Chapter 3

Forces

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Forces
A force is either a push or pull that one object exerts on
another object. A force is a vector quantity. The SI unit is
Newton (N). It can:
 Produce motion

 Stop a moving object

 Slow down or speed up a moving object

 Change the direction of motion

Types of Forces:
Forces are produced by the interaction between objects. There are
two types of forces:
Contact Forces, which exist between objects that are in contact. 2
Forces
Non-contact Forces,
which do not require
objects to be in
contact to exist.

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Vector Diagrams
At any time, two or more forces may be acting on an object. The
forces may have different magnitudes and directions. In such
cases, vector diagrams can be used to add up these vectors.
In a vector diagram, a vector quantity is represented by an
arrow. The length of the arrow is
proportional to the magnitude
of the vector. The direction of
the arrow indicates the
direction of the vector.
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Addition of Parallel Vectors

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Addition of Non-Parallel Vectors (at right angle)

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Addition of Non-Parallel Vectors (at right angle)

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Balanced Forces & Newton’s First Law
Newton’s first law states that every object will continue in its
state of rest or uniform
motion in a straight
line unless a resultant
force acts on it.
If the resultant force
acting on an object is
zero, the forces acting
on the object are
balanced.
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Unbalanced Forces & Newton’s Second Law
Newton’s second law states that when a resultant force acts
on an object of a constant mass, the object will accelerate in
the direction of the resultant force. The product of the mass
and acceleration of the object gives the resultant force.
When there is a resultant force acting on an object, the object
will accelerate in the direction of the resultant force. The
relationship between resultant force, mass and acceleration is
described by Newton’s Second Law of Motion. Its
mathematical expression is:
F=ma where F= resultant force (in N), m= mass of object , a= acceleration of object (m/s2)
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Unbalanced Forces & Newton’s Second Law
If the resultant force acting on an object is not zero, the forces
acting on the object are unbalanced.

 Worked example 3.2, 3.3 10

Newton’s Third Law
Newton’s third law of motion states that when object A exerts a
force on object B, then object B exerts an equal and opposite
force on object A. They act on mutually opposite bodies.
Newton’s third law of motion tells
us four characteristics of forces:
 Forces always occur in pairs.
Each pair is made up of an
action and a reaction.
 Action and reaction are equal
in magnitude.
 Action and reaction act in
opposite directions.
 Action and reaction act on
mutually opposite bodies. 11
Newton’s Third Law

 Test Yourself 3.312

Friction & its Effects
Friction is the contact force that oppose or tends to oppose
motion between surfaces in contact. It is the result of
irregularities of the surfaces.
Effects of Friction:

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Friction & its Effects

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Friction & Braking
Static Friction:
Static friction is the force of friction on an object that is not
moving. If you push on a stationary block and it doesn't move,
it is being held by static friction which is equal and opposite to
your push.
Dynamic Friction:
Kinetic friction, also known as dynamic friction or sliding
friction, occurs when two objects are moving relative to each
other and rub together.
Static friction is always more than the kinetic friction. 15
Friction & Braking
Stopping Distance:
Stopping distance is the distance covered during the time
that it takes to bring a moving car to a complete stop. This
includes
 Thinking distance. This is how far the car travels before the

brakes are applied or the time it takes you to react.

 Braking distance. This is how far the car travels after the brakes

have been applied or the time it takes for the brakes to stop the
car.
It can be calculated with this stopping distance formula:
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Stopping distance = thinking distance + braking distance
Friction & Braking
It takes an average person about half a second to react and
press a brake pedal. This is the driver’s reaction time. However;
it can be affected by:
 Drugs and alcohol

 Distractions

 Tiredness

The braking distance however depends upon following factors:

 Brakes: The condition of the car's brakes will affect braking

distance. Worn out brakes will increase the distance.

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Friction & Braking
 Tyres: New tyres have better wet and dry grip than old, worn
out tyres; due to more friction between tyre and road.
 Weather conditions: If the road is wet or icy, friction will
decrease and this will significantly increase braking distances.
 Road conditions: A damaged or muddy road surface will
increase braking distance; due to less friction.
 Weight: Braking distance will also increase if the car is heavier.
 Speed: Greater the speed of the car, greater would be the
braking distance.

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Free-Body Diagrams
Simple block diagrams with arrows to represent forces acting on
individual objects, are
called free-body diagrams.
While solving problems
about forces, forces acting
on individual objects
need to be identified.
Drawing a free-body
diagram of an object helps
to identify and visualise
the forces and their effects 19

on the object.
Moving in Circles
Centripetal Force:
An object moving in a circle is experiencing an acceleration. Even if
moving around the perimeter of the circle with a constant speed,
there is still a change in velocity and subsequently an acceleration.
This acceleration is directed towards the center of the circle. And in
accord with Newton's second law of motion, an object which
experiences an acceleration must also be experiencing a net force.
The direction of the net force is in the same direction as the
acceleration. So for an object moving in a circle, there must be an
inward force acting upon it in order to cause its inward acceleration.
This force is referred to as the centripetal force. 20
Moving in Circles
The inward force needed to make an object move in a circle is
called the centripetal force.
More centripetal force is needed if:
 The mass of the object is increased.

 The speed of the object is increased.

 The radius of the circle is reduced.

Moving objects will tend to naturally travel in

straight lines; an unbalanced force is required
to cause it to turn. Thus, the presence of an unbalanced force is
required for objects to move in circles.
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Stretching & Compressing
Elasticity is a physical property of a material whereby the material
returns to its original shape after having been stretched out or
altered by force. Substances that display a high degree of elasticity
are termed "elastic." However, they stop being elastic if bent or
stretched too far. They either break or become permanently
deformed.

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Hooke’s Law
In the 1660s, Robert Hooke investigated how springs and wires
stretched when loads were applied. He found, that for many
materials, the extension and load were in proportion, provided the
elastic limit was not exceeded..
Hooke's law states that the deformation (extension) of an elastic
object or material is proportional to the stress (load) applied to
it; within the elastic limit.
Mathematically, Hooke’s law can be stated as:
Load  Extension
Load = spring constant x extension
In symbols: F = kx 23
Hooke’s Law
A material obeys Hook’s law if, beneath its elastic
limit, the extension is proportional to load.
Steel wires do not stretch as much as the steel
springs, but they obey Hooke’s law. Glass and
wood also obey the law, but rubber does not!

 https://youtu.be/yAIb3T9DPyE 24
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