Matric Notes Physics 9th Ch 3 Dynamics Long Questions

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Q.1 Define and explain force?

Ans. Force:-an agent which moves or tends to move a stationary body stops or tends to stop a moving body or tends to change the speed or direction of moving body is known as force.

Force is a vector quantity and its unit is Newton. One Newton is that force that moves a body of one kg with the acceleration of 1m/sec2.

Q.2 State and explain or Newton 1st law of motion?

Ans. Newton’s 1st law:-this law states that in the absence of any external force a body in rest will always remain in rest and a body in motion will always continue its motion in a straight line with uniform speed.

Explanation:-this law has two parts which are explained below.

In the 1st part, it is stated that in the absence of any external force a stationary body will always be at rest, for example, a book is laying on the table so it will remain stationary and will not change it’s by itself unless someone moves it by applying a force.

In 2nd parts it is stated that moving will always move along a straight line unless someone stops it by applying a force but when a ball is kicked it comes to the state of rest after covering some distance it is because the force friction of air and the earth acts upon the ball each moment which compels it to stop.

Q.3 state and explain law of inertia?

Ans.1 inertia:-Newton’s 1st law of motion is also known as inertia “inertia” is the property of a body which opposes any change in its state of rest or motion.

Explanation:-Inertia depends upon the mass of a body in fact mass is the measure of inertia. Greater the mass of a body greater will be inertia greater force will be required to stop it e.g the inertia of hard cricket ball is greater than the inertia of soft ball and greater force will require to stop hard ball.

Examples:-(1) When a jerk is given to the branch of a tree all fruits falls down from it because when branch comes into motion at on at that time the fruits in rest and tends remain in rest as a result + they separated and fall down.

(2) When a carpet is beaten with a stick. The carpet set into motion suddenly while the dust particles are at rest and tends to remain at rest and tends to remain at rest as a result they fly off from carpet.

(3) When bus suddenly stops the lower past of passenger comes into rest while the upper past is still in motion and tends to remain in motion as a result the passenger falls forward.

(4) When a person jumps from a moving vehicle, his feet come in contact with ground and comes to state of rest while the upper part is still in motion and tends to remain in motion, As a result person gets hurt.

(5) A parcel laying on the seat of a car move with same as that of the car whe breaks are suddenly applied the comes into rest but parcel is still in motion and tends to remain in motion due to inertia as a result the parcel falls into the floor.

Q.4 State and explain Newton’s 2nd law in detail?

Ans. Statement:-this law states that when a force acts on a body on acceleration is produced in direction of force which is directly proportional to the applied force but inversely proportional to the mass of the body.

Explanation:- if we apply a force “f” on a body of mass “m” and it produces acceleration “a” in the direction of applied force, the it mathematical from is given as

            a  f……… (i)     a 1/m……….(ii)

Comparing eq (i) and eq(ii) we get,

a  f. q/m => a f/m

             =>a = (k)f/m ………….(iii)

If f = 1 N mass = 1kg then k = 1 so by putting

            k = 1 we get a = f/m =>f/,m => = ma ……(iv)

            Equation (iv) represents the mathematical form of 2ndeq: of motion.

If we apply equal force on two bodies of different masses, the acceleration of the lighter body will be greater than the acceleration of the heavy body. Similarly greater the applied force greater will be the acceleration produced.

Example:-when a cricketer catches the ball he moves his hands backwards. By doing so the acceleration of ball decreases and the force of ball caching in hand is also decreases and by catching the ball the cricketer feels less fain.

Q.5 State and explain Newton’s 3rd law with the help of examples?

Ans. Newton s third law:- states that for every action there is a reaction which is equal in its magnitude but opposite in direction.

Explanation:-when a body exerts some force on another body. Then the 2nd body will also exert a force on 1st body the force exerted by 1st body on 2nd body is action and let it Is equal to →F12. And the force exerted of 2nd body on its body is known as reaction and let it is equal toF21. These forces are equal in magnitude but their direction is opposite and the mathematical from is given as:-

Example(1): When air is released from balloon the balloon pushes the air in back ward direction which is action while the air pushes the balloon in forward direction which is reaction.

Examples(2): When a fireman directs a hose pipe toward fire the water shoots out from the pipe in forward direction which is action on water. And water pushes the pipe in back ward direction which is reaction.

Example(3): When a passenger jumps from a boat they pushes the boat with their feet in the backward direction which is action. Because of this reason the boats mans generally tie the boat on the river on the bank and then the passenger are allowed to step out.

Q.6 Differentiates b/w mass and weight?

Mass:

1. The quantity of matter in a body is called mass.
2. Mass is constant everywhere.
3. Mass is measured by ordinary balance I-ie beam balance.
4. It is represented by “m”.
5. It is scalar quantity.
6. Its unit is kg.

Weight: 

1. The force with which earth attracts things toward its center is called force.
2. Weight is variable quantity.
3. Weight is measured by spring balance.
4. It is denoted by w.
5. It is vector quantity.
6. Its unit is Newton.

Q.7 Defines and explain tension?

Ans. Tension:- The force exerted by string on the hand is called tension . It is denoted by “T”.

Explanation:- suppose a man is holding a body of mass “m” in his hands with the help of string

as shown in figure.

In his case the weight of object exerts a force on our hands in down direction which is action as a reaction the hands also exerts an equal force on object in upward direction .so in this case the force which is exerted by string on our hands is known as tension. Which is denoted by “T” in state of rest tension in string is equal to its weight ie. T = w = mg.

Q.8 Define momentum give its unit?

Ans. Momentum:- The quantity of motion in a body is called momentum (OR) the product of mass and velocity of a body is known as momentum it is denoted by “P” if mass of a body is “m” and velocity is “v” then the mathematical can be written as:-

p = m.v

Momentum is the product of mass and velocity of a body. Greater mass and velocity greater will be a momentum produced it is a vector quantity and its unit is kg/sec.

Q.9 Explains the relation of momentum with 2nd law of motion?

Ans. Let a force “ F” is acting on a body of mass “m” and its velocity changes from “vi” to “vf” in time "∆t".

Then initial momentum of body = pi = mvi...........(i)

Final momentum of body = pi = mvf.......... (ii)

And change in momentum = ∆p = pf - pi = mvf– mvi

=>∆p = m (vf – vi)

÷ing b/s on ∆t, we get, ∆p/∆t = m (vf – vi)/∆t.......(iii)

But we also know that :- a = (vf – vi)/ ∆t.........(iv)

Putting eq (iv) in eq (iii) we get.

∆p/∆t = ma ...........(v)

As f = ma so eq (v) become

∆p/∆t = F

Or [F = ∆p/∆t]................(vi)

Equation (vi) shows that the time rate of change of momentum is equal to applied force.

Q. 10 Define friction and give its types?

Ans. Friction:- a force which always opposes the motion of one body over another body in contact with it is called friction. (OR) the resistive force b/w any two surfaces during motion is known as friction.

It has two types which are given below.

(1) Sliding friction:- The force which opposes the sliding friction of a body we see that when a body is

laying on surface and we apply some force on it. Then forces are acting on body its weight “W” in

down ward direction and force of surface on body in upward direction. These both forces balance

each other. Now we apply force “Fa” on it then it is still in state because of resistive force of surface

“Fs” friction during this time is known as state friction. When “Fa” is increased a point comes at

which body tends to start motion. Friction during this time is known as limiting friction. When friction

is increased more than body starts moving on surface. And friction of body it is observed that

limiting friction also increases with on increase in normal force. 

Mathematically. F φFn => f = μ Fn

Where μ is constant of proportionality and known as co – efficient of friction.

Which depends on nature of surfaces.

(2) Rolling friction:- Friction produced during rolling of one body over another body is known as rolling friction. Rolling friction is smaller than sliding friction because the contact area b/w surfaces in rolling are less than that of in sliding friction. For example the rolling friction of steel and steel.

Q. 11 discuss advantages and disadvantages of friction:

Ans.(1) Advantages of friction:

i) We walk on ground with the help of friction.

ii) The automobile vehicles move with the help of friction.

iii) We can write on black board and pages because of friction.

iv) Brake of cycles and vehicles and words with the help of friction.

v) Due to friction nails remains fined in walls and woods.

vi) We can climb on tree due to friction.

(2) Disadvantages of friction :-

i) Because of friction heat is produced in various parts of machine which is the waste of usefully energy

ii) Due to friction parts of machine get wear and tear.

iii) Due to friction our shoes becomes useless.

iv) Because of friction the efficiency of machine is reduced.

Q.12 Describe methods of reducing friction?

Ans. We can reduce friction by following methods:-

i) By lubricating the parts of machine with grease and oil.

ii) By using rolling techniques instead of sliding techniques.

iii) To polish well the contact surface.

iv) To make the front side of the objects oblong eg airplane.

Q.13 Define and explain centripetal acceleration?

Ans. Centripetal acceleration:- the acceleration which is produces due to centripetal force is known as centripetal acceleration. Which is denoted by ac.

Explanation:-When a force compels a body to move in a circle the force is called centripetal forces and the acceleration which is produced because of this force is call centripetal acceleration.

Now. As we know that centripetal force “fc” is given by

Fc = mv(2)/r.................(i)

But according to Newton’s 2nd law of motion we have fc = mac..............(ii)

Combining eq (i) ε eq (ii) we get.

Mac = mv(2)/r

ac = av(2)/r

Eq (iii) is the mathematical form of centripetal acceleration a.

Q.14 Define and explain centrifugal force?

Ans. Centrifugal force: - the force which equal in magnitude but opposite in direction to the centripetal force is known as centrifugal force. (OR) the reaction of centripetal force is called centrifugal force.

Explanation:- when a stone is whirled in a circle through a string. Then we provide it a force to move it

along a curculio path which is known as centripetal force and direction toward the center of the circle as

shown in figure. But according to Newton s 3rd law of motion the stone also exerts a force on our hands

which is equal in magnitude to centripetal force but opposite in the direction of centripetal force but

opposite in the direction of centripetal force. And this opposite and equal force is called centrifugal force. Mathematically:-Fcentrigul= - mv(2)/r

Q.15 Give some practical applications of centripetal force?

Ans. Some practical applications of centripetal force are given below.

(1) Banking of road:- Usually the outer edge of round bank is kept a little higher than the inner edge. It is called banking of road. When it is raining then the friction b/w the roads and tires does not provide the necessary centripetal force. And it there is no banking in road then the car will slip so the banking provides necessary centripetal force to car to move easily on a round track.

(2) Centrifuge:- A device with the help of which we can separate heavy and light particles from mixtures is known as centrifuge. It is based on the principal that if the magnitude of applied force becomes shorter than the required centripetal force. Then the object will move away from the center of the mixture .If a mixture of unequal densities is allowed to rotate rapidly in centrifuge then the heavy particles of liquid remains farther from the axis of rotation while the lighter liquids remain near to the axis of rotation and this these are separated from each other.

(3) Dryer of washing machine:- The dryer of washing machine is also based on the principle of centrifuge. Dryer consists of a cylinder which possess thousands of small holes when wet clothes are rotated in dryer then the drops from wet cloths drained out through holes and the cloths becomes dry.

(4) Cream separator:- Speed centrifuge with the help of which cream is separated from milk. A simple cream separator is an electric madani. The turning blades of this devise spin the milk due to which the light and heavy component of milk is separated.

Q.16 how does a gunman get jerk on firing a bullet?

Ans. When bullet is fired from a gum the gun pushes the bullet in forward direction which is action and as a reaction the bullet pushes the gun in backward direction .that’s why the gun man get a jerk on firing a bullet.

Q.17 why are the long jump athletes are made to jump in sand pits?

Ans. The long jump athletes are made to jump in sandy pits for softy because when they jump on sandy pits then the time of impact is increased and the impact force decreases and thus the athletes may not be hurt.

Q.18) What is force? What are its units? Distinguish between contact and non-contact forces?

Answer: Force: “A force is a kind of a push or a pull on an object”. OR “Force is a physical quantity which moves or tends to move a body, stops or tends to stop a moving body”.

Unit of Force: In SI the unit force is Newton ‘N’. One newton is defined as “the force that produces an acceleration of one meter per second squared (a = 1m/s2) in a body of mass one kilogram (1 kg).

                                    1 N = 1 kg × 1 m / s2  or    N = kg m / s2 

Contact and Non-Contact forces:

We can think of different ways in which we can move textbook. We can push or pull it, or we can tie a string around it and pull on the string. We often call these contact forces because the force is exerted when one object comes in contact with another object. As we are holding physics textbook right now, our hands are exerting a contact force on it. There are other ways in which we can change the motion of the textbook. It would accelerate as if it falls to the ground. This time the gravitational force of Earth (which is not a contact force) is acting on the book which is causing this acceleration. We call such forces as non- contact forces, field forces or action-at-a-distance forces.

Q.19) State Newton’s three laws of motion. Give one example for each.

Answer: First law of motion: Statement: “If the net (external) force acting on an  object is zero, the object will maintain its state of rest or of uniform motion (constant velocity)”.

Explanation: The study of first law of motion can be divided into two parts.

Bodies At Rest: The first part of the law sates that a body at rest will remain at rest if no net force acts on it. This part of first law is easy to understand and goes with our common observation. For example a chair laying in a room will remain stationary and will not start moving or flying around by itself unless some one moves it by applying a net force.

Bodies In Motion: The second part of the law states that a body in motion will continue to move in a straight in line with uniform speed if no net force acts on it. However our daily observation is against this. For example if we roll a ball it comes to rest after some time. But careful study of the moving ball shows that there are forces (like friction and air resistance) which oppose the motion of the ball.

This means that object would continue to move in a straight line for ever, with uniform speed if the forces opposing the motion of the object are removed.

Newton’s second law of motion: Statement: “A net force(unbalanced force) applied on the body produces an acceleration ‘a’ in the body. This acceleration is directly proportional to the magnitude of the net force and inversely proportional the as of the object”.