FORCES
We cannot see forces, but we can see the results of their actions.
A force can cause an object to move or it can stop it from moving.
It can change the speed of a moving object
It can change the direction in which it is moving.
A force acting on an object can cause the object to change its shape or
size,
It can cause the object to rotate.
Sometimes two forces can act on an object in opposite directions.
WHAT IS A FORCE?
• DEFINITION: A force is the result of the interaction
between two objects and can be a push, a pull or a
twist. (2)
• Force is measured in newtons (N), named after the
English scientist Sir Isaac Newton.
• Force is measured with an instrument called a
spring balance / newton meter.
EFFECTS OF FORCES
A force can cause:
• a change in speed of an object (a stationery object starts
moving, a moving object stops or a moving object moves
faster or slower.
• a change in direction in which an object is moving
• a change in state of an object, shape changes or object
breaks
• an object to rotate or twist
• no visible effect
1. CHANGE IN SPEED…
A stationary object
starts moving
A moving object
moves even faster
(acceleration)
A moving object
moves slower
(deceleration) or stop
2. CHANGE IN
STATE- SHAPE
CHANGES OR
OBJECT BREAKS
3. CHANGE OF
DIRECTION
4. TWIST OR
ROTATE
5. NO VISIBLE EFFECT
(THE FORCE IS TOO SMALL OR THE FORCES EXERTED BY
THE TWO OBJECTS ON EACH OTHER ARE EQUAL BUT IN
OPPOSITE DIRECTIONS – THEY CANCEL OUT)
CLASSIFICATION OF FORCES
Forces can be classified into two main categories:
A. Contact forces
B. Non-contact forces / Field forces
A. CONTACT FORCES
In order for a contact force to work, the two objects
must be touching each other.
Examples of contact forces:
(take note of the symbols used for the various forces)



Push or compression, (𝑭𝑨 𝐟𝐨𝐫 𝐚𝐩𝐩𝐥𝐢𝐞𝐝 𝐟𝐨𝐫𝐜𝐞)
Pull (𝐅𝐀 ) or tension 𝐓 in a rope or string,
Friction (𝑭𝒇 ) (brake pads).
B. FIELD FORCES / NON-CONTACT FORCES
A field forces will have an effect even if the two objects
are not touching. These forces act at a distance or
through space.
Examples of field forces:
Gravity,
Magnetism,
Electrostatic fields.
OTHER CONTACT FORCES
FRICTION FORCE (𝑭𝒇 , 𝒇𝒐𝒓 𝒇𝒐𝒓𝒄𝒆 𝒐𝒇 𝒇𝒓𝒊𝒄𝒕𝒊𝒐𝒏)
• Definition: Friction is the force that resists the motion caused by
another force on an object.
Two objects must be in contact with each other and an external force
must be applied on the object in an attempt to move it.
If the object does not move, the friction is considered static. If the
force is sufficient to move the object, the friction is called kinetic.
Friction can be very useful, without friction we
will not be able to walk across a surface.
A disadvantage of friction is that a lot of heat
can be produced.
Friction is caused by a combination of (1) texture of the
surface, (2) what the object is made of (how hard or soft it is)
and (3) how its shape changes due to forces.
Three types of friction
1. Sliding friction
When two objects slide across each other.
Examples: rubbing your hands together, pushing furniture across
the floor
2. Rolling friction
When a rolling object moves across a surface.
(Only the area of the round object that is in contact with the
surface causes rolling friction.)
Examples: marbles, bowling ball or soccer ball rolling across the
pitch, wheels of a vehicle.
3. Fluid friction
When an object moves through a liquid or a gas. (A fluid can be a
liquid or a gas.)
Examples: swimming, a ball in the air, airplanes, birds flying,
oil-lubricated moving parts
MORE CONTACT FORCES
Tension (𝑻, 𝒇𝒐𝒓 𝒕𝒆𝒏𝒔𝒊𝒐𝒏)
Definition: Tension force is the pulling force that is transmitted
through a string, rope, cable or wire when it is pulled tight
by forces acting from opposite ends.
The tension force is directed along the length of the wire and
pulls equally on the objects on the opposite ends of the wire.
Compression (𝑭𝑨 , 𝒇𝒐𝒓 𝒂𝒑𝒑𝒍𝒊𝒆𝒅 𝒇𝒐𝒓𝒄𝒆)
Definition: Compression Force is the pushing force against an
object that causes it to become squeezed, squashed, or
compacted.
Objects routinely subjected to compression forces include
columns, gaskets, disc brakes, and the components of fuel cells.
TENSION AND COMPRESSION WORKING TOGETHER
ANOTHER CONTACT FORCE
Normal force (𝐹𝑁 𝑜𝑟 𝑁)
Definition: The Normal force is the perpendicular force (90°)
exerted by a surface on an object in contact with it.
(It can also be understood as the reaction force of a surface
perpendicular to an object that is in contact with it.)
REPRESENTING FORCES
Forces can be represented graphically by an arrow, where the
length of the tail of the arrow shows the size of the force and the
arrowhead points in the direction in which the force is exerting an
influence.
Tail
Arrowhead
FREE-BODY DIAGRAMS
(IEB)
• Draw the object of interest by making use of a dot.
• Draw the forces that act on the object by making use of arrows.
• The arrows are drawn from the dot pointing in the direction of
influence.
Example: Draw a free-body diagram
showing the forces on the book.
Reaction of table surface on
book (Normal force / N)
The dot represents the book.
Weight of book
(Fg = Gravitational force)
BALANCED & UNBALANCED FORCES AND
PAIRED FORCES
A. PAIRED FORCES
Forces occur in pairs
• Forces that two objects exert on each other, always occurs in
pairs.
• When an object A exerts a force on object B then object B
always exerts a force of equal magnitude (size) on object A, but
in the opposite direction (Newton’s 3rd Law: for every action
there is an equal and opposite reaction).
• The two forces act on different objects, A acts on B and B acts on
A
In the previous example of the book lying on the table, the
paired forces can be shown as follows:
𝑵
These two forces are paired
Force of table on book
Force of book on table
Force of earth’s gravity
Force of book on earth
on book
𝑭𝒈
Examples of forces that occur in pairs:
Example 1
Huang sits on a chair and exerts a downwards force (weight) on the
chair. The chair also exerts an upward force (normal force) on Huang.
The forces are:
• Equal in magnitude (size)
𝐹𝑐ℎ𝑎𝑖𝑟 𝑜𝑛 𝐻𝑢𝑎𝑛𝑔
• Acting in opposite directions
• Acting simultaneously on Huang and the chair
𝐹𝐻𝑢𝑎𝑛𝑔 𝑜𝑛 𝑐ℎ𝑎𝑖𝑟
The two forces act on different objects – one force
acts on the chair and the other force acts on Huang.
Example 2
If a book lies on a table, the force that the book exerts on the table is
the same size but in the opposite direction than the force that the table
exerts on the book.
The two forces act on different objects – one force acts on the book
and the other force acts on the table.
N
Fg
B. BALANCED FORCES
Two forces of equal magnitude (size) act on an object in opposite
directions. The forces are balanced and there is no visible effect
on the object.
Both the forces act on the same object: force A acts on the
object and force B acts on the same object.
Object
Force A
12 N
Force B
12 N
The resultant (sum) of the two forces is zero and there is no
visible effect on the object.
Choose direction towards the right as positive
Resultant force = +12 – 12 = 0 N
C. UNBALANCED FORCES
Two forces of different magnitude act on an object in opposite
directions. The forces are unbalanced and there is a visible effect
on the object.
Both the forces act on the same object: force A acts on the object
and force B acts on the same object.
Object
Force A
5N
Force B
8N
The resultant (sum) of the two forces is greater than zero and the
object will change its speed and position towards the right.
Choose direction to the right as positive (direction of largest
force)
Resultant force = +8 – 5 = +3 N towards the right
NON CONTACT FORCES
Definition: A force that pushes or pulls on an object without touching it
1. GRAVITY
• Would you be surprised if you let go of a pen you were holding
and it did not fall?
• You are so used to objects falling that you may not have
thought about why they fall.
• One person who thought about it was Isaac Newton.
• He concluded that a force acts to pull objects straight down
toward the center of Earth.
• Gravity is a force that pulls objects toward each other.
• Newton realized that gravity acts everywhere in the universe,
not just on Earth.
• It is the force that makes an apple fall to the ground.
• It is the force that keeps the moon orbiting around Earth.
• It is the force that keeps all the planets in our solar system
orbiting around the sun.
• What Newton realized is now called the law of universal
gravitation.
• The law of universal gravitation states that the force of
gravity acts between all objects in the universe.
• This means that any two objects in the universe, without
exception, attract each other.
• You are attracted not only to Earth but also to all the
other objects around you.
• However, you do not notice the attraction among
objects because these forces are small compared to the
force of Earth’s attraction.
Weight and Mass
• Mass is sometimes confused with weight.
• Mass is a measure of the amount of matter in an object; weight is the
measure of the gravitational force exerted on an object.
• The force of gravity on a person or object at the surface of a planet is
known as weight.
• So, when you step on a bathroom scale, you are determining the
gravitational force Earth is exerting on you.
• Weight varies with the strength of the gravitational force but mass does
not.
• Suppose you weighed yourself on Earth to be 450 newtons.
• Then you traveled to the moon and weighed yourself again.
• You might be surprised to find out that you weigh only about 75 newtons –
the weight of about 8 kilograms on Earth!
• You weigh less on the moon because the moon’s mass is only a fraction
of Earth’s.
• On Earth, gravity is a downward force that affects all objects.
• When you hold a book, you exert a force that balances the
force of gravity.
• When you let go of the book, gravity becomes an unbalanced
force and the book falls.
• When the only force acting on an object is gravity, the object is
said to be in free fall.
• An object in free fall is accelerating.
• Do you know why?
• In free fall, the force of gravity is an unbalanced force, which
causes an object to accelerate.
• How much do objects accelerate as they fall?
• Near the surface of Earth, the acceleration due to gravity is
9.8 m/s2.
• This means that for every second an object is falling, its
velocity increases by 9.8 m/s.
• For example, suppose an object is dropped from the top of a
building.
• Its starting velocity is 0 m/s.
• After one second, its velocity has increased to 9.8 m/s.
• After two seconds, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s)
• The velocity continues to increase as the object falls.
• While it may seem hard to believe at first, all objects in free fall
accelerate at the same rate regardless of their masses.
BUT…….
• Despite the fact that all objects are supposed to fall at the same
rate, you know that this is not always the case.
• For example, an oak leaf flutters slowly to the ground, while an
acorn drops straight down.
• Objects falling through air experience a type of fluid friction called
air resistance.
• Remember that friction acts in the direction opposite to motion,
so air resistance is an upward force exerted on falling objects.
• Air resistance is not the same for all objects.
• Falling objects with a greater surface area experience more air
resistance.
• That is why a leaf falls more slowly than an acorn.
• In a vacuum, where there is no air, all objects fall with exactly
the same rate of acceleration.
• You can see the effect of air resistance if you drop a flat piece of paper
and a crumpled piece of paper at the same time.
• Since the flat paper has a greater surface area, it experiences greater
air resistance and falls more slowly.
• In a vacuum, both pieces of paper would fall at the same rate.
• Air resistance increases with velocity.
• As a falling object speed up, the force of air resistance becomes greater
and greater.
• Eventually, a falling object will fall fast enough that the upward force of
air resistance becomes equal to the downward force of gravity acting on
the object.
• At this point the forces on the object are balanced.
• Remember that when forces are balanced, there is no acceleration.
• The object continues to fall, but its velocity remains constant.
• The greatest velocity a falling object reaches is called its
terminal velocity.
• Terminal velocity is reached when the force of air resistance
equals the weight of the object.
2. MAGNETISM




Magnets have 2 poles (north and south)
Like poles repel
Unlike poles attract
Magnets create a MAGNETIC FIELD
around them
Properties of Magnets
They attract many, but not all metals (iron, steel, cobalt and nickel)
They exert a force at a distance. They do not need to touch an object to
move it.
They always have two ends – a north pole and a south pole.
They can attract or repel another object.
They can make another object magnetic.
Magnetic Field: The space around a magnet in which a
magnetic force is exerted
The shape of a magnetic field is revealed by magnetic
field line, directed away from north poles and toward
south poles
The Earth’s Magnetic Field
• Currents in the molten part of Earth beneath the crust
create the Earth’s magnetic field - Moving charges loop
within the Earth
• Earth’s magnetic field is not stable - Magnetic pole and
geographic pole are offset
3. ELECTROSTATIC FORCE
All
matter is made up of atoms
Atoms contain
Protons (+)
Neutrons (0)
Electrons (-)
The law of electric charges states that like charges repel, and
opposite charges attract.
 Protons are positively charged and electrons are negatively
charged, so they are attracted to each other.
 Without this attraction, electrons would not be held in atoms.

ELECTRIC FORCE: The force between the charged objects
The size of the electric force depends on 2 things:
1.
2.
The amount of charge (the greater the charge, the greater the
force)
The distance between charges (the further the distance, the
less the force)
ELECTRIC FIELD is the region around a charged object where
electric forces can be exerted on another charged object.
(Repelled or attracted)
How Can You Charge Objects?
There are 3 ways objects can be charged:
1. Friction
2. Conduction
3. Induction
**In each of these, only the electrons move. The protons
stay in the nucleus**
CONDUCTION
INDUCTION
FRICTIONN
Charging by conduction
Charging by induction
Charging by friction
happens when electrons
happens when charges in
occurs when electrons
an uncharged object are
are “wiped” from one
rearranged without direct
object onto another.
move from one object to
another through direct
contact (touching).
eg. Suppose you touch an
contact with a charged
uncharged piece of metal
object.
with a positively charged
eg. If you charge up a balloon
glass rod. Electrons from
through friction and place the
the metal will move to the
balloon near pieces of paper, the
glass rod. The metal loses
electrons and becomes
positively charged.
charges of the paper will be
rearranged and the paper will be
attracted to the balloon.
eg. If you use a cloth to
rub a plastic ruler,
electrons move from
the cloth to the ruler.
The ruler gains
electrons and the cloth
loses electrons.
LIGHTNING
Research and write a
paragraph on what
lightning is and how to
stay safe during a
lightning storm