49.235 --> 00:02:51.368 Not all metals are magnetic. 00:02:54.033 --> 00:02:57.672 Like magnetic poles repel; unlike poles attract. 00:02:57.822 --> 00:03:03.997 An initially unmagnetised magnetic material can be attracted as magnetism is induced in it. 00:03:06.667 --> 00:03:10.133 A field can exist even when no test object is present. 00:03:10.283 --> 00:03:14.395 A force occurs when a suitable second object is placed in that field. 00:03:17.067 --> 00:03:21.509 A free-body diagram shows all the forces acting on one chosen object. 00:03:21.659 --> 00:03:24.846 Use labelled arrows pointing in the correct directions. 00:03:24.996 --> 00:03:28.526 Their lengths can show the sizes (magnitudes) of the forces. 00:03:31.200 --> 00:03:36.287 For a book resting on a horizontal table, draw weight downwards and normal force upwards. 00:03:36.437 --> 00:03:39.303 If no other vertical force acts, these balance. 00:03:41.967 --> 00:03:48.045 Weight and support on that book are not a third-law pair: both act on the book and are different interaction types. 00:03:48.195 --> 00:03:50.935 The partner forces act on Earth and the table. 00:03:53.600 --> 00:04:02.666 For a car travelling at constant velocity on level ground, driving force balances drag or friction horizontally and support balances weight vertically. 00:04:02.816 --> 00:04:05.342 Only forces acting on the car are shown. 00:04:10.000 --> 00:04:12.816 Zero resultant means no acceleration. 00:04:12.966 --> 00:04:19.354 This can describe rest or straight-line motion at constant velocity; it does not imply there are no forces. 00:04:22.033 --> 00:04:25.601 An unbalanced resultant can change speed or direction. 00:04:25.751 --> 00:04:31.489 Even if perpendicular forces have equal sizes, they do not cancel because they are not opposite. 00:04:34.167 --> 00:04:39.903 An object hanging from two angled strings has two tension forces and its weight acting on it. 00:04:40.053 --> 00:04:43.889 Each tension has a horizontal and vertical part (component). 00:04:44.039 --> 00:04:48.701 The horizontal parts can cancel while the vertical parts together balance the weight. 00:04:51.367 --> 00:04:55.520 Scalars such as mass, time, distance and speed have magnitude only. 00:04:55.670 --> 00:05:01.043 Force, weight, displacement and velocity have magnitude and direction and are vectors. 00:05:03.700 --> 00:05:12.179 For forces in one straight line, add those in the same direction and subtract those in opposite directions, stating the resultant's direction. 00:05:14.833 --> 00:05:22.789 For angled forces, choose a scale such as 1 centimetre equals 2 newtons, draw accurate lengths and use a protractor for directions. 00:05:25.467 --> 00:05:30.693 In the head-to-tail method, draw the second vector from the head of the first without rotating it. 00:05:30.843 --> 00:05:33.861 The resultant joins the first tail to the final head. 00:05:34.011 --> 00:05:38.387 Preserve each vector’s magnitude and direction when moving it head to tail. 00:05:43.067 --> 00:05:48.368 In the parallelogram method, draw both forces from one point and complete the parallelogram. 00:05:48.518 --> 00:05:51.377 Its diagonal from that point is the resultant. 00:05:51.527 --> 00:05:54.449 Both construction methods give the same resultant. 00:05:59.100 --> 00:06:05.616 Measure the resultant's length and direction on the drawing and convert length back to force using the chosen scale. 00:06:05.766 --> 00:06:08.878 Drawing thickness and protractor reading limit precision. 00:06:11.533 --> 00:06:16.802 The equilibrant is a force the same size as the resultant but in the opposite direction. 00:06:16.952 --> 00:06:20.647 Adding it balances the forces, giving a zero resultant. 00:06:23.300 --> 00:06:27.933 To resolve a force means to split it into two parts (components) at right angles. 00:06:28.083 --> 00:06:33.613 On a scale drawing, make the force the diagonal of a rectangle: its sides show the components. 00:06:33.763 --> 00:06:40.205 Together they have the same effect as the original force, but their lengths do not simply add to the diagonal’s length. 00:06:40.355 --> 00:06:44.644 The components form the sides; the original force is the diagonal. 00:06:49.300 --> 00:06:56.369 A 3 newtons horizontal force plus 4 newtons vertical force has a 5 newtons diagonal resultant. 00:06:56.519 --> 00:07:01.895 A scale drawing also gives its direction, roughly 53 degrees above the horizontal. 00:07:04.567 --> 00:07:08.286 In equilibrium, forces form a closed head-to-tail polygon. 00:07:08.436 --> 00:07:14.046 Check both horizontal and vertical balance; cancelling one direction alone is insufficient. 00:07:16.700 --> 00:07:19.431 A force can rotate an object about a pivot. 00:07:19.581 --> 00:07:26.917 Its moment equals force times perpendicular distance from the pivot to the force’s line of action, measured in newton metres. 00:07:27.067 --> 00:07:31.285 The lever’s total length is not automatically the perpendicular distance. 00:07:31.435 --> 00:07:35.533 Distances are measured perpendicular to the force lines from the pivot. 00:07:40.200 --> 00:07:48.370 For an object with no overall turning effect, total clockwise moment equals total anticlockwise moment: rotational equilibrium. 00:07:48.520 --> 00:07:54.227 To stay completely at rest (static equilibrium), its resultant force must also be zero. 00:07:54.377 --> 00:07:58.023 Balanced moments alone do not stop it moving in a straight line. 00:08:00.700 --> 00:08:07.877 A longer perpendicular lever arm gives a larger moment for the same force, so a long spanner can make a nut easier to turn. 00:08:08.027 --> 00:08:11.891 If the line of action passes through the pivot, the moment is zero. 00:08:14.567 --> 00:08:20.369 A lever can give a larger output force by placing that force nearer the pivot than the input force. 00:08:20.519 --> 00:08:25.137 The input side then moves farther; the lever does not create extra energy. 00:08:27.800 --> 00:08:30.667 Meshing gears transmit rotational effects. 00:08:30.817 --> 00:08:40.821 Two directly meshing gears turn in opposite directions; a small gear driving a larger one generally gives slower rotation and a larger torque, ignoring losses. 00:08:43.500 --> 00:08:47.874 Ignoring losses, a gear with twice as many teeth turns at half the speed. 00:08:48.024 --> 00:08:53.815 For example, a 20-tooth gear driving a 40-tooth gear makes the larger gear turn at half the speed. 00:08:53.965 --> 00:08:58.423 This is an inverse relationship between rotation speed and number of teeth. 00:08:58.573 --> 00:09:01.547 Real gears also have friction and other losses. 00:09:04.200 --> 00:09:06.483 That completes Forces and their effects. 00:09:06.633 --> 00:09:09.978 Revisit the notes and test yourself on the revision website.