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CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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Page 1: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

CIE IGCSE PHYSICS

Forces

Physics for IGCSE Physics pages 36-41

July 13th 2011

Page 2: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

CIE IGCSE SpecificationSection 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

Page 3: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Homework due Wednesday 19th 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

Page 4: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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 K1, K2, K3

• One interesting fact - 1 mark I• 100 handwritten words – 1 mark 100• Quality 1 or 2 marks ( Plagiarism 1 mark max)

Page 5: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

ForceA 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 newtonmeter.

newtonmeters

Page 6: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

What is a force?What is a force?A force is a “push” or a “pull” or a “twist”.

Q. How many different forces do you know?

Page 7: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

Learning today Learning today Newton 1

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/s2

Newton 3 For every action there is an opposite and equal reaction

Page 8: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Some types of force1. GravitationalThis 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 1kg weighs about 10N

1 stone is about 63N.

Page 9: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

2. Normal reaction or contactThis is the repulsive force that stops two touching bodies moving into each other.

The word ’normal’ means that this force acts at 90° to the surfaces of the bodies.

It is caused by repulsive molecular forces.

normal reaction forces

weight

The two upward reaction forces on the tyres balance the

downward weight of the car

Page 10: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

3. FrictionThis 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!

Page 11: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

4. Air resistance or dragThis 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 cara parachute

maximises drag force

Page 12: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

5. UpthrustThis 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.

Page 13: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

6. MagneticBetween magnets but also the force that allows electric motors to work.

7. ElectrostaticAttractive and repulsive forces due to bodies being charged.

Electrostatic force causes the girls’ hair to rise when they

touch the Van der Graaff generator.

Page 14: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

What is a force?What is a force?A force is a “push” or a “pull”. Some common examples:

________ – pulls things downwards

___ ________ (drag) – acts against anything moving through air

______ – keeps things afloat_____ – acts against anything moving

Words – upthrust, air resistance, friction, weight

Page 15: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

What would happen if we took the road away?

Weight

Reaction

Page 16: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

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

Page 17: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

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 and unbalanced Balanced and unbalanced forcesforces

Page 18: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Balanced forces- Newton 1Balanced forces- Newton 1stst 19/04/23

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.

Page 19: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

InertiaInertia19/04/23

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

Page 20: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

Learning today Learning today Newton 2

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/s2

Newton 3 For every action there is an opposite and equal reaction

Page 21: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

1) This animal is either ________ or moving with _____ _____…

4) This animal is…

2) This animal is getting _________…

3) This animal is getting _______….

Page 22: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

Page 23: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Newton 2Newton 219/04/23

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

Page 24: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

Force and accelerationForce and acceleration

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/s2)

F

AM

Page 25: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23Force, mass and Force, mass and accelerationacceleration

1) A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate?

2) A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car?

3) A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying?

4) A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?

F

AM

Page 26: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Resultant forceA 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.

This overall force is called resultant force.

In the example opposite, 5N is the resultant force of the 3N and 2N forces.

3N2N

5N

Page 27: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Determine the resultant force in the cases below:

4N

6N1.

4N3N2.

7N

3N3.

2N

6N4.

4N

4N5.

10N 1N

4N

4N

There is no resultant force in this case

TRIPLE ONLY

Page 28: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Mass and weightMass 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

Page 29: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

The acceleration due to gravity (g)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 m/s2 Location m/s2

Earth 10 Jupiter 24

Moon 1.6 Pluto 0.7

Mars 3.7 The Sun 270

Page 30: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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/s2, hence they both use the same symbol ‘g’.

Page 31: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Calculating weight

weight = mass × gravitational acceleration

W = m x g

weight is measured in N

mass is measured in kg

gravitational acceleration is measured in m/s2

On the Earth’s surface a mass of 1kg

has a weight of 10N.

Page 32: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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/s2.

Page 33: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

19/04/23

Learning today Learning today 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

Page 34: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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.

Page 35: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Terminal VelocityVelocity-time graphs for a falling object in fluids

The forces are Weight and Drag

Page 36: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Newton 3Newton 3• 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 liftEarth and the moonTask 1Explain how a rocket produces a force.Task 2 Question 4

19/04/23

Page 37: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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.

Page 38: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Scalar and Vectors Scalar and Vectors

Draw a T-table at the back of your books Scalar Vectors

19/04/23

Page 39: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Scalar and Vectors Scalar and Vectors

Now draw a T-table in the front of your books. Scalar Vectors

19/04/23

Page 40: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

ParachutingA 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.)

Page 41: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

velocity

time

first terminal velocity

initial acceleration

= 10 m/s2

Velocity-time graph of a parachutist

second terminal velocity

parachute opened

ground reached

Page 42: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Choose appropriate words to fill in the gaps below:

A _____ is a push or a pull. A force can cause an object to ___________ or change shape.

Force is measured in _______ (N) with a newtonmeter.

There are many types of force. ________ force occurs when two bodies touch each other.

Friction is a force that _______ the _______ of one body relative to another. It is caused by the _________ forces between ___________.

attractive

motioncontact

newtons

opposes

forceaccelerate

WORD SELECTION:

molecules

attractive

motion

contact

newtons

opposes

force

accelerate

molecules

Page 43: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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.

Page 44: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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 50m EAST

Displacement 25m at 45o North of East

Page 45: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Addition of vectors 1

The original vectors are called COMPONENT vectors.

The final overall vector is called the RESULTANT vector.

4N

6Nobject

4N6N

object

resultant = 10N

object

4N 6N

object

4N6N

object

resultant = 2N

object

Page 46: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Resultant force and motion

Resultant force Effect on the motion of an object

Zero Object’s velocity stays the same including staying stationary

In the direction the object is moving

Object accelerates

In the opposite direction in which the object is

moving

Object decelerates

Page 47: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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.

Page 48: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

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.

Page 49: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Choose appropriate words to fill in the gaps below:

A single force, called _________ force, can be used to replace a _______ of forces that act on a body.

If the resultant force is _____ then the body will either remain at _____ or continue to move at a constant ________.

If the resultant force is in the same _________ as an object’s motion, the object will __________. A car is decelerated when the braking force acts in the _________ direction to the car’s motion.

acceleratevelocity

rest direction

resultant

number zero

WORD SELECTION:

opposite

accelerate

velocityrest

direction

resultant

number

zero

opposite

Page 50: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Changing shapeForce 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.

Page 51: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Brittle materials such as glass do not change shape easily and break before noticeably stretching.

Resilient materials do not break easily.

Page 52: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Elastic materials return to their original shape when the forces on them are removed.

Plastic materials retain their new shape.

Page 53: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Stretching SpringsExperimental procedure:1. Place the weight holder only on the spring and note the position of the pin against the metre rule.2. Add 1N (100g) 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 2N, 3N, 4N, 5N and 6N. DO NOT EXCEED 6N. metre rule

pin

spring

weights

Page 54: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Typical resultsPin 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

Page 55: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Force against extension graph

0

Fo

rce

(N

)

0Extension (mm)

Page 56: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Hooke’s lawHooke’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.

Page 57: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

QuestionA spring of original length 150mm is extended by 30mm by a force of 4N. Calculate the length of the spring if a force of 12N is applied.

12N is three times 4N

Therefore the new extension should be 3 x 30mm

= 90mm

New spring length = 150mm + 90mm

= 240mm

Page 58: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Elastic limitUp 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.

Hooke’s law is no longer obeyed by the spring if its elastic limit is exceeded.

The right hand spring has been

stretched beyond its elastic limit

Page 59: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Force against extension graph if the elastic limit is exceeded

0

Fo

rce

(N

)

0Extension (mm)

elastic limit

Page 60: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Stretching an elastic band

0

Fo

rce

0Extension

An elastic band does not obey Hooke’s law.

Page 61: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Choose appropriate words to fill in the gaps below:

Hooke’s law states that when a wire or spring is _________ the increase in length or _________ is proportional to the load ______ applied.

This law is not obeyed if the spring is taken beyond its ______ limit after which it will become _____________ stretched.

A ________ band does not obey Hooke’s law.

A graph illustrating Hooke’s law will have a line that is ___________ and passes through the _______.

straight

elastic

origin

stretched

rubberforce

extension

WORD SELECTION:

permanently

straight

elastic

origin

stretched

rubber

force

extension

permanently

Page 62: CIE IGCSE PHYSICS Forces Physics for IGCSE Physics pages 36-41 July 13th 2011

Online SimulationsEffect of forces on motion using a space module - Freezeray.com Force combination balloon game - eChalk Electric & Magnetic Forces - 'Whys Guy' Video Clip (3:30mins) - Shows Charged Balloon & Effect of a magnet on a TV screen. Resultant of two forces - Fendt Forces on objects immersed in liquids - NTNU BBC KS3 Bitesize Revision: What is a force Balanced forces Unbalanced forces BBC AQA GCSE Bitesize Revision: Resultant force Types of forces

Vector Addition - PhET - 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 - eChalk

Vectors & Scalars - eChalk

Vector addition - eChalk

Vector Chains - eChalk

Fifty-Fifty Game on Vectors & Scalars - by KT - Microsoft WORD

Vector addition - Explore Science

Stretching Springs - PhET - 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.