Common Misconception Fact Objects which are not moving are at rest. Objects which are not moving with time and with respect to the observer are a rest. E.g. Earth appears to be at rest to us, but for an observer in the outer space the earth is in motion. Distance and displacement travelled by the body depends 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 life, we observe vehicles moving on a road, the motion of the birds in the sky, the motion of the blades of fan etc., in all these examples we observe that the bodies are under motion. In the contrary, a body can also be under rest.

To understand rest and motion their basic definition need to be defined. The basic definition 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 with respect to surroundings and time is said to be motion.

Rest

When a body is not changing the position with respect to 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).

However, to a person in a moving train, both of you appear to be moving (this surrounding is different). In a moving bus, the passenger who sits along with you is at rest, however to a person on the road, both of you appear to be in motion. Therefore, rest and motion are relative terms. They are not absolute terms.

Different kinds of motion

Motion can be classified based on periodicity (repetitiveness) as periodic motion 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.

c)     Heart beats of a normal healthy person.

d)    The needle of a sewing machine running at constant speed.

Periodic motion can be 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 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 which 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 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 as translator motion.

Translatory motion

The motion in which all the particles of a body move through the same distance in 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 body is the shortest distance travelled in a direction from initial to final point. SI unit of displacement is the meter (m) while its CGS unit is centimeter (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 balls is thrown vertically up about 5m high and catch 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 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 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 CGS system, the unit of acceleration is cm s-2. In 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 takes 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 with respect to time, the acceleration is said to be negative, which is called deceleration or retardation.

Concepts of Rest, Motion and Speed Reviewed by knowledge people creators on November 18, 2021 Rating: 5