# FBISE 9th Class Physics Chapter 3 Dynamics Short Questions Answers

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Class/Subject: 9th Class Physics

Chapter Name: Dynamics

Board: Federal  Board

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## FBISE 9th Class Physics Chapter 3 Dynamics Short Questions Answers

Define Inertia?

The inertia of a body its property due to which it any change in its state of rest or motion.

Define Force?

A force that moves or tends to move stops or tends to stop the motion of a body. The force can also change the direction of motion of the body.

What is the difference between Mass and Weight?

Mass: The quantity of matter contained in a body is a called mass
Weight: Weight is a force with which the earth attracts a body towards its center.

What do you know about tension in a string?

The force exerted by a string when it is subjected to pull is called tension in the string.

What is the limiting force of friction?

The maximum value of friction is known as the limiting friction (Fs). It depends on the normal reaction (pressing force) between the two surfaces in contact Fs=µR

What is momentum?

The momentum of a body is the quantity of motion it possesses due to its mass and velocity.

What is Centripetal Force?

Centripetal force is a force that keeps a body moving in a circle. The center-seeking force is called the centripetal force. It keeps the body moving in a circle. Centripetal force always acts perpendicular to the motion of the body.

What is sliding friction?

A force between the sliding objects which opposes the relative motion between them is called sliding friction.

What is rolling friction?

Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than sliding friction.

Define Dynamics?

The branch of mechanics that deals with the study of the motion of an object and the cause of its motion are called dynamics.

State Newton’s First law of motion.

A body continues in its state of rest or of uniform motion in a straight line provided no net force acts on it.

State Newton’s Second law of motion.

When a net force acts upon a body, it produces acceleration in the body direction of force and the magnitude of the acceleration is directly proportional to the force and is inversely proportional to the mass of the body.

State Newton’s Third law of motion.

To every action, there is always a reaction that is equal in magnitude but opposite in direction.

State law of conservation of momentum.

The momentum of an isolated system of two or more than two interacting bodies remains constant.

Define Circular motion?

The motion of the body moving in the circular path is known as circular motion.

Define Centripetal Force?

A force that keeps a body moving in a circle is known as centripetal force.

Define centripetal acceleration?

The acceleration produced by the centripetal force which is always directed towards the center of the circle is known as centripetal acceleration. It is represented by ac.

Define Centrifugal force.

According to Newton’s third law of motion, there exists a reaction to centripetal force. The centripetal reaction that pulls the bodies in an outwards direction is called the centrifugal force.

What is Centrifuge?

A centrifuge is one of the most useful laboratory instruments. It helps to separate heavy and light particles from the mixture.

When a gun is fired, it recoils, why?

Before firing the gun, both the gun and bullet are at rest, so the total momentum of the system is zero. As the gun is fired, a bullet shoots out of the gun and acquires momentum. To conserve the momentum of the system, the gun recoils.

Why the spinner of the washing machine is made to spin at a very high speed?

The dryer of a washing machine is basket spinners. They have a perforated wall having large numbers of fines holes in the cylindrical rotor. The lid of the speed, the water from wet clothes is forced out through these holes due to lack of centripetal force.

Why it is dangerous to travel on the roof of a bus?

Because of the friction force due to air acting on the upper part of the body the person who traveling on the roof of running bus try to turn over which is dangerous for passengers while traveling on the lower portion remain at rest w.r.t. the roof on the bus due to inertia.

How can you relate a force with the change of momentum of a body?

Consider a body of mass m moving with initial velocity vi, let a force acts on the body which produces an acceleration an in it. This changes the velocity of the body. Let’s its final velocity after time t becomes vf. If pi and pf be the initial momentum and final momentum of the body related to initial and final velocities respectively then.
Pi = mvi
Pf = mvf

What will be the tension in a rope that is pulled from its ends by two opposite forces 100N each?

Tension in the rope and its force pulls equally at both ends if no forces acting on the rope except its ends, and the rope itself is in equilibrium, the tension is the same throughout the rope.
∑F = 0

Why is the law of conservation of momentum important?

The Law of conservation of momentum applies to all objects in the universe. A rocket and jet engine taking off. The recoil of a gun and bank shot in a pool are examples that demonstrate the importance of the law of conservation of momentum.

Describe two situations in which force of friction is needed.

i. It is risky to run on a wet floor with shoes that have smooth soles. Athletes use special shoes that have extraordinary ground grip. Such shoes prevent them from slipping while running fast.
ii. To stop our bicycle we will apply brakes. The rubber pads pressed against the rims provide friction that stops the bicycle.

How does oiling the moving parts of a machine lower frictions?

The friction can be reduced by lubricating the sliding surface. The oil helps slick (polished) the two surfaces so that the molecular surfaces become easier to side on with less friction.

Describe ways to reduce friction?

i. The friction can be reduced by making the sliding surfaces smooth.
ii. The friction can be reduced by making the fast-moving objects streamline shape (fish shape) such as cars, airplanes, etc. This causes the smooth flow of air and thus minimizes air resistance at high speeds.
iii. The friction can be reduced by using ball bearings because the rolling friction is lesser than the sliding friction.
iv. The friction can be reduced by lubricating the sliding surfaces.

What is braking force?

Friction between a rotation component (the drum or disc) and a stationary force is called braking force.

What would happen if all friction suddenly disappears?

If there was no friction then we could not walk, we would keep slipping. Nothing would be steady on the ground; many things would be just sliding and sliding.

Why rolling friction is less than sliding friction?

When a wheel moves on a smooth surface, it has contact with the surface only at a single point. As there is no relative motion between the two surfaces at this point. Therefore, sliding friction is zero. However, particularly, the wheel is compressed a little at the contact point of the two surfaces under stress. Because of that little sliding friction, the rolling friction is produced and less than sliding friction.

The moon revolves around the earth, from where it gets necessary centripetal force?

The gravitational force between the earth and the moon provides the necessary centripetal force to the moon for revolving around the earth.

Can a body move with uniform velocity in a circle? It not, why?

When a body is moving in a circle it may have uniform speed but its velocity is non-uniform because the direction of the body is changing at every instant.

Write down the disadvantages of friction.

i. Friction is undesirable when moving at high speed because it opposes the motion and thus limits the speed of the moving objects.
ii. Most of our useful energy is lost as heat and sound due to the friction between various moving parts of machines.
iii. In machines, friction also causes wear and tear of their moving parts.

What are the advantages of Friction?

i. We cannot write if there would be friction between paper and the pencil.
ii. Friction enables us to walk on the ground.
iii. Birds could not fly if there is no air resistance. The reaction of pushed air enables the birds to fly.

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