# KPK 11th Class Physics Chapter 5 Rotational and circular Motion Short Questions Answers

KPK 11th Class Physics Chapter 5 Rotational and circular Motion Short Questions with answers are combined for all 11th class(Intermediate/hssc) Level students.Here You can prepare all Physics Chapter 5 Rotational and circular Motion short question in unique way and also attempt quiz related to this chapter. Just Click on Short Question and below Answer automatically shown. After each question you can give like/dislike to tell other students how its useful for each.

Class/Subject: 11th Class Physics

Chapter Name: Rotational and circular Motion

Board: All KPK  Boards

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## KPK 11th Class Physics Chapter 5 Rotational and circular Motion Short Questions Answers

Why is the fly wheel of an engine made heavy in the rim?

The fly wheel of an energy is made heavy in the rim for increasing the moment of inertia of the flywheel. We know that the moment of inertia is given by,

I  =  mr2  __________ (1)

Eq (1) shows that if we increase the mass, the moment of inertia also increases. Now if the flywheel gains or loses some rotational kinetic energy [K.E = 1/2Iw2], then due to large value of moment of inertia, the angular velocity w’ change very slowly. The result is that the fly wheel helps in maintaining uniform rotational motion.

Why is a rifle barrel ‘rifled’?

A rife barrel has a spiral groove which imparts spin to the bullet, due to which the bullet acquires angular momentum about an axis parallel to the barrel.

Since angular momentum is conserved which keeps the bullet pointing stably in this direction and so improve the accuracy. That is the distant target is hit accurately.

Is it possible for a person to distinguish b/w a raw egg and a hard boiled one by spinning each on a table? Explain?

Yes, it is possible for a person to distinguish b/w a raw egg and a hard boiled one by spinning each on a table. When a raw egg is spun, the yoke of the egg being denser moves away from the axis of rotation. Now according to the relation I=mr2, the moment of inertia of raw egg increase. Therefore, it is much more difficult to spin the egg. On the other hand, the yoke of a hard boiled egg is fixed when the egg is spun, its moment of inertia does not increase and hence it spins more easily.

Why is the acceleration of a body moving uniformly in a circle, directed towards the centre?

When a body moves in a circle with uniform speed, then the direction of its velocity changes from point to point. This change in direction of velocity is always directed towards the centre of the circle. The re fore, the centripetal acceleration is always directed towards the center of the circle.

Consider a body is moving along a circular path of radius ‘r’ and let its velocity changes from ‘VA’ to ‘VA’ as shown in the figure. The change in velocity is given by,

∆V= VBVA

• ∆V= VB=(VA)

Now if ∆θ              0,  then CD or BO.

CD is perpendicular to VA or VB i.e. along AO or BO.

thus the change in velocity ∆V [hence centripetal acceleration]is directed towards the centre of the circle.

A ball is just supported by a string without breaking. If it is set swinging, it breaks? Why?

Consider a ball of mass ‘m’ is suspended from a string as shown in the figure. Point ‘o’ represents the equilibrium position at which the weight of the ball ‘W’ is just supported by the tension ‘T’ in the string. Now if the ball is set swinging, the tension in the string increases. The string cannot bear the increasing tension and thus it breaks.

An insect is sitting close to the axis of a wheel. If the friction b/w the insect and the wheel is very small, describe the motion of the insect when the wheel starts rotating?

Consider an insect of mass of ‘m’ is sitting on a wheel closer to the axis ‘o’ as shown in the figure.

Now when the wheel starts rotation, the centripetal and centrifugal force comes into action. As the friction b/w the insect and the wheel is very small, so the insect will move away from the axis ‘O’ of the wheel due to centrifugal force.

Explain how many minimum number of geo-stationary satellite are required for global coverage of T.V transmission?

The minimum number of geo-stationary satellites required for global coverage of T.V tra – nsmission is three. Since one geo-stationary satellite covers 120° of longitude [360°/3 = 120°], so for the whole pop-ulated. Earth, surface there must be 360° of longitude. Hence, the whole of earth’s surface for global coverage of T.V transmission can be covered by three correctly position geo-stationary satellites.

Explain the significance of moment of inertia in rotator motion?

The moment of inertia plays the same role in rotational motion as the mass plays in linear motion. The greater the moment of inertia of a body, the greater is the difficulty in starting or stopping its rotation. That is, it is a measure of inertia in rotational motion and is also called rotational inertia. As in case of linear motion, the inertia measures the opposition of a body to change its state of rest or its uniform linear motion.

In the same way, by virtue of moment of inertia or rotational inertia a body opposes and change in the rotational motion of a body. Using momentum, K.E etc of a rotating body.

Why does the casting rotating system slow down as water drops into the beaker?

We know that the moment of inertia depends upon mass of the body and distance ‘r’ from the axis of rotation. In case of mass, we can say that greater the mass of a body, greater will be the moment of inertia and vice versa.

Now when water is added to the beaker, the mass of the system increases due to which the moment of inertia of the system increases. Thus the coasting rotating system becomes slowed down as water drops into the beaker.

A body will be weightless when the elevator falls down just like a free falling body. Explain?

For downwards motion of an elevator, the tension or normal reaction is giving by, T = mg – ma ………………………(1)

Now when the elevator falls down just like a free falling body, then we put a = g, so eq (1) becomes,

T = mg – mg   =>  T=0 ……………..(2)

Eq (2) shown that, when the elevator falls down freely, then the apparent weight of the object will be shown by the scale to be zero i.e. the body will be weightless.

When a tractor moves with uniform velocity, its heavier wheel rotates slowly than its lighter wheel, why? Explain?

We know that the moment of inertia of a rotating body depends upon the mass ‘m’ of a body and distance ‘r’ from the axis of rotation. i.e. I = mr2 ………………(1)

The mass of heavier wheel is large due to which its rotational inertia is also greater. So when a tractor moves with uniform velocity, its heavier wheel rotates slowly than its lighter wheel due to large amount of moment of inertia.

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