CIE IGCSE PHYSICS
Forces
Physics for IGCSE Physics pages 36-41
July 13th 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 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
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)
Force
newtonmeters
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
newtonmeter.
What is a force?
22/03/2016
A force is a “push” or a “pull” or a “twist”.
Q. How many different forces do you know?
Newton 1
Learning today
22/03/2016
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
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 1kg weighs
about 10N
1 stone is about 63N.
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?
22/03/2016
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
22/03/2016
Reaction
What would happen if we took the
road away?
Weight
Balanced and unbalanced forces
22/03/2016
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
22/03/2016
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 1st
22/03/2016
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
22/03/2016
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
22/03/2016
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
Balanced and unbalanced forces
22/03/2016
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
22/03/2016
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
22/03/2016
Force and acceleration
22/03/2016
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
M
A
Force, mass and acceleration
22/03/2016
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
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?
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.
3N
2N
This overall force is called
resultant force.
In the example opposite, 5N
is the resultant force of the
3N and 2N forces.
5N
TRIPLE ONLY
Determine the resultant force in the cases below:
4N
6N
10N
1.
3.
3N
4.
2.
6N
4N
4N
7N
2N
3N
4N
1N
5.
4N
There is no resultant
4N 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)
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
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’.
Calculating weight
weight = mass × gravitational acceleration
W=mxg
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.
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.
Learning today
22/03/2016
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
Velocity-time graphs for a falling object in fluids
The forces are Weight and Drag
Newton 3
22/03/2016
• 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
22/03/2016
Scalar and Vectors
Now draw a T-table in the front of
your books.
Scalar
Vectors
22/03/2016
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/s2
second terminal
velocity
time
Choose appropriate words to fill in the gaps below:
force is a push or a pull. A force can cause an object to
A _____
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 50m EAST
Displacement 25m at
45o North of East
Addition of vectors 1
4N
object
4N
6N
6N
object
resultant = 10N
object
The original vectors are called COMPONENT vectors.
The final overall vector is called the RESULTANT vector.
4N
6N
6N
object
4N
object
resultant = 2N
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 when
accelerate
opposite
the braking force acts in 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 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.
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
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 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
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.
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
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 the
Hooke’s law states that when a wire or spring is _________
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 - 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.