WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.907 Unit C P 2: Forces and motion. 00:00:09.567 --> 00:00:18.003 The resultant force is the overall force on one object after all its forces are combined, taking account of their directions (their vector sum). 00:00:18.153 --> 00:00:21.459 Choose positive and negative directions consistently. 00:00:24.133 --> 00:00:31.983 A 1000 newtons driving force forward and 500 newtons drag backward give a resultant of 500 newtons forward. 00:00:32.133 --> 00:00:36.613 Subtract opposing forces; retain the direction of the larger force. 00:00:41.267 --> 00:00:45.002 Balanced forces give zero resultant and no acceleration. 00:00:45.152 --> 00:00:50.687 A stationary object stays stationary; a moving object continues at constant velocity. 00:00:53.367 --> 00:00:58.869 Newton’s first law describes this behaviour in the absence of a resultant external force. 00:00:59.019 --> 00:01:02.493 Motion does not require a continuing resultant force. 00:01:05.167 --> 00:01:10.379 Unbalanced forces cause acceleration: a change in speed, direction or both. 00:01:10.529 --> 00:01:14.904 A moving object can slow down if the resultant opposes its velocity. 00:01:17.567 --> 00:01:22.730 Circular motion at constant speed requires a resultant force towards the centre. 00:01:22.880 --> 00:01:28.265 This centripetal force changes the direction of velocity, so the object accelerates. 00:01:30.933 --> 00:01:35.647 Mass describes how much matter an object contains and is measured in kilograms. 00:01:35.797 --> 00:01:40.672 It also measures how difficult it is to change the object’s velocity (inertia). 00:01:40.822 --> 00:01:45.143 Weight is the gravitational force on the object, measured in newtons. 00:01:47.800 --> 00:01:53.123 Weight equals mass times gravitational field strength, W equals M times G. 00:01:53.273 --> 00:01:59.053 Near Earth use G equals 10 newtons per kilogram in these examples or the value given. 00:02:01.733 --> 00:02:07.066 A 6 kilograms object weighs 60 newtons where G equals 10 newtons per kilogram. 00:02:07.216 --> 00:02:13.497 Its mass stays 6 kilograms on the Moon, but its weight is smaller because the gravitational field is weaker. 00:02:16.167 --> 00:02:18.983 Measure weight with a calibrated newton meter. 00:02:19.133 --> 00:02:23.490 On a weight, mass graph, the gradient is gravitational field strength. 00:02:26.167 --> 00:02:32.139 Air or water resistance (drag) opposes an object’s movement relative to the fluid around it. 00:02:32.289 --> 00:02:34.743 Drag generally increases with speed. 00:02:34.893 --> 00:02:36.855 Streamlining reduces it. 00:02:39.533 --> 00:02:43.571 A falling object initially accelerates because weight exceeds drag. 00:02:43.721 --> 00:02:47.886 As speed increases, drag increases and the resultant becomes smaller. 00:02:50.567 --> 00:02:55.037 At terminal velocity, drag balances weight and acceleration is zero. 00:02:55.187 --> 00:03:00.159 The object continues falling at constant velocity; the forces have not disappeared. 00:03:00.309 --> 00:03:03.579 At terminal velocity, resultant force is zero. 00:03:08.233 --> 00:03:14.920 Opening a parachute increases drag: the falling person slows until reaching a new, lower terminal velocity. 00:03:17.600 --> 00:03:27.167 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. 00:03:27.317 --> 00:03:31.683 For the same mass, increasing resultant force increases acceleration. 00:03:34.333 --> 00:03:38.791 For the same resultant force, a larger mass has a smaller acceleration. 00:03:38.941 --> 00:03:41.894 Inertial mass equals F divided by A. 00:03:42.044 --> 00:03:45.945 It measures how strongly an object resists a change in velocity. 00:03:48.600 --> 00:03:56.396 In the trolley core practical, vary pulling force while keeping total moving mass constant, or vary mass while keeping force constant. 00:03:56.546 --> 00:04:00.477 Use light gates or a motion sensor to calculate acceleration. 00:04:03.133 --> 00:04:08.146 Keep track and release conditions consistent, minimise friction and repeat measurements. 00:04:08.296 --> 00:04:13.725 A hanging mass and pulley can provide a pulling force; the hanging mass belongs to the moving system. 00:04:16.400 --> 00:04:23.151 Newton’s third law: when two objects interact, each exerts an equal and opposite force of the same type on the other. 00:04:25.833 --> 00:04:29.654 A foot pushes a ball forward; the ball pushes the foot backward. 00:04:29.804 --> 00:04:34.265 These forces act on different objects and do not balance each other on the ball. 00:04:36.933 --> 00:04:40.971 A book’s weight and the table’s upward normal force can balance on the book. 00:04:41.121 --> 00:04:45.973 They are not a third-law pair: the partner to Earth pulling the book is the book pulling Earth. 00:04:46.123 --> 00:04:49.568 Balanced forces on the book are not a Newton’s third-law pair. 00:04:54.233 --> 00:05:00.610 Momentum equals mass times velocity, P equals M times V, in kilogram metres per second. 00:05:00.760 --> 00:05:03.899 Momentum is a vector, so direction matters. 00:05:06.567 --> 00:05:17.103 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. 00:05:19.767 --> 00:05:26.540 A 2 kilograms trolley at 3 metres per second hits a stationary 1 kilogram trolley and they stick together. 00:05:26.690 --> 00:05:35.435 Initial momentum is 6 kilogram metres per second; final combined mass is 3 kilograms, so final velocity is 2 metres per second. 00:05:35.585 --> 00:05:40.235 Negligible external resultant force allows momentum conservation. 00:05:44.900 --> 00:05:55.309 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. 00:05:55.459 --> 00:05:57.805 Use the sign of the velocity change. 00:06:00.467 --> 00:06:05.596 For the same change in momentum, increasing stopping time reduces average force. 00:06:05.746 --> 00:06:10.875 Crumple zones, airbags and seat belts extend stopping time and reduce injury risk. 00:06:13.533 --> 00:06:18.502 Momentum conservation does not mean kinetic energy is always conserved. 00:06:18.652 --> 00:06:23.016 In a sticking collision, some kinetic energy is transferred to other stores. 00:06:25.667 --> 00:06:29.420 Stopping distance equals thinking distance plus braking distance. 00:06:29.570 --> 00:06:34.117 Thinking distance is how far a vehicle travels before the driver starts braking. 00:06:34.267 --> 00:06:37.189 A delay before braking adds to the distance travelled. 00:06:41.867 --> 00:06:48.361 Thinking distance equals speed times reaction time, assuming speed stays constant during the reaction interval. 00:06:48.511 --> 00:06:53.289 At 20 metres per second and 0.5 seconds, it is 10 metres. 00:06:55.967 --> 00:07:02.755 Reaction time varies between people and conditions; drugs, alcohol, tiredness and distractions can increase it. 00:07:02.905 --> 00:07:07.155 A simple ruler-drop test can investigate reaction time with repeats. 00:07:09.833 --> 00:07:16.364 Braking distance depends on speed, mass, braking force, tyre condition, brake condition and road surface. 00:07:16.514 --> 00:07:19.532 Wet or icy roads reduce available friction. 00:07:22.200 --> 00:07:27.267 Higher speed increases thinking distance and increases braking distance more strongly. 00:07:27.417 --> 00:07:31.772 With fixed braking force, braking distance is proportional to speed squared. 00:07:34.433 --> 00:07:38.679 Large decelerations require large forces and can cause injury. 00:07:38.829 --> 00:07:44.768 Safety features reduce force by increasing collision time, but do not remove the momentum change. 00:07:47.433 --> 00:07:49.567 That completes Forces and motion. 00:07:49.717 --> 00:07:53.062 Revisit the notes and test yourself on the revision website.