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CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36 -41 July 13 th 2011
CIE IGCSE Specification Section 1: Effects of Force • • State that a force may produce a change in size and shape of a body. Describe the ways in which a force may change the motion of a body. Find the resultant of two or more forces acting along the same line. Demonstrate an understanding that mass is a property which ‘resists’ change in motion. Inertia • Recall and use the relation between force, mass and acceleration (including the direction). • Describe, and use the concept of, weight as the effect of a gravitational field on a mass. Vectors and Scalers • Demonstrate an understanding of the difference between scalars and vectors and give common examples • Add vectors by graphical representation to determine a resultant • Determine graphically the resultant of two vectors
Homework due Wednesday 19 th October • To write in your books 100 words each on the lives of Galileo and Newton 200 word total Include the following • When he was born • When he died • Where he lived • What three things was he known for • One interesting fact
Homework Markscheme • To write in your books 100 words each on the lives of Galileo and Newton 200 word total • When he was born – 1 mark B • When he died – 1 mark D • Where he lived - 1 mark L • What three things was he known for – I mark for each mentioned K 1, K 2, K 3 • One interesting fact - 1 mark I • 100 handwritten words – 1 mark 100 • Quality 1 or 2 marks ( Plagiarism 1 mark max)
Force A force is a push or a pull, it is a vector it has magnitude and direction. A force can cause an object to: – speed up – slow down – change direction – change shape Force is measured in newtons (N). Force is measured with a newtonmeters
What is a force? 6/14/2021 A force is a “push” or a “pull” or a “twist”. Q. How many different forces do you know?
Newton 1 Learning today 6/14/2021 Balanced Forces What inertia is Newton 2 Unbalanced Forces cause a body to speed up slow down or change direction. Also given as F=ma Unit N Gravity Mass and Weight W=mg Unit Newton – N g= 10 m/s 2 Newton 3 For every action there is an opposite and equal reaction
Some types of force 1. Gravitational This is the attractive force exerted between bodies because of their masses. This force increases if either or both of the masses is increased and decreases if they are moved further apart. Weight is the gravitational force of the Earth on an object. Bathroom scales measure weight. A mass of 1 kg weighs about 10 N 1 stone is about 63 N.
2. Normal reaction or contact This is the repulsive force that stops two touching bodies moving into each other. normal reaction forces The word ’normal’ means that this force acts at 90° to the surfaces of the bodies. weight It is caused by repulsive molecular forces. The two upward reaction forces on the tyres balance the downward weight of the car
3. Friction This is the force that opposes motion. The kinetic energy of the moving object is converted to heat energy by the force of friction. Friction is needed for racing cars to grip the road Friction is needed for walking!
4. Air resistance or drag This is the force that opposes the movement of objects through air. Drag is a more general term used for the opposition force in any gas or liquid. Objects are often streamlined to reduce this force. streamlined car a parachute maximises drag force
5. Upthrust This is the force experienced by objects when they are placed into a fluid (liquid or gas). An object will float on a liquid if the upthrust force equals its weight. A hot air balloon rises when the upthrust from the surrounding air is greater than the balloon’s weight.
6. Magnetic Between magnets but also the force that allows electric motors to work. 7. Electrostatic Attractive and repulsive forces due to bodies being charged. Electrostatic force causes the girls’ hair to rise when they touch the Van der Graaff generator.
What is a force? 6/14/2021 A force is a “push” or a “pull”. Some common examples: ____ – pulls things downwards _____ – acts against anything moving ________ (drag) – acts against anything moving through air ______ – keeps things afloat Words – upthrust, air resistance, friction, weight
Balanced and unbalanced forces 6/14/2021 Reaction What would happen if we took the road away? Weight
Balanced and unbalanced forces 6/14/2021 What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards… Weight
Balanced and unbalanced forces 6/14/2021 What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…
Balanced forces- Newton 1 st 6/14/2021 When forces are balanced, it means that: If an object is stationary, it will continue to be stationary. If you are sitting on a chair, your weight is balanced by an upwards force from the chair. If an object is moving, it will move at constant velocity. If you are pedalling a bicycle at a constant speed, the force you put into the pedals is balanced by the air resistance and friction.
Inertia 6/14/2021 When is at rest it takes a force to make it move. All objects resist a change in velocity, this is called inertia. Mass and paper demonstration
Newton 2 Learning today 6/14/2021 Unbalanced Forces cause a body to speed up slow down or change direction. Also given as F=ma What a resultant force is Gravity Mass and Weight W=mg Unit Newton – N g= 10 m/s 2 Newton 3 For every action there is an opposite and equal reaction
Balanced and unbalanced forces 6/14/2021 1) This animal is either ____ or moving with _____… 3) This animal is getting _______…. 2) This animal is getting _____… 4) This animal is…
Balanced and unbalanced forces 6/14/2021
Newton 2 Unbalanced Forces cause a body to speed up slow down in the direction of the force or they change direction. Also given as F=ma Resultant force is the overall sum of many different forces 6/14/2021
Force and acceleration 6/14/2021 If the forces acting on an object are unbalanced then the object will accelerate or change direction, like these wrestlers: Force (in N) = Mass (in kg) x Acceleration (in m/s 2) F M A
Force, mass and acceleration 6/14/2021 1) A force of 1000 N is applied to push a mass of 500 kg. How quickly does it accelerate? F 2) A force of 3000 N acts on a car to make it accelerate by 1. 5 m/s 2. How heavy is the car? 3) A car accelerates at a rate of 5 m/s 2. If it weighs 500 kg how much driving force is the engine applying? 4) A force of 10 N is applied by a boy while lifting a 20 kg mass. How much does it accelerate by? M A
Resultant force A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together. 2 N This overall force is called resultant force. In the example opposite, 5 N is the resultant force of the 3 N and 2 N forces. 5 N 3 N
TRIPLE ONLY Determine the resultant force in the cases below: 4 N 6 N 10 N 1. 3 N 4. 2. 6 N 4 N 4 N 7 N 2 N 3 N 4 N 1 N 5. 4 N There is no resultant 4 N case force in this
Mass and weight Mass is the amount of matter in an object. Mass is measured in kilograms. The mass of an object is the same on the Moon as on the Earth. Weight is the force of gravity on an object. Weight is measured in newtons. The weight of an object on the Moon is about one sixth that on the Earth. A newtonmeter is used to determine the weight of the parcel
The acceleration due to gravity (g) varies with planet, moon and star and depends on the height of an object. Some examples of the acceleration due to gravity: Location Earth m/s 2 10 Location Jupiter m/s 2 24 Moon 1. 6 Pluto 0. 7 Mars 3. 7 The Sun 270
Gravitational field strength (g) This is an alternative way of measuring the strength of gravity. The gravitational field strength is equal to the gravitational force exerted per kilogram. Near the Earth’s surface, g = 10 N/kg In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s 2, hence they both use the same symbol ‘g’.
Calculating weight = mass × gravitational acceleration W=mxg weight is measured in N mass is measured in kg gravitational acceleration is measured in m/s 2 On the Earth’s surface a mass of 1 kg has a weight of 10 N.
Falling objects When an object falls through air or some other fluid initially the only significant force acting on it is the downward pull of gravity. On Earth, it will initially accelerate downwards at 10 m/s 2.
Learning today 6/14/2021 Terminal Velocity in a fluid Forces acting – Weight and Drag Newton 3 For every action there is an opposite and equal reaction Explain using Newton 3 how a rockets produces a force The difference between vectors and scalars Yacapaca results
As the object speeds up frictional forces such as air resistance become greater the faster the object moves. Eventually the resultant force on the object will be zero when the frictional forces equal the weight of the object. The object then moves at a constant speed called terminal velocity.
Terminal Velocity-time graphs for a falling object in fluids The forces are Weight and Drag
Newton 3 6/14/2021 • For every action there is an equal and opposite reaction. • Forces act in pairs Two bodies are involved so forces do not cancel out A person in a lift Earth and the moon Task 1 Explain how a rocket produces a force. Task 2 Question 4
Vectors and Scalars All physical quantities (e. g. speed and force) are described by a magnitude and a unit. VECTORS – have magnitude, direction and a unit. examples: displacement, velocity, acceleration, force. SCALARS – have magnitude only examples: distance, speed, mass, work, energy.
Scalar and Vectors Draw a T-table at the back of your books Scalar Vectors 6/14/2021
Scalar and Vectors 6/14/2021 Now draw a T-table in the front of your books. Scalar Vectors
Parachuting A parachutist will have two different terminal velocities. Before opening the parachute it is about 60 m/s (140 m. p. h. . ). Afterwards, due the much greater drag force, the terminal velocity is about 5 m/s (12 m. p. h. )
Velocity-time graph of a parachutist velocity first terminal velocity parachute opened ground reached initial acceleration = 10 m/s 2 second terminal velocity time
Choose appropriate words to fill in the gaps below: A force _____ is a push or a pull. A force can cause an object to accelerate ______ or change shape. newtons (N) with a newtonmeter. Force is measured in _______ contact force occurs when There are many types of force. ____ two bodies touch each other. motion of one body opposes the _______ Friction is a force that _______ attractive forces relative to another. It is caused by the _____ molecules between ______. WORD SELECTION: newtons opposes accelerate force molecules contact attractive motion
Vectors and Scalars All physical quantities (e. g. speed and force) are described by a magnitude and a unit. VECTORS – have magnitude and direction examples: displacement, velocity, acceleration, force. SCALARS – have magnitude only examples: distance, speed, mass, work, energy.
Representing Vectors An arrowed straight line is used. The arrow indicates the direction and the length of the line is proportional to the magnitude. Displacement 50 m EAST Displacement 25 m at 45 o North of East
Addition of vectors 1 4 N object 4 N 6 N 6 N object resultant = 10 N object The original vectors are called COMPONENT vectors. The final overall vector is called the RESULTANT vector. 4 N 6 N 6 N object 4 N object resultant = 2 N object
Resultant force and motion Resultant force Effect on the motion of an object Zero Object’s velocity stays the same including staying stationary Object accelerates In the direction the object is moving In the opposite direction in which the object is moving Object decelerates
Examples 1 & 2 The box will move when the man’s push force is greater than the friction force. The plane will accelerate provided that the engine force is greater than the drag force.
Examples 3 & 4 The brakes exert a resultant force in the opposite direction to the car’s motion causing the car to decelerate. Once released, the glider moves at a near constant velocity as it experiences a nearly zero horizontal resultant force.
Choose appropriate words to fill in the gaps below: resultant force, can be used to replace A single force, called _____ number of forces that act on a body. a _______ zero then the body will either If the resultant force is _____ rest or continue to move at a constant ____. velocity remain at _____ direction as an object’s If the resultant force is in the same _____ motion, the object will _____. A car is decelerated accelerate when the braking force acts inopposite the _____ direction to the car’s motion. WORD SELECTION: number rest direction zero opposite velocity accelerate resultant
Changing shape Force can also change the shape of an object. A stretching force puts an object such as a wire or spring under tension. A squashing force puts an object under compression.
Brittle materials such as glass do not change shape easily and break before noticeably stretching. Resilient materials do not break easily.
Elastic materials return to their original shape when the forces on them are removed. Plastic materials retain their new shape.
Stretching Springs Experimental procedure: 1. Place the weight holder only on the spring and note the position of the pin against the metre rule. 2. Add 1 N (100 g) to the holder and note the new position of the pin. 3. Calculate the extension of the spring. 4. Repeat stages 1 to 3 for 2 N, 3 N, 4 N, 5 N and 6 N. DO NOT EXCEED 6 N. spring weights pin metre rule
Typical results Pin position with holder only (mm) Added weight or Force (N) Pin position with weight (mm) Extension (mm) 450 1 480 30 450 2 509 59 450 3 541 91 450 4 570 120 450 5 601 151 450 6 629 179
Force (N) Force against extension graph 0 0 Extension (mm)
Hooke’s law states that the extension of a spring force is proportional to the force used to stretch the spring. ‘Proportional’ means that if the force is doubled then the extension also doubles. The line on a graph of force against extension will be a straight AND go through the origin.
Question A spring of original length 150 mm is extended by 30 mm by a force of 4 N. Calculate the length of the spring if a force of 12 N is applied. 12 N is three times 4 N Therefore the new extension should be 3 x 30 mm = 90 mm New spring length = 150 mm + 90 mm = 240 mm
Elastic limit Up to a certain extension if the force is removed the spring will return to its original length. The spring is behaving elastically. If this critical extension is exceeded, known as the elastic limit, the spring will be permanently stretched. The right hand spring has been Hooke’s law is no longer obeyed by the stretched beyond its elastic limit spring if its elastic limit is exceeded.
Force (N) Force against extension graph if the elastic limit is exceeded elastic limit 0 0 Extension (mm)
Force Stretching an elastic band An elastic band does not obey Hooke’s law. 0 0 Extension
Choose appropriate words to fill in the gaps below: stretched Hooke’s law states that when a wire or spring is _____ the extension is proportional to the load increase in length or _____ force applied. ______ elastic This law is not obeyed if the spring is taken beyond its ______ permanently limit after which it will become _______ stretched. A ____ rubber band does not obey Hooke’s law. A graph illustrating Hooke’s law will have a line that is straight origin ______ and passes through the _______. WORD SELECTION: stretched elastic permanently extension origin force rubber straight
Online Simulations Effect of forces on motion using a space module Freezeray. com Force combination balloon game - e. Chalk Electric & Magnetic Forces - 'Whys Guy' Video Clip (3: 30 mins) - Shows Charged Balloon & Effect of a magnet on a TV screen. Resultant of two forces - Fendt Forces on objects immersed in liquids - NTNU BBC KS 3 Bitesize Revision: What is a force Balanced forces Unbalanced forces BBC AQA GCSE Bitesize Revision: Resultant force Types of forces Vector Addition - Ph. ET - Learn how to add vectors. Drag vectors onto a graph, change their length and angle, and sum them together. The magnitude, angle, and components of each vector can be displayed in several formats. Representing vectors - e. Chalk Vectors & Scalars - e. Chalk Vector addition - e. Chalk Vector Chains - e. Chalk Fifty-Fifty Game on Vectors & Scalars - by KT Microsoft WORD Vector addition - Explore Science Stretching Springs - Ph. ET - A realistic mass and spring laboratory. Hang masses from springs and adjust the spring stiffness and damping. You can even slow time. Transport the lab to different planets. A chart shows the kinetic, potential, and thermal energy for each spring.