CBSE Class 6 Science Motion and Measurement of Distances Exam Notes

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Motion and Measurement of Distances

WHAT IS MOTION
We see many things moving around us. A boy walking along a road, a man running a race, a boy driving a car, a speeding truck.
In all these examples you will observe that the bodies keep on changing their position with respect to other objects. When we consider whether a body is moving or not, we usually take stationary objects such as trees, houses, electric poles on the surface of the earth as references.

REST
A body is said to be at rest (or in a state of rest) when it does not change its position with respect to its surrounding objects. For example, buildings, plants are all at rest.

TYPES OF MOTION

Translatory Motion: Translatory motion is that in which all the particles of a body moves along a straight path with same speed. The motion of a drawer of a table, a moving car or train, the movement of an aeroplane, a boy sliding down a slope, stone hurled from a catapult are some examples of translatory motion.

When a body moves along a straight line, it is called rectilinear motion. When a body moves along a curved
line, it is called curvilinear motion.

Circular Motion: When an object moves about a fixed point keeping the distance same, then the object
is said to be an circular motion. Motion of earth about sun, motion of moon about earth are the examples of circular motion.

Rotatory Motion: Rotatory motion is that in which a body moves about a fixed axis without changing its
position.

In some cases, you find that translatory and rotatory motions occur simultaneously. The wheels of a bicycle, car, train, bus, truck, perform rotatory motion along the axis and at the same time perform translatory motion on the road. Rolling is a combination of rotational and translatory motions.

Oscillatory Motion: Oscillatory motion is that in which a body moves to and fro about its mean position.
Observe the things that perform oscillatory motion in your surroundings.

The motion of a swing and the pendulum of a clock show oscillatory motion.

Periodic Motion: Periodic motion is that which repeats itself after regular intervals of time. The motion of the Moon round the Earth, the motion of a clock pendulum, heartbeat, and pulse movement are a few examples of periodic motion. Periodic motion can be used to measure time intervals. Can you say
why?

Measurements
Measurement is one of the most useful processes in science and in our daily life. We all make measurements in our daily life whether we are in a factory, laboratory or a shop or at home.

Quantities: Any thing that we measure is called as quantity. Like we measured distance and time so, these are quantities.
The quantities are of two types depending on their dependency. These are
i) Fundamental quantities (independent)
ii) Derived quantities (dependent on fundamental quantities)
S.I. system is known as the standard system of measurement or International system of measurement.

Following are the fundamental quantities in this system:  

But some other quantities like speed, density, force are depending on the fundamental quantities, like:

So, these quantities are known derived quantities. The other derived quantities are, acceleration, volume, pressure, power work, energy etc.

Depending on direction, quantities are further classified as,

a) Vectors (has a sense of direction and magnitude) b) Scalars (has no sense of direction but have magnitude only)

More about some quantities to understand motion

Distance and Displacement: Consider an object moves in a curvilinear path from A to B as shown below:

Here the object moves along A x y z B. The total length of the path is called distance.

But the shortest distance from the critical position A to the final position B is called the displacement of the object. There is a clear sense of direction when we describe the term displacement. Hence displacement is a vector quantity where as distance is a scalar quantity.

Speed and Velocity:

Speed: It is the distance travelled by any object in unit time.

So, v = d/t

For example, an object travels 10 m in 5 sec. then its speed = 10m / 5sec= 2 m/s

Velocity is the speed in a particular direction.

Consider these two cares are moving in opposite directions. One may say, one of them is moving in (+) direction and the other in (–) direction. Thus, speed and direction together is called as velocity. Therefore, we say theri speeds are respectively 8 m/s and 10 m/s but the velocities are +8 m/s and –10 m/s. Note that speed is scalar quantity and velocity is a vector quantity.

Acceleration: If in a motion the velocity of an object changes then it is called a non-uniform motion or accelerated motion. Here, acceleration is the rate of change of velocity.

For example, let an object is moving with an initial velocity (u) of 5 m/s and after some time t = 5 sec its final velocity (v) becomes 15 m/s. The rate of change in velocity.

= (v - u) /t = (10 m / s) / 5s= 2 m/s2

Thus acceleration (a) can be expressed as a = (v - u) / t , and its unit is m/s².

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