NCERT Solutions For Class 7 Science

Class 7 science 1. INTRODUCTION Oscillation Oscillation is the repetitive motion performed about a central point called as mean position.

Vibration The to and fro or back and forth motion of an object is called vibration Note: Oscillation and vibration are same.

Example: pendulum of wall clock oscillating about the lower most point.

2. WAVES In physics, a wave is a disturbance (an oscillation) that travels through space as time passes. It is accompanied by transfer of energy.

When a pebble is thrown into still water circular ripples are formed which spread out in all directions on the surface of water from the point where the stone hit the water surface. Thus, the kinetic energy of the stone is transferred to the water and that energy is distributed to the entire water in the pond in the form of ripples or waves. To check whether water moves along with ripples produced or not, we can observe a floating object like cork or a leaf placed on the surface of water. As the ripple move in all possible direction on the surface of water from the point where the disturbance is produced, the leaf which is floating on the surface of water vibrates up and down, but does not have lateral translatory motion along the surface of water.

We even observe that the leaf does not start vibrating till the first ripple reaches it from the point of disturbance.

This is the characteristic of the propagation of waves.

The energy is transmitted from one point to another without actual translator motion or transport of the particles across the medium. Thus a “wave is a disturbance produced at a point in a medium or a field and is transmitted to other parts of the medium or the field without the actual translatory motion of the particles” The transfer of energy in the form of waves is known as “wave motion”.

3.1 Classification of Waves We can classify waves in two categories on the basis of medium for their propagation where medium of wave is the region in which wave exists. For example: air, water, iron etc.

Some wave requires medium to propagate whereas some propagate without any medium i.e. in vacuum as well.

Vacuum is medium with no matter. It is also referred as no medium.

(i) Mechanical Waves: The waves which require medium for their propagation are called mechanical waves or elastic waves. For example: waves in stretched string or water waves. (ii) Non Mechanical Waves: The waves which do not require medium for their propagation are called nonmechanical waves. For example: Light and radio waves, �-rays, X-rays etc, are common examples of such waves.

Now we can further classify waves in two parts on the basis of particle and wave motion.

We can also classify waves on the basis of nature of propagation as Transverse Waves and Longitudinal Waves. (i) Transverse Waves: If the particles of medium vibrate about their mean position in a direction perpendicular to the direction of wave propagation, wave is called transverse wave. For example: wave produced in string is transverse in nature.

It propagates in the form of crests and troughs. The particles having maximum upward displacement are called crests and particle having maximum downward displacement are called troughs. All electromagnetic waves e.g. light and radio waves are transverse in nature. (ii) Longitudinal Waves: If the particles of medium vibrate about their mean positions in the direction of wave propagation, the wave is called longitudinal waves. These are propagated in the form of compression and rarefaction and also known as pressure or compression waves. Waves on spring and sound waves in air are common examples of longitudinal waves.

A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium. We will see further analysis while studying sound because sound wave is a longitudinal wave.

2.2 Terms Related to Wave Motion The snap shot (photograph of an instant) of a transverse wave can be represented as shown in the figure

Crest: It is the point of maximum displacement of a particle in upward direction.

Trough: It is the point of maximum displacement of a particle in downward direction.

Amplitude: It is the maximum displacement of the particles either upwards or downwards.

Wavelength ( l ): It is the distance between any two successive crests or troughs. "

Wave number: It is the reciprocal of wavelength Îť

Time period (T): It is the time required for one oscillation.