WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.945 Unit S P 2: Motion and forces. 00:00:09.600 --> 00:00:18.036 The resultant force is the overall force on one object after all its forces are combined, taking account of their directions (their vector sum). 00:00:18.186 --> 00:00:21.493 Choose positive and negative directions consistently. 00:00:24.167 --> 00:00:32.017 A 1000 newtons driving force forward and 500 newtons drag backward give a resultant of 500 newtons forward. 00:00:32.167 --> 00:00:36.646 Subtract opposing forces; retain the direction of the larger force. 00:00:41.300 --> 00:00:45.035 Balanced forces give zero resultant and no acceleration. 00:00:45.185 --> 00:00:50.720 A stationary object stays stationary; a moving object continues at constant velocity. 00:00:53.400 --> 00:00:58.902 Newton’s first law describes this behaviour in the absence of a resultant external force. 00:00:59.052 --> 00:01:02.527 Motion does not require a continuing resultant force. 00:01:05.200 --> 00:01:10.412 Unbalanced forces cause acceleration: a change in speed, direction or both. 00:01:10.562 --> 00:01:14.937 A moving object can slow down if the resultant opposes its velocity. 00:01:17.600 --> 00:01:22.763 Circular motion at constant speed requires a resultant force towards the centre. 00:01:22.913 --> 00:01:28.299 This centripetal force changes the direction of velocity, so the object accelerates. 00:01:30.967 --> 00:01:35.681 Mass describes how much matter an object contains and is measured in kilograms. 00:01:35.831 --> 00:01:40.705 It also measures how difficult it is to change the object’s velocity (inertia). 00:01:40.855 --> 00:01:45.177 Weight is the gravitational force on the object, measured in newtons. 00:01:47.833 --> 00:01:53.156 Weight equals mass times gravitational field strength, W equals M times G. 00:01:53.306 --> 00:01:59.087 Near Earth use G equals 10 newtons per kilogram in these examples or the value given. 00:02:01.767 --> 00:02:07.099 A 6 kilograms object weighs 60 newtons where G equals 10 newtons per kilogram. 00:02:07.249 --> 00:02:13.531 Its mass stays 6 kilograms on the Moon, but its weight is smaller because the gravitational field is weaker. 00:02:16.200 --> 00:02:19.016 Measure weight with a calibrated newton meter. 00:02:19.166 --> 00:02:23.523 On a weight, mass graph, the gradient is gravitational field strength. 00:02:26.200 --> 00:02:32.173 Air or water resistance (drag) opposes an object’s movement relative to the fluid around it. 00:02:32.323 --> 00:02:34.776 Drag generally increases with speed. 00:02:34.926 --> 00:02:36.889 Streamlining reduces it. 00:02:39.567 --> 00:02:43.605 A falling object initially accelerates because weight exceeds drag. 00:02:43.755 --> 00:02:47.919 As speed increases, drag increases and the resultant becomes smaller. 00:02:50.600 --> 00:02:55.071 At terminal velocity, drag balances weight and acceleration is zero. 00:02:55.221 --> 00:03:00.192 The object continues falling at constant velocity; the forces have not disappeared. 00:03:00.342 --> 00:03:03.612 At terminal velocity, resultant force is zero. 00:03:08.267 --> 00:03:14.953 Opening a parachute increases drag: the falling person slows until reaching a new, lower terminal velocity. 00:03:17.633 --> 00:03:27.200 Newton’s second law is F equals M times A: resultant force in newtons equals mass in kilograms times acceleration in metres per second squared. 00:03:27.350 --> 00:03:31.717 For the same mass, increasing resultant force increases acceleration. 00:03:34.367 --> 00:03:38.825 For the same resultant force, a larger mass has a smaller acceleration. 00:03:38.975 --> 00:03:41.927 Inertial mass equals F divided by A. 00:03:42.077 --> 00:03:45.979 It measures how strongly an object resists a change in velocity. 00:03:48.633 --> 00:03:56.429 In the trolley core practical, vary pulling force while keeping total moving mass constant, or vary mass while keeping force constant. 00:03:56.579 --> 00:04:00.511 Use light gates or a motion sensor to calculate acceleration. 00:04:03.167 --> 00:04:08.179 Keep track and release conditions consistent, minimise friction and repeat measurements. 00:04:08.329 --> 00:04:13.759 A hanging mass and pulley can provide a pulling force; the hanging mass belongs to the moving system. 00:04:16.433 --> 00:04:23.184 Newton’s third law: when two objects interact, each exerts an equal and opposite force of the same type on the other. 00:04:25.867 --> 00:04:29.687 A foot pushes a ball forward; the ball pushes the foot backward. 00:04:29.837 --> 00:04:34.298 These forces act on different objects and do not balance each other on the ball. 00:04:36.967 --> 00:04:41.005 A book’s weight and the table’s upward normal force can balance on the book. 00:04:41.155 --> 00:04:46.007 They are not a third-law pair: the partner to Earth pulling the book is the book pulling Earth. 00:04:46.157 --> 00:04:49.601 Balanced forces on the book are not a Newton’s third-law pair. 00:04:54.267 --> 00:05:00.643 Momentum equals mass times velocity, P equals M times V, in kilogram metres per second. 00:05:00.793 --> 00:05:03.933 Momentum is a vector, so direction matters. 00:05:06.600 --> 00:05:17.137 Total momentum is conserved in a closed system with no resultant external force: add the momenta of all objects before and after a collision, using signed velocities. 00:05:19.800 --> 00:05:26.573 A 2 kilograms trolley at 3 metres per second hits a stationary 1 kilogram trolley and they stick together. 00:05:26.723 --> 00:05:35.468 Initial momentum is 6 kilogram metres per second; final combined mass is 3 kilograms, so final velocity is 2 metres per second. 00:05:35.618 --> 00:05:40.268 Negligible external resultant force allows momentum conservation. 00:05:44.933 --> 00:05:55.342 Average resultant force equals change in momentum divided by time, F equals the quantity M times V minus M times U, divided by T for constant mass. 00:05:55.492 --> 00:05:57.839 Use the sign of the velocity change. 00:06:00.500 --> 00:06:05.629 For the same change in momentum, increasing stopping time reduces average force. 00:06:05.779 --> 00:06:10.909 Crumple zones, airbags and seat belts extend stopping time and reduce injury risk. 00:06:13.567 --> 00:06:18.535 Momentum conservation does not mean kinetic energy is always conserved. 00:06:18.685 --> 00:06:23.049 In a sticking collision, some kinetic energy is transferred to other stores. 00:06:25.700 --> 00:06:29.453 Stopping distance equals thinking distance plus braking distance. 00:06:29.603 --> 00:06:34.151 Thinking distance is how far a vehicle travels before the driver starts braking. 00:06:34.301 --> 00:06:37.222 A delay before braking adds to the distance travelled. 00:06:41.900 --> 00:06:48.395 Thinking distance equals speed times reaction time, assuming speed stays constant during the reaction interval. 00:06:48.545 --> 00:06:53.323 At 20 metres per second and 0.5 seconds, it is 10 metres. 00:06:56.000 --> 00:07:02.788 Reaction time varies between people and conditions; drugs, alcohol, tiredness and distractions can increase it. 00:07:02.938 --> 00:07:07.188 A simple ruler-drop test can investigate reaction time with repeats. 00:07:09.867 --> 00:07:16.397 Braking distance depends on speed, mass, braking force, tyre condition, brake condition and road surface. 00:07:16.547 --> 00:07:19.565 Wet or icy roads reduce available friction. 00:07:22.233 --> 00:07:27.300 Higher speed increases thinking distance and increases braking distance more strongly. 00:07:27.450 --> 00:07:31.805 With fixed braking force, braking distance is proportional to speed squared. 00:07:34.467 --> 00:07:38.713 Large decelerations require large forces and can cause injury. 00:07:38.863 --> 00:07:44.801 Safety features reduce force by increasing collision time, but do not remove the momentum change. 00:07:47.467 --> 00:07:53.957 Braking transfers the vehicle’s kinetic energy into thermal energy in brakes, tyres, road and surroundings. 00:07:54.107 --> 00:08:02.575 With an approximately constant braking force, work done F times D equals initial kinetic energy one half times M times V squared. 00:08:05.233 --> 00:08:12.859 Rearrange to braking distance d equals mass times initial speed squared divided by the quantity two times braking force. 00:08:13.009 --> 00:08:22.116 If mass and braking force stay unchanged, braking distance is proportional to the square of initial speed: doubling speed gives four times the braking distance. 00:08:22.266 --> 00:08:29.465 The curve follows relative braking distance equals relative initial speed squared; the axes use relative quantities. 00:08:34.133 --> 00:08:37.363 Thinking distance equals speed times reaction time. 00:08:37.513 --> 00:08:41.465 With the same reaction time, doubling speed doubles thinking distance. 00:08:41.615 --> 00:08:44.825 Stopping distance adds thinking and braking distances. 00:08:44.975 --> 00:08:50.168 It does not exactly follow speed squared, because only the braking part does in this model. 00:08:52.833 --> 00:09:04.448 For a 1000 kilograms car travelling at 20 metres per second with 5000 newtons braking force, kinetic energy is 200000 joules and braking distance is 40 metres. 00:09:04.598 --> 00:09:12.660 With reaction time 0.7 seconds, thinking distance is 14 metres and total stopping distance is 54 metres. 00:09:15.333 --> 00:09:21.337 Use a realistic speed range to estimate emergency stopping distances and state assumptions. 00:09:21.487 --> 00:09:29.508 Wet or icy roads can reduce available braking force and lengthen braking distance; vehicle condition and driver response also matter. 00:09:32.167 --> 00:09:37.582 In this model, increasing mass with an unchanged braking force increases braking distance. 00:09:37.732 --> 00:09:45.923 In real vehicles the available force can also change with mass, so distinguish the specified calculation model from a universal rule. 00:09:48.600 --> 00:09:50.797 That completes Motion and forces. 00:09:50.947 --> 00:09:54.293 Revisit the notes and test yourself on the revision website.