WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.223 Unit C P 8: Forces and their effects. 00:00:09.900 --> 00:00:13.087 Objects interact by exerting forces on each other. 00:00:13.237 --> 00:00:17.536 A force is a vector with magnitude and direction, measured in newtons. 00:00:20.200 --> 00:00:23.951 Contact forces require physical contact between objects. 00:00:24.101 --> 00:00:29.773 Examples include normal contact force, friction, tension, drag and upthrust from a fluid. 00:00:32.433 --> 00:00:38.373 The normal contact force is the support force acting at right angles (perpendicular) to a surface. 00:00:38.523 --> 00:00:43.994 It acts upwards on a horizontal table and at right angles to the slope on a sloping surface. 00:00:44.144 --> 00:00:48.783 The perpendicular arrow rises leftwards because the slope rises rightwards. 00:00:53.433 --> 00:00:58.467 Friction opposes relative sliding or the tendency to slide between surfaces. 00:00:58.617 --> 00:01:02.718 It can also provide useful grip, such as between shoes and the ground. 00:01:05.400 --> 00:01:09.721 Tension acts along a taut string or cable, pulling the attached object. 00:01:09.871 --> 00:01:12.707 A string does not push an object along its length. 00:01:15.367 --> 00:01:20.988 Air and water resistance are drag forces associated with relative motion through a fluid. 00:01:21.138 --> 00:01:26.088 Upthrust is an upward force from a fluid and does not require an object to be moving. 00:01:28.767 --> 00:01:35.293 Newton’s third law: two interacting objects exert equal, opposite forces of the same type on each other. 00:01:35.443 --> 00:01:37.469 These act on different objects. 00:01:40.133 --> 00:01:44.977 Earth attracts the Moon and the Moon attracts Earth with equal gravitational forces. 00:01:45.127 --> 00:01:48.305 Their different masses mean their accelerations are different. 00:01:50.967 --> 00:01:53.589 Non-contact forces act at a distance. 00:01:53.739 --> 00:01:59.294 Gravitational, electrostatic and magnetic interactions are represented using fields. 00:02:01.967 --> 00:02:06.469 A field is a region in which an appropriate object experiences a force. 00:02:06.619 --> 00:02:09.773 It is not a material string connecting the objects. 00:02:12.433 --> 00:02:16.413 All masses have gravitational fields and attract other masses. 00:02:16.563 --> 00:02:21.057 Weight is the gravitational force on an object in a gravitational field. 00:02:23.733 --> 00:02:26.622 Charged objects have electrostatic fields. 00:02:26.772 --> 00:02:33.629 Like charges repel and unlike charges attract: two electrons repel; an electron and a proton attract. 00:02:33.779 --> 00:02:36.723 Charge signs determine attraction or repulsion. 00:02:41.400 --> 00:02:49.051 Magnetic fields affect other magnets and magnetic materials such as iron, nickel and cobalt; steel can also be magnetic. 00:02:49.201 --> 00:02:51.335 Not all metals are magnetic. 00:02:54.000 --> 00:02:57.639 Like magnetic poles repel; unlike poles attract. 00:02:57.789 --> 00:03:03.963 An initially unmagnetised magnetic material can be attracted as magnetism is induced in it. 00:03:06.633 --> 00:03:10.099 A field can exist even when no test object is present. 00:03:10.249 --> 00:03:14.361 A force occurs when a suitable second object is placed in that field. 00:03:17.033 --> 00:03:21.475 A free-body diagram shows all the forces acting on one chosen object. 00:03:21.625 --> 00:03:24.813 Use labelled arrows pointing in the correct directions. 00:03:24.963 --> 00:03:28.493 Their lengths can show the sizes (magnitudes) of the forces. 00:03:31.167 --> 00:03:36.253 For a book resting on a horizontal table, draw weight downwards and normal force upwards. 00:03:36.403 --> 00:03:39.269 If no other vertical force acts, these balance. 00:03:41.933 --> 00:03:48.011 Weight and support on that book are not a third-law pair: both act on the book and are different interaction types. 00:03:48.161 --> 00:03:50.902 The partner forces act on Earth and the table. 00:03:53.567 --> 00:04:02.633 For a car travelling at constant velocity on level ground, driving force balances drag or friction horizontally and support balances weight vertically. 00:04:02.783 --> 00:04:05.309 Only forces acting on the car are shown. 00:04:09.967 --> 00:04:12.783 Zero resultant means no acceleration. 00:04:12.933 --> 00:04:19.321 This can describe rest or straight-line motion at constant velocity; it does not imply there are no forces. 00:04:22.000 --> 00:04:25.567 An unbalanced resultant can change speed or direction. 00:04:25.717 --> 00:04:31.455 Even if perpendicular forces have equal sizes, they do not cancel because they are not opposite. 00:04:34.133 --> 00:04:39.870 An object hanging from two angled strings has two tension forces and its weight acting on it. 00:04:40.020 --> 00:04:43.856 Each tension has a horizontal and vertical part (component). 00:04:44.006 --> 00:04:48.667 The horizontal parts can cancel while the vertical parts together balance the weight. 00:04:51.333 --> 00:04:55.487 Scalars such as mass, time, distance and speed have magnitude only. 00:04:55.637 --> 00:05:01.009 Force, weight, displacement and velocity have magnitude and direction and are vectors. 00:05:03.667 --> 00:05:12.145 For forces in one straight line, add those in the same direction and subtract those in opposite directions, stating the resultant's direction. 00:05:14.800 --> 00:05:22.755 For angled forces, choose a scale such as 1 centimetre equals 2 newtons, draw accurate lengths and use a protractor for directions. 00:05:25.433 --> 00:05:30.659 In the head-to-tail method, draw the second vector from the head of the first without rotating it. 00:05:30.809 --> 00:05:33.827 The resultant joins the first tail to the final head. 00:05:33.977 --> 00:05:38.354 Preserve each vector’s magnitude and direction when moving it head to tail. 00:05:43.033 --> 00:05:48.335 In the parallelogram method, draw both forces from one point and complete the parallelogram. 00:05:48.485 --> 00:05:51.343 Its diagonal from that point is the resultant. 00:05:51.493 --> 00:05:54.416 Both construction methods give the same resultant. 00:05:59.067 --> 00:06:05.583 Measure the resultant's length and direction on the drawing and convert length back to force using the chosen scale. 00:06:05.733 --> 00:06:08.845 Drawing thickness and protractor reading limit precision. 00:06:11.500 --> 00:06:16.769 The equilibrant is a force the same size as the resultant but in the opposite direction. 00:06:16.919 --> 00:06:20.614 Adding it balances the forces, giving a zero resultant. 00:06:23.267 --> 00:06:27.899 To resolve a force means to split it into two parts (components) at right angles. 00:06:28.049 --> 00:06:33.580 On a scale drawing, make the force the diagonal of a rectangle: its sides show the components. 00:06:33.730 --> 00:06:40.172 Together they have the same effect as the original force, but their lengths do not simply add to the diagonal’s length. 00:06:40.322 --> 00:06:44.611 The components form the sides; the original force is the diagonal. 00:06:49.267 --> 00:06:56.336 A 3 newtons horizontal force plus 4 newtons vertical force has a 5 newtons diagonal resultant. 00:06:56.486 --> 00:07:01.861 A scale drawing also gives its direction, roughly 53 degrees above the horizontal. 00:07:04.533 --> 00:07:08.253 In equilibrium, forces form a closed head-to-tail polygon. 00:07:08.403 --> 00:07:14.013 Check both horizontal and vertical balance; cancelling one direction alone is insufficient. 00:07:16.667 --> 00:07:18.949 That completes Forces and their effects. 00:07:19.099 --> 00:07:22.445 Revisit the notes and test yourself on the revision website.