Common Misconception |
Fact |
Objects which are
not moving are at rest. |
Objects that are
not moving with time and concerning the observer are a rest. The Earth
appears to be at rest to us, but for an observer in outer space, the Earth
is in motion. |
Distance and
displacement travelled by the body depend on the path that it follows. |
Only the distance
travelled by the body depends on the path that it follows. It will be equal
to the magnitude of the displacement only if the path is straight. |
In our daily lives, we observe vehicles moving on a road, the
motion of the birds in the sky, and the motion of the blades of fans etc., in all
these examples we observe that the bodies are in motion. On the contrary, a
body can also be under rest.
To understand rest and motion their basic definition need to
be defined. The basic definitions of motion and rest are defined in this
chapter. Apart from this, the physical quantities related to the motion are
studied under this topic.
Motion
A body is said to move only when it changes its position in
space from one point to another point. In physics, just changing the position
is not motion. Change of position concerning surroundings and time is said
to be motion.
Rest
When a body is not changing its position concerning
surroundings and time it is said to be at rest. To make it clear, it can be
understood by the following example: when you are standing on a railway
platform in front of another standing person, you are at rest (in this
surrounding).
Different kinds of
motion
Motion can be classified based on periodicity
(repetitiveness) as periodic motion or non-periodic motion
Periodic motion
A repetitive motion, which repeats itself after a fixed
interval (or regular interval) of time is called the periodic motion.
Examples of periodic
motion
a)
The
swinging pendulum of a wall clock.
b)
The
piston of a motor car engine running at a constant speed.
d) The needle of a sewing machine running at a constant speed.
Periodic motion can be of two types:
1.
Rotatory
motion
2.
Oscillatory
motion
Rotatory motion
When the moving body describes a curve, the body is under
rotatory motion. It is the motion in which the body moves about a fixed axis
without changing its position is called the rotatory motion.
Examples of Rotatory
motion
a)
Motion
of the blades of the ceiling fan.
b)
Motion
of the potter’s wheel.
c)
Motion
of a spinning top.
d)
Motion
of planets around the sun.
Oscillatory/vibratory
motion
The ‘to-and-fro’ or ‘back and forth’ motion described by an
object as a whole, along the same path, without any change in the shape of the
object is called oscillatory motion.
Examples of Oscillatory
motion
a)
The
motion of the bob of the simple pendulum.
b)
The
up and down motion of the needle of the sewing machine.
c)
The
vertical oscillation of a loaded spring (weight attached to a spring).
d)
Motion
of the molecules in a solid.
Vibratory motion
Vibratory motion is another kind of oscillatory motion in
which the body does not move as a whole, i.e., the entire body does not move.
The moving object undergoes a change in shape or size.
Examples of Vibratory
motion
Musical instruments such as guitar, violin, sitar, drums,
etc., produce vibratory motion.
Non-periodic motion
A motion that is not repetitive or repeats at irregular
intervals of time is called non-periodic motion.
Examples of
Non-periodic motion
a)
A
football player running in the field up and down is performing non-periodic
motion as sometimes he runs fast and sometimes slows.
b)
When
the brakes are applied to a moving vehicle, the motion described by the wheels is
non-periodic.
c)
A
cricket ball rolling down the ground gradually slows down and finally stops and
hence, is performing non-periodic motion.
Non-periodic motion is also termed translator motion.
Translatory motion
The motion in which all the particles of a body move through
the same distance at the same time is called translator motion.
Examples of Translatory
motion
a)
A
car or a train moving along a road.
b)
A
ball rolling on the ground.
c)
A
girl sliding down a slope.
d)
Pulling
out a drawer of a table.
e)
Firing
of a bullet from a gun.
f)
A
stone hurled from a sling or catapult.
The motion of translation is of two
kinds:
1. Rectilinear motion
2. Curvilinear motion
Rectilinear motion
When a body moves along a straight line, the motion described
by the body is rectilinear motion.
Examples of Rectilinear
motion
a)
A
train moving on a straight railway track.
b)
A
car moving on a straight road.
c)
A
freely falling stone.
d)
A
coin moving over a carom board.
Curvilinear motion
When a body moves along a curved line, the motion described
by the body is called curvilinear motion.
Examples of curvilinear
motion
a)
A
ball thrown upwards at an angle.
b)
A
car or train moving along a curved road or track.
Distance
Distance travelled by a body is the length of the path taken
irrespective of the direction of motion. SI unit of distance travelled is meter
(m).
Displacement
Displacement of the body is the shortest distance travelled in a
direction from the initial to the final point. SI unit of displacement is the meter (m)
while its CGS unit is the centimetre (cm).
If your house is situated far away from your school,
different paths (routes) you travel to reach the school constitute distances.
On the other hand the straight line path (actual distance) between your house
and school can be termed as displacement.
Few more examples
When the ball is thrown vertically up about 5m high and catches it
again, the total distance the body travels is 10m and the total displacement is
zero, as it returns to the same point.
Speed
The distance covered by a body in unit time. It is the rate
at which the body is moving. It means how fast or slow the body is moving.
Speed is measured in S.I. units, as mps (meter per second) or
m s-1.
Though we can measure distance in several units, the standard
expressions used are as follows:
Meter per second - mps
(m s-1)
Kilometer per hour - kmph (km h-1)
Kinds of Speed
Average speed
When a body starts with a certain speed and changes its
speed after some time and completes the journey. Then the average speed implies
the overall rapidity or speed of the body.
Uniform speed (constant speed)
If a body covers equal distances in equal intervals of
time however small these intervals may be, the body is said to be under uniform
speed or constant speed.
Nonuniform speed
(Variable speed)
When a body covers either unequal distances in equal
intervals of time or equal distances in unequal intervals of time or both, the
body is said to be under nonuniform or variable speed. For example, A body
starts at a point ‘p’ and travels as shown in the figure.
Kinds of velocity
a) Uniform Velocity
When a body covers equal distances in
equal intervals of time in a specified direction is called uniform velocity.
However small these time intervals may be, the body is said to be moving with a
uniform velocity.
A body will have a uniform velocity only, if
It covers equal distances in equal intervals of time, i.e., the magnitude does not change. Its direction remains the same.
If any of the two conditions is not fulfilled,
then the body will not be moving with a uniform velocity, but with a variable
velocity.
b Variable Velocity
When a body covers unequal distances
in equal intervals of time in a specified direction or equal distances in equal
intervals of time, but its direction changes then the body is said to be moving
with a variable velocity.
c) Average Velocity
The average displacement of a body
per unit time, when the body is actually moving with variable velocity, is
called average velocity.
Acceleration
The rate of change of velocity is called the acceleration.
In the CGS system, the unit of acceleration is cm s-2.
In the SI system, the unit of acceleration is m s-2.
When the velocity changes either increasing or decreasing
then the body is said to be accelerating or retarding (decelerating).
Kinds of Acceleration
Uniform acceleration
If equal changes in velocity take place in equal intervals
of time, however small the time intervals may be, the body is said to be
travelling with uniform acceleration.
Example: A freely falling body possesses uniform
acceleration.
Non-uniform
acceleration
If an unequal change in velocity takes place in equal
intervals of time, then the body is said to be moving with non-uniform
acceleration.
Negative acceleration
If the velocity of the body gradually decreases concerning time, the acceleration is said to be negative, which is called deceleration or retardation.