# Physics O Level Short Note 1

```1. Physical Quantities, Units and
Measurement
 Three basic quantities we measure in physics
are length, mass and time. Units for other
quantities are based on them. The SI (System
International) system is a set of metric units now
used in many countries. It is a decimal system in
which units are divided or multiplied by 10 to
give smaller or larger units.
 Many length measurements are made with rulers.
Our eye must be directly over the mark on the
scale or the thickness of the ruler causes a
parallax error. Lengths can be measured with a
ruler to an accuracy of about 1 mm.
 The number of figures, called significant figures,
given for a measurement indicates how accurate
we think it is and more figures should not be
given than is justified. In deciding the least
significant figure, rounding off should be used. If
a number is expressed in standard notation, the
number of significant figures is the number of
digits before the power of ten. For example,
2.73 × 103 has three significant figures.
 The volume of a liquid may be obtained by
pouring it into a measuring cylinder. When
making a reading both vessels must be upright
and your eye must be level with the bottom of the
curved liquid surface, i.e. the meniscus.
 The meniscus formed by
mercury is curved oppositely to
that of other liquids and the top
 A scalar quantity has only magnitude. A vector
quantity has both magnitude and direction. Work
Done, Kinetic Energy (KE), Gravitational Potential
Energy (GPE) and Power are all Scalar quantities.
 The main use of the Vernier calliper is to measure
the internal and the external diameters of an object.
Vernier scales are also used on barometers,
travelling microscopes and spectrometers. The
simplest type enables a length to be measured to
0.01 cm.
 If Main scale reading: 10.0 cm (Immediate left of
zero) and Vernier scale reading: 0.02 cm (Alignment
of scale lines) the Measurement reading is : 10.02 cm
 Zero error is defined as such a condition when a
measuring instrument registers a reading when
there should not be any reading. In case of Vernier
callipers it occurs when a zero on main scale does
not coincide with a zero on Vernier scale. Rather the
zero error may be of two types i.e. when the scale is
towards numbers greater than zero it is positive else
negative. The method to use a vernier scale or
caliper with zero error is to use the formula:
Actual reading = main scale + vernier scale
− (zero error)
 Zero error may arise due to knocks that cause the
calibration at the 0.00 mm when the jaws are
perfectly closed or just touching each other.
 Micro meter screw gauge measures very small
objects to 0.001 cm. Before making a measurement,
check to ensure that the reading is zero when the
jaws are closed.
Prepared by OIA Lecturer: Mrs. Ruvini Wanniarachchi (University of Kelaniya) / 2017 September OL Revision
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 A tickertape timer also enables us to measure
speeds and hence accelerations. The ‘tentick’ (1/ 5
s) is also used as a unit of time.
 To convert km/h into m/s, multiply the number by 5
and then divide it by 18.
 stopping distance = thinking distance + braking
distance
 The area under a velocity–time graph measures
the distance travelled. The slope or gradient of a
velocity–time graph represents the acceleration of
the body
 The slope or gradient of a distance–time graph
represents the velocity of the body.
2. Kinematics
 Motion can be described in terms of
displacement, velocity, acceleration and so on.
This is known as kinematics.
 Distance moved in a stated direction is called the
displacement.
 Speed is the distance travelled in unit time
 Velocity is the distance travelled in unit time in a
stated direction.
 Acceleration is the change of velocity in unit
time. Acceleration is positive if the velocity
increases and negative if it decreases. A negative
acceleration is also called a deceleration or
retardation.
3. Dynamics
Whenan object moves in terms of the forces which
change its motion that is known as dynamics.
 Gravitational acceleration is the acceleration on
an object caused by
the
force
of gravitation.
 The velocity of a
free-falling
body
increases by 10 m/s
every second. In
calculations
using
the equations of
motion, g replaces a.
It is given a positive sign for falling bodies (i.e. a = g
= +10 m/s2) and a negative sign for rising bodies
since they are decelerating (i.e. a = −g = –10 m/s2).
 The acceleration of free fall on the Moon is about
2
one sixth of that on the Earth
Prepared by OIA Lecturer: Mrs. Ruvini Wanniarachchi (University of Kelaniya) / 2017 September OL Revision
 A tendency to do nothing or to remain unchanged
is called inertia.
Newton's
first
law of motion is
sometimes
referred to as
the law
of
inertia.
 F M*A is a simpliﬁed version of Newton’s second
law of motion.
 Newton's first law states that every object will
remain at rest or in uniform motion in a straight
line unless compelled to change its state by the
action of an external force.
 Newton's second law of motion can be formally
stated as follows: The acceleration of an object as
produced by a net force is directly proportional
to the magnitude of the net force, in the same
direction as the net force, and inversely
proportional to the mass of the object.
 Newton's third law is: For every action, there is
an equal and opposite reaction.
 The constant speed that a freely falling object
eventually reaches when the resistance of the
medium through which it is falling prevents
further acceleration is terminal velocity.

Friction is the force resisting the relative motion of
solid surfaces, fluid layers, and material elements
4. Mass, Weight and Density
 The weight of a body is the force of gravity on it.
 The weight of a body can be measured by hanging it
on a spring balance marked in newtons and letting
the pull of gravity stretch the spring in the balance.
The greater the pull, the more the spring stretches.
 Density is the mass per unit volume. The SI unit of
density is the kilogram per cubic metre. To convert
a density from g/cm3, normally the most suitable
unit for the size of sample we use, to kg/m3, we
multiply by 103. For example the density of water is
1.0 g/cm3 or 1.0 × 103
kg/m3.
 In Regularly shaped solid
the mass is found on a
balance and the volume by
measuring its dimensions
with a ruler.
sliding against each other.
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Prepared by OIA Lecturer: Mrs. Ruvini Wanniarachchi (University of Kelaniya) / 2017 September OL Revision
 Irregularly shaped solid, such as a pebble or
glass stopper the mass of the solid is found on a
balance. Its volume is measured by measuring
cylinder or Water displacement Method can be
used.
 An object sinks in a liquid of lower density than
its own; otherwise it floats, partly or wholly
submerged. An iron nail sinks in water but an
iron ship floats because its average density is less
than that of water.
 A force is a push or a pull. The weight of a body
of mass 1 kg is 9.8 N
 The centre of gravity of an object is deﬁned as
the point where all the weight of the object may
be considered to act.
from diﬀerent holes. The centre of gravity will be
at the point of intersection of the lines drawn on
the object.
5. Turning Effect of Forces
 The quantity which tells us about the turning eﬀect
of a force is its moment. The moment of a force
depends on two quantities: the magnitude of the
force (the bigger the force, the • greater its moment)
& the perpendicular distance of the force from the
pivot • (the further the force acts from the pivot, the
greater its moment).
 Moment of a force = F × perpendicular distance
Of the pivot from the line of action of the force
 When finding the centre of gravity of a thin
sheet, or lamina, of cardboard or metal can be
found by suspending it freely from two or three
points. Small holes are made round the edge of the
irregularly shaped object. A pin is put through one
of the holes and held ﬁrmly in a clamp and stand so
the object can swing freely. A length of string is
attached to the pin. The other end of the string has
a heavy mass attached to it. This arrangement is
known as a plumb line. The object will stop
swinging when its centre of gravity is vertically
below the point of suspension. A line is drawn on
the object along the vertical string of the plumb
line. The centre of gravity must lie on this line. To
ﬁnd the position of the centre of gravity, the
process is repeated with the object suspended
 Principle of moments: For any object that is in
equilibrium, the sum of the clockwise moments
about any point provided by the forces acting on the
object equals the sum of the anticlockwise moments
 A couple is two equal forces which act in opposite
directs on an object but not through the same point
so they produce a turning effect. The moment
(or torque) of a couple is calculated by multiplying
the size of one of the force (F) by the perpendicular
distance between the two forces (s). To form a
couple, the two forces must be: equal in magnitude•
parallel, but opposite in direction• separated by a
distance •
 The work done by a force is deﬁned as the product
of the force and the distance moved in the direction
of the force:
W=F×S
where s is the distance moved in the direction of the
force.
 work done = energy transferred
 A joule is deﬁned as the work done (or energy
transferred) when a
force of 1 N moves a
distance of 1 m in
the direction of the
force.
 Internal energy is
the sum of the
random
potential
and kinetic energies
of all the molecules
in a body.
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Prepared by OIA Lecturer: Mrs. Ruvini Wanniarachchi (University of Kelaniya) / 2017 September OL Revision
 Energy which a body possesses by virtue of being
in motion is known as Kinetic Energy
stretching
a
material. The
elastic limit is
the
point
beyond which
the material
you
are
stretching
becomes
permanently
stretched so
that
the
when the force is removed.
7. Pressure
 Principle of conservation of energy: Energy
cannot be created or destroyed. It can only be
converted from one form to another.
 The watt is deﬁned as a rate of working of 1 joule
per second.
 In all collisions (where no external force acts)
there is normally a loss of kinetic energy, usually
to heat energy and to a small extent to sound
energy. The greater the proportion of kinetic
energy lost, the less elastic is the collision, i.e.
the more inelastic it is. In a perfectly elastic
collision, kinetic energy is conserved.
 Pressure is deﬁned as the normal force acting per
unit cross-sectional area: ( F/A)
 Pressure in a ﬂuid increases with depth
 Pressure in a fluid at one depth acts equally in all
directions.
 Pressure in a fluid at same level is equal
 A barometer is a manometer which measures
atmospheric pressure. (76 cmHg)
 Pressure is transmitted in hydraulic systems with
particular reference to the hydraulic press and
hydraulic brakes on vehicles.
6. Deformation
 Hooke's law is a principle of physics that states
that the force (F) needed to extend or compress a
spring by some distance X scales linearly with
respect to that distance.
 The limit of proportionality is the point beyond
which Hooke's law is no longer true when
Prepared by OIA Lecturer: Mrs. Ruvini Wanniarachchi (University of Kelaniya) / 2017 September OL Revision
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