FORCE

What is a Force?

◦Push and Pull are called

forces
◦When you push or pull
something you exert a
force on it
 Squeezing a tube of toothpaste

Example of Turn the cap of a bottle

Forces in our
daily lives Stretch a rubber band

Kick a soccer ball

 When a person studies forces, it is important to
know what is exerting the force and on what it is
being exerted as well as in what direction.
 What forces do is called the effect of a force

Forces can not be seen, but the effect of it can

The effect of a be observed

force Sometimes a force exerted onto an object is too

small to have a visible effect

Two or more forces can be exerted in opposite

directions so that they neutralise eachother
 Forces can have the following effects:
Change the
Movement: Deformation: Rotation:
direction:
• A stationary • Change the • A force can • Can cause an
object to start shape of an change the object to rotate
moving object direction od an
• A moving object
object to move
faster
• A moving
object to move
slower
THE UNIT OF FORCE

◦ Newton (N) is the unit

that we use to measure
force
◦ This unit is derived from
the name of the English
scientist, Sir Isaac Newton
The measuring of
force
◦ A Newton meter (Newton
spring balance) is used to
measure force
◦ It has a scale marked or
calibrated in Newton
 ◦ https://phet.colorado.edu/sims/html/force
Simulation s-and-motion-basics/latest/forces-and-
motion-basics_en.html
Forces work in pairs
Forces always work in pairs and simultaneously on various objects and never as a single force

Example 1: if you push a against a wall, Example 2: if you sit on a chair, you are
your hands exert a force onto the wall. exerting a downwards force on the chair..
The wall also exerts a force onto your The chair also exerts an upwards force on
hands (in the opposite direction) you.
• The Forces: • The Forces:
• Act in pairs • Act in pairs
• Are equal in size • Are equal in size
• Act in opposite directions • Act in opposite directions
• Act simultaneously on your hands • Act simultaneously on your hands
and the wall and the wall
BALANCED AND
UNBALANCED
FORCES
Balanced and
unbalanced forces
◦ The effect of forces that act together
depends on the amount and direction
in which each force acts
◦ When two or more forces act together
on an object the forces combine to
form a net force
◦ Note: these forces are not force pairs
since they act on the same object
 ◦ When the two forces are equal, but in
opposite directions, they balance each
other
◦ We say that the net force is 0 N (zero
Newtons)
◦ When the two forces act in opposite
Balanced directions and are not equal, we say the
net force is greater than 0 N
Forces
 ◦ Think of tug of war where people pull on
opposite ends of the rope
◦ If they exert a force of equal magnitude,
the rope will stand still
◦ The forces balance one another and the
Balanced net force is 0 N

Forces
 ◦ If one group can exert a stronger force
than the other, the rope will move in the
direction of the stronger force.
◦ This is because the forces are unbalanced
and there is a net force working in the
direction of the greater force.
Unbalanced
Forces
 Types of forces

Non-contact
Contact forces forces (field
forces)
 A contact force is exerted
when two objects are in
contact with one another
(touch each other)
Contact Forces
Examples: frictional force,
tensile force, compression
force, thrust force, tension, air
resistance
 A non-contact force is exerted
over a distance. The two
objects do not touch one
Non-contact another.
Forces (field
forces)
Examples: Gravitational force,
magnetic force, electrostatic
force.
Homework/Classwork

◦Exercise 1 page 171 in your

textbooks
◦Questions 1, 2, 3, 4, 5, 6
 FIELD FORCES
(NON-CONTACT)
GRAVITATIONAL FORCE
Gravitational force
◦ When one throws a ball into the air, it always
comes down to the surface of the earth
again
◦ The ball falls back to the earth because there
is a force of attraction between the ball and
the Earth that is called gravitational force.
◦ Gravitational force is always a force of
attraction
◦ All objects that have mass, exert a
gravitational force on one another. Even you,
however it is too small to observe.
Gravitational Force

Definition: A Force of
attraction that is exerted by
objects on one another as a
result of their mass
 Gravitational Force
◦ Gravitational force keeps everything on
earth in place
◦ Without gravitational force, things would
drift away from the surface of the earth
◦ There is gravitational force between the
earth and the moon – it keeps the moon
in its orbit around the earth
◦ The pull between the moon and the earth
causes the tides of the sea
◦ The gravitational force of the earth keeps
satellites in their orbits above earth
 Gravitational Force
◦ The Earth and other planets in our solar
system are kept in their places around the
sun by gravitational force
◦ Gravitational force is always directed at
the centre point of the mass of the object
◦ The magnitude (size) of the gravitational
force is determined by two factors:
1. The mass of the object: Objects with a
greater mass have more force of attraction
on one another.
2. The distance between objects: The force
of attractions becomes weaker as the
distance between objects become greater.
Homework
◦Textbook page 176 – 178
◦Numbers 2, 3, 4, 5
 In everyday language, we confuse mass
and weight as the same thing

In science, the two terms are related but

mean two different things
Mass versus
Weight The mass of an object is the amount of
matter in the object and is measured in
kilograms (kg)

The weight of an object is the gravitational

force that is exerted by the Earth (or Moon
or another planet) on the object
Mass
◦ The mass of an object stays the
same everywhere, even if it can
be measured at any place on
the earth or any place in the
universe.
◦ The unit of mass is kilogram (kg)
or gram (g) and milligram (mg)
for smaller units. A grater unit
like a ship can be measured in
ton
◦ We use a scale to measure mass
 Weight
◦ Weight (Fg) is a force and is measured in Newton
◦ The weight of an object will change if the gravitational
force (gravity) on the object changes
◦ On the moon the gravitational force is one sixth of the
gravitational force on Earth
◦ The weight of an object on the moon will therefore be
one sixth of its weight on earth
◦ The weight of an object with mass (m) on the surface of
the earth is obtained by multiplying the mass (m) with
the gravitational acceleration (g) on the surface of the
earth.
◦ Weight = mass x gravitational force
◦ Gravitational acceleration for earth is g = 9,8 m/s²
◦ Weight can also be measured with a Newton spring
balance (Newton meter)
Gravitational
Acceleration
◦ Gravitational acceleration (g) is the
acceleration as a result of
gravitational force and amounts to
how fast objects are attracted to the
Earth or other planets.
◦ It differs from planet to planet
because not all planets have the
same magnitude of mass
MAGNETIC FORCE
 Magnetic Forces
◦ Magnetic forces or magnetism is
the force that magnets exert on
magnetic materials that contain
iron steel cobalt and nickel
◦ Objects that are attracted by a
magnet are called magnetic
material
◦ Objects that are not attracted my
magnets are nonmagnetic
◦ Magnets can attract magnetic
material through other materials
such as paper, carton, plastic and
glass
Magnets
◦ There are different types of
magnets in different forms
◦ Although they come in different
forms, they all have the same
thing in common – they exert
forces on other magnets and
magnetic materials
◦ Magnets can magnetise
another object
 At each end of a magnet, there is a pole

We call them the north and the south pole

Magnetic We say that magnets have a definite polarity

Poles Each magnet has both north and south poles, never
just one
 SIMILAR
POLES
REPEL ONE
ANOTHER
DIFFERENT
POLES
ATTRACT
ONE
ANOTHER
Magnets indicate
direction
◦ Magnets that is suspended
freely, and is horizontal,
comes to rest in a North
South direction
◦ Magnets are therefore used
in compasses to determine
direction
◦ Magnets can also interfere
with a compass
 Magnetic Fields
◦ Area around the magnet in which magnetic
forces is exerted
◦ Surrounds the entire magnet
◦ Stronger at the poles
◦ Invisible
◦ Can be seen through iron fillings that arrange
themselves around the magnet, forming lines
◦ Lines are called magnetic field lines
◦ Magnetic field lines never cross
◦ Magnetic fields are stronger when the field
lines are closer together or more
◦ The direction of magnetic field lines are from
North to South
 IRON
FILINGS TO
SEE
MAGNETIC
LINES
 PATTERN OF
MAGNETIC
FIELDS
BETWEEN
DIFFERENT
POLES
(ATTRACTION)
 PATTERN OF
MAGNETIC
FIELDS
BETWEEN
SIMILAR POLES
(REPULSION)
 Simulations

MAGNETS AND MAGNET AND COMPASS

ELECTROMAGNETS
ELECTROSTATIC FORCE
 Electrostatic Force
◦ When certain materials are rubbed against each other, they
are able to get electrostatic charge.
◦ Electricity generated through friction is called static electricity
◦ Only electrons can be transferred between atoms (protons
and neutrons don’t move)
◦ During the friction process, electrons move from one
material to the next
◦ This causes a positive charge on the one material (lost
electrons) and a negative charge on the other (gained
electrons)
◦ Objects with similar charges will repel one another
◦ Objects with opposite charges will attract one another
◦ A charged object will also attract a neutral object
 When two neutral materials are rubbed against
eachother, friction results

Friction between
The friction between the two materials causes
materials electrons to move from the on the surface of
transfer the one material to the atoms on the surface of
the other material
electrons
The first material have more protons than
electrons (positively charged), and the second
material has more electrons than protons
(negatively charged)
Only electrons can
move
◦ Protons and neutrons cannot move
◦ Only electrons can be transferred from one
atom to the next
◦ Protons and neutrons cannot be transferred
between atoms/materials
◦ Positive charges are due to a loss of
electrons
◦ Negative charges are due to a gain in
electrons
◦ The adding or removing of electrons from a
material is known as electrification
Charged objects has an
influence on eachother

◦ Two positively charged objects or

two negatively charged objects will
repel each other
◦ A positively charged object and a
negatively charged object will repel
each other
Shocks, Sparks and Static
electricity
◦ The shock or spark that you feel or see when you touch
a metal door handle is the discharge of static electricity.
◦ When you walk over a nylon or wool carpet, the
electrons from the carpet will transfer to you.
◦ You gained electrons and thus have a negative charge.
◦ When you touch the door handle, which conducts
electrons, the electrons move from your hand to the
handle.
◦ You will feel a static shock and if it is dark a spark of
static electricity will be seen.
 Static electricity will usually only be noted when
the air is very dry.

Shocks, Sparks When the air is moist, the water in the air helps
and Static the electrons move from your body quicker.
Therefore no charge builds up in your body.
electricity
The air is dryer in the winter so we notice this
more in the winter.
Shocks, Sparks and Static
electricity
◦ Lightning is nature’s most spectacular
demonstration of static electricity and it is a huge
spark.
◦ When two objects are rubbed together, they are
charged. Likewise, thunder clouds are charged
when they rub against each other.
◦ Thunder clouds release large amounts of charge
in which energy is accumulated until the energy
releases a spark which results in a lightning bolt.
 How Lightning Originates
◦ During a storm, the clouds are charged electrostatically
◦ Friction between clouds and moisture in clouds causes clouds to
become charged
◦ The underside of the clouds become negatively charged, and the top
part becomes positively charged
◦ When the charge build-up becomes too great, the electrons on the
underside of the cloud move down to the ground where they are
being earthed
◦ The transfer of energy is enormous and leads to bright light, heat and
sound
◦ The flash of the lightning is a giant spark of electricity between
charged areas in the cloud or between clouds and the ground
◦ The lightning heats the air very quickly so that the air has no chance to
expand so it gets compressed
◦ When the compressed air expands it creates a sound wave. This is the
clap of thunder.
◦ If the electrons of a flash moves through a person, the person can get
seriously hurt or be killed.
 Lightning Safety
◦ Keep indoors
◦ Keep away from windows
◦ Do not use electric appliances or telephones
◦ Avoid standing or running water
◦ Avoid trees, towers, fences and telephone
poles
◦ Do not bath or shower
◦ Make the surface of your body as small as
possible
◦ Vehicles are safe shelters against lightning
◦ Look for a low laying part away from trees, tall
objects, metal fences and pipes
 Page 183-186

• Questions 1, 2, 3, 4, 5
Classwork
Page 192

• Questions 2 and 3