# KPK Board 10th Class Physics Ch 1 Simple Harmonic Motion & Waves Short Questions Answers

## KPK Board 10th Class Physics Ch 1 Simple Harmonic Motion & Waves Short Questions Answers

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Ans. No, it is not necessary for an oscillatory motion to be simple harmonic. In oscillatory motion a body moves to and fro about a fixed point periodically. e . g. The motion of tuning fork, swing etc. While for SHM, the following two conditions must be satisfied.

The acceleration of the vibrating body is directly proportional to the displacement from the mean position at any instant, and

The acceleration is always directed towards the mean position. For example

i Vibratory motion of simple pendulum. ii Vibratory motion of a mass-spring system.

It is to be noted that every SHM is oscillatory but every oscillatory motion is not necessary a SHM.

Ans For a particle execution SHM its total energy at any instant of time is constant. That is the sum of K.E and P.E remains the same all the time. Since K.E of the particle passing through mean position is maximum ( equal to total energy) so at this position the velocity of the particle will be maximum.

When the particle is the either extreme position. The total energy of the particle is in the form of P.E and its K.E is equal to zero. As K.E is zero at other extreme position, so the velocity of the particle is also zero at these positions.

Ans. Yes the restoring force in SHM becomes zero at the mean position, According to hook’s law, we have

F = – Kx —————–> (1)

In equation (1) ‘x’ represents the displacement to vibrating body from mean position. Now at the mean position, we have, x = 0

So equation (1) becomes

F = – K ( 0 ) => F = 0 —————–> ( 2 )

Equation (2) shows that the restoring force is zero at the mean position.

Ans. By decreasing length of a simple pendulum to half of its original length, the time period will decrease √2 time while frequency will increase √2 time.

Mathematical proof: Time period of simple pendulum is given by,

Ans. The length of rope can be found by finding time period of pendulum and then use the using the mathematical relation for the time period of simple pendulum to find length of the rope.

Explanation: The time period of simple pendulum is given by

In equation1, g and are constants. T is +found experimentally by attaching a small bob to the lower and of the rope and starting oscillatory motion in it.

Ans. The frequency of oscillation will increase when you stand on a swing instead of sitting on it.

Explanation: As you stand on the swing the length of the pendulum decreases because you are a part of the bob (mass of person in this case). By the mathematical relation of frequency, length and frequency are inversely proportional to each other given by,

F = 2

So decreasing length will increase frequency of the swing and decrease its time period.

Ans. The speed of the wave is how fast the disturbance moves along the cord. This speed is constant for a uniform cord as long as the tension remains constant. As the wave travels along the card it disturbs tiny parts of the card which move perpendicularly to the direction of move motion. The tiny part of the cord continue its motion up and down and do not change their positions.

So, motion of waves and motion of small parts are perpendicular to each other as shown in the figure.

Ans. Water waves refract at the boundary of shallow and deep water because the speed of waves in water is influenced by the depth of water.

Explanation: When water enters from deep to shallow part of the river / pond, then its speed and wavelength decreases while frequency remains the same as shown in the figures. When water enters from shallow part of the river / pond to deeper part, its speed and wavelength increases and frequency remains the same as shown in the figure.

Ans. When the opening is made small (but comparable with the wavelength of opening) will became greater as shown in the below figure.

Explanation: As the opening is made smaller in figure “B” as compared to figure “A”, large diffraction effect is observed.