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Force and acceleration IGCSE 1 Forces • What is a force? • A force is a push or a pull. • An objects behaviour is dependant on all the forces acting on it. • Force is measured in Newtons (N) Effects of forces • The size and direction of the forces acting on an object will determine how that object moves or behaves. Forces can: 1. Change the speed of an object 2. Change the direction of movement of an object 3. Change the shape of an object Types of forces Weight – the gravitational force between the earth and an object. What is the difference between mass and weight? Weight = mass x gravitational acceleration = The weight of an object changes according to its position in the universe because the value of g changes. Other forces • Tension:This is the force that acts through a rope. • Friction: Friction is a force that acts to oppose motion. • Normal:The normal force is the force exerted by a surface on an object with which it is in contact. Adding Forces Forces acting in a straight line: 10N 5N 10N 4N 10N What will be the total Force? 10N 5N 4N Adding Forces Forces acting in a straight line: 8N 3N 10N 4N What will be the total Force? 10N 8N 3N 4N A car of mass 900kg is travelling along a road at 20. −1 . Forces that act on the car: Accelerating force of the engine ( ) Weight of the car ( ) Friction of road on tyres ( ) Normal force of the road ( ) Adding Forces: Parallelogram Law Go to page 81 in your textbook and follow the instructions on the Parallelogram Law. Worked Example • Find the resultant of two forces of 4.0 and 5.0 acting at an angle of 45° to each other. • Using a scale of 1cm = 1N, draw parallelogram ABDC with AB = 5cm, AC = 4N and the angle = 45°. • Use the parallelogram law to find the resultant of these two forces. (it must have magnitude and direction). • Answer = 8.3N acting at an angle of 20° to the 5.0N force. Vectors and Scalars • Scalars are quantities with magnitude ONLY e.g. speed, distance • Vectors are quantities with BOTH magnitude AND direction e.g. velocity, displacement, acceleration • A vector is represented by an arrow, where the line indicates the magnitude and the arrow points in the direction of motion. • Questions • Do the questions on pg 70 Newton’s 1st Law • What causes a car to come to rest when the engine is switched off? • What would happen if these forces weren’t around? Newton’s 1st Law • Force is not needed to keep a body with uniform velocity in motion as long as no opposing forces act on it. • This is summed up in Newton’s First Law A body stays at rest, or continues to move at constant velocity, unless acted upon by an external force. Mass and Inertia • Inertia is the property of a body that resists a change in it’s motion. i.e. if it is at rest, it wants to stay at rest. The Egg Experiment Mass and Inertia • The larger the mass of a body, the larger its inertia (resistance to movement) Newton’s Second Law If there is an unbalanced force acting on an object, the object will accelerate in the direction of the force. The acceleration is directly proportional to the force and inversely proportional to the mass of the object. = Newton’s Second Law • A CONSTANT force causes a CONSTANT acceleration • CONSTANT VELOCITY means there is NO resultant force acting on the object. • CONSTANT ACCELERATION = CONSTANT FORCE acting on the object Worked Example 1. A block of mass 2kg is pushed along a table with a constant velocity by a force of 5N. When the push is increased to 9N, what is: a) The resultant force b) The acceleration Worked Example 1. A car of mass 1200kg is travelling at 72/ℎ is brought to rest in 4s. Find: a) The average deceleration b) The average braking force c) The distance moved during the deceleration. Newton’s Third Law If body A exerts a force on body B, then body B will simultaneously exert a force on body A. The forces are equal in magnitude but opposite in direction. These forces are called an ACTION-REACTION pair. Note: the forces are applied to different objects so no resultant force can be found. Newton’s Third Law F2: Table pushes book up F1: Earth pulls book down F3: Book pushes table down F4: Book pulls earth up • The book does not move because F1 = F4 (balanced forces) • What are the action-reaction pairs? (F1+ F2 and F3+F4) Newton’s Third Law Newton’s Third Law Example 3: Horse pulling a cart Cart pulls horse backwards Horse pulls cart forward Friction The cart will only move when: pull on cart > friction on cart The cart will not move if pull on cart = friction on cart Newton’s Third Law Example 4: Motion of a rocket Exhaust gases push rocket up = THRUST Rocket pulls earth up Earth pulls rocket down Engine pushes exhaust gases down Questions • Do the questions on page 108 Hookes Law When an elastic object - such as a spring - is stretched, the increased length is called its extension. The extension of an elastic object is directly proportional to the force applied to it: = × • F is the force in newtons, N • k is the 'spring constant' in newtons per metre, N/m • is the extension in metres, m • This equation works as long as the elastic limit (the limit of proportionality) is not exceeded. If a spring is stretched too much, for example, it will not return to its original length when the load is removed. The spring constant The spring constant k is different for different objects and materials. It is found by carrying out an experiment. For example, the unloaded length of a spring is measured. Assuming the limit of proportionality (elastic limit) is not exceeded, a graph of force against extension produces a straight line that passes through the origin. The gradient of the line is the spring constant, k. The greater the value of k, the stiffer the spring. How are materials affected by stretching? • A spring (or length of wire) will stretch if weight is added . • The wire will stretch in proportion to the load up to a certain point (the limit of proportionality) • What happens after this point? Moments • Moments make things turn or rotate. • A moment is the turning effect of a force around a fixed point called a pivot. Moments The size of a moment depends on two factors: 1. the size of the force applied 2. the perpendicular distance from the pivot to the line of action of the force Moments SMALL MOMENT The distance from the fulcrum to the line of action of force is very small LARGE MOMENT The distance from the fulcrum to the line of action of force is large Moments • M = the moment of the force in newtonmetres, Nm • F = the force in newtons, N • d = the perpendicular distance from the line of action of the force to the pivot in metres, m Worked example A spanner is used to undo a nut. A force of 25 N is applied to the end of the spanner, which is 10 cm away from the centre of the nut. Calculate the moment when the spanner is horizontal. Balancing moments • Where an object is not turning around a pivot, the total clockwise moment must be exactly balanced by the total anti-clockwise moment. sum of the clockwise moments = sum of the anti-clockwise moments Are they balanced?? Levers • A lever is a simple machine that makes work easier to do. Examples of simple levers include cutting with scissors, or lifting the lid on a tin of paint with a screwdriver. • Levers reduce the force needed to perform these tasks. • When someone uses a lever, they exert a force (the effort) around a pivot to move an object (the load). Levers Practical: Verifying the Principle of Moments • Follow the instructions on pg 60 of your textbook to verify the principle of moments When a body is in equilibrium the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point. × = × Questions • Do the questions on pg 62 Centre of Mass • Mass is the amount of matter an object has. • Centre of Mass: This is the point at which any object with mass will balance. Finding the centre of mass of an irregular object: plumb line 1. Drill a small hole in the object and hang it up so that it is free to swing without obstruction. 2. Hang a plumb line (a piece of string with a weight hanging from it) from the same suspension point. This lets you mark the vertical line directly below the suspension point. 3. Drill another hole at a different location within the object. 4. Again hang a plumb line to determine the vertical and mark it on. 5. The point at which the two marked lines cross is the centre of mass. Toppling • A body topples when the vertical line through its centre of mass falls outside its base. Which is more stable? Why? What makes objects stable? Toppling The stability of a body is increased by: 1. Lowering its centre of mass. 2. Increasing the area of its base. Motion in a curved path due to a perpendicular force. • Newton’s 1st law – an object with no resultant forces acting on it will continue to move in a straight line at constant velocity. Motion in a curved path due to a perpendicular force. • If an object is moving in a circular path the direction is always changing – but a force must be present to create this change! • If the direction is constantly changing then the force must also be constantly changing. Motion in a curved path due to a perpendicular force. • Centripetal force - the force that always acts towards the centre of the circle. • This force acts perpendicularly to the motion of the object Motion in a curved path due to a perpendicular force. Examples of circular motion: 1. The moon orbiting the earth 2. A car turning a corner 3. A train going around a bend Motion in a curved path due to a perpendicular force. What would happen if the string that was spinning an object around broke? Questions • Do questions 1, 3, 4 and 5 on page 67