NCERT 9th class solution : Force and law of motion notes

Updated Tue, 20 Jun 2023 04:23 PM IST


If you are searching for answers to questions related to Gravitational Force and Newton's Law, then this article can be important for you, so read this article carefully.

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Force and Newton's law of motion

Force is that factor which changes the state of the object from the initial state to the final state till the time it is applied, that is, there is a kind of push or pull which changes the position of the object.
There are three types of force, (1)magnetic force, (2) electrostatic force and. (3) gravitational force. We have now read about these three forces in detail in the previous classes.
The change in any thing is only due to the force applied by it, if the object is in motion, then its place changes, that too is due to the force, the laws related to which were propounded by the great scientist Newton. This three law is very important law in physics.

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Newton’s first law of motion and inertia


It is a common observation that a book place on a table remains in the same place and it the same position a body tends to retain its position unless some external force act on it similarly if you rule a smooth ball on a Grass it will continue rolling unless some external force stop it. This property is called inertia hence a body at rest remains at rest and a body in motion continues to be in uniform motion in straight line unless some external force act on it to produce our change.

By Newton’s first law, States a body continues in its state of rest or the uniform motion along a straight line and list and external force acts on it.  For example bullet fired, bicycle riding etc

Know more:

Remember that the equation if equal to ma is not a statement of Newton second law it is only derived result.
 In CGI system the unit of force is dyne.
 In SI unit the unit of force is Newton denoted by the letter N.
 1N= 10⁵
 The gravitational unit of force are KGF and GF
 1 kgf= 9.8 N
 1gf= 980dynes

Qualitative definition of force Through Newton's first law

It is evident that force is an external agent which changes or tense to change the position of rest or of uniform motion of a body along a straight line it is the quantitative definition of force.
Mass of inertia
“The mass of inertia the mass of a body is the characteristic that relates of force and the body to the resulting acceleration.” It can be concluded that more the mass the more difficult is to move a body from rest or modified or stop the motion of a moving body since the resistant of motion is inertia we can say the greater the mass greater in the inertia therefore mass is a directly proportional to inertia .hence mass is a measure of inertia.

Newton's law Types of inertia

Inertia is three type…..

 Inertia in rest

According to the law of inertia of rest “ A continue to be in a state of rest unless I am unbalanced internal force act on it.”
For example..

Source: Amar ujala

Bina horse start riding suddenly the rider falls back down.
Bina carpet is suddenly jald that dust particles fly off.

 Inertia in motion

According to the law of inertia of motion “A body continues to be in the state of motion with the same speed in the same direction in a straight line unless and unbalanced external force is applied to it to change its state.”
For example: Bina moving train or bus suddenly stop the passenger fall down.

 Inertia of direction

According to the law of inertia of direction of “A body continues to move in the same direction unless sum and balance external force is applied to it.”
For example: I stone tied on string which is being rotated in a horizontal direction flies of tangentially win the string breaks.

Newton's second law of motion

When two bodies heavy and other light are acted upon by the same force for the same time the light body builds up a higher velocity then the heavier one but the momentum they gain is the same in both cases the important relation between force and momentum was recognised by Newton and express by him as a second law of motion by second law of motion is the rate of change of momentum of a body is directly proportional to the net force acting on it and it takes place in the direction of force.
 consider a body of mass m moving with initial velocity u is straight line when a force F is applied on the body for time t let it its velocity changes from initial velocity u to final velocity v……
Momentum M = mv & final momentum = mv
Then Change in momentum= mv-mu
Rate of change = mv- mu÷ t
Then F prapotional to ma ; F = ma
Because v- u÷t = a
1N =10⁵ dynes
1kgf= 9.8 N

Linear momentum

Linear momentum: force is needed to stop a moving body difference directly on the product of mass of the body and its velocity which we call linear momentum linear momentum ian s defin as the product of mass (m) and velocity(v).
P = mv

Newton's third law of motion

Forces come in pairs. If a hammer exerts a force on a nail the nail, exerts a force of equal magnitude on the hammer. Whenever a force acts on a body an equal and opposite force acts on the body applying force.
One of the two forces is called the action force and the other is called the reaction force their force Newton's third law maymay beaten as” to every action, there is always an equal and opposite reaction.”


some  Examples of Newton’s law with explanation

1. When a train or a bus suddenly starts the passengers tend to fall back down. This happens because the lower part of the body which is in contact with the floor of the traiofuthe s begito move while the upper part of the body tends to remain in the position of rest and hence tpper part of the body tells ttoldl backward.
2. When a carpet is suddenly jerked the dust particle flparticleifliessoffe off his of beating the carpet comes in motion but the dust particles remis enclosedis enclose in contact with the horse shares the motion but the upper part remains at the rest due to inertia and has the rider falls back down.
3. A person riding a bicycle along a level road does not come to the rest image deeply after he or she stops b pedalling. The bicycle continues to move forward due to inertia of motion. Finally, it comes to rest due to the force of friction.
4. Mud sticking to cycle or scooter tyres fly off tangentially. Because of this reason mudguards are provided on the front and rear wheels of a bicycle or scooter.
5. Stone type to a string which is being rotated in a horizontal direction flies tangentially win the string break. The happiest due to properly inertia of direction. The direction of motion of stone is along the tangent to the string and hence, it flies tangentially when the string breaks.
6. When a voting man says at the water backward with his own the water exerts an equal and opposite force on the ore which is the reaction force and makes the boat move. Here the action is the force exerted by the ore on the water and reaction is the force exerted by the sour on the oar.
Related to your course:

Q1-what do you mean by force and law of motion?

Answer -force means push or pull on an object to change it's velocity (to accelerate). laws of motion are the laws proposed by newton.

Q2-what is the formula for force and law of motion?

Answer -Newton's second law, which states that the force F acting on a body is equal to the mass m of the body multiplied by the acceleration a of its centre of mass, F = ma, is the basic equation of motion in classical mechanics.

Q-3 what is a force explain?

Answer-A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects.

Q4-what are the two type of forces?

Answer -Forces can be divided into primarily into two types of forces:
Contact Forces.
Non-contact Forces.

Q5-what are the Newton's third law?

Answer-His third law states that for every action (force) in nature there is an equal and opposite reaction. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A. In other words, forces result from interactions.

Q6-What is the SI unit for force?

Answer- The International System of Units (SI) is widely used for trade, science and engineering. The SI unit of force is the newton, symbol N. The base units relevant to force are: The metre, unit of length — symbol m. The kilogram, unit of mass — symbol kg.

Q7-what are the difference between motion and force?

Answer- Force is basically a push or pull which acts on an object or energy as an attribute of physical action or movement. It happens when two entities are in contact. Further, motion is when a body is moving, it is in motion.

Q8-what does Newton stand for?

Answer- The newton is the Standard International (SI) unit of force. In physics and engineering documentation, the term newton(s) is usually abbreviated N. One newton is the force required to cause a mass of one kilogram to accelerate at a rate of one meter per second squared in the absence of other force-producing effects.

Q9-what is the value of 1 Newton?

Answer-It is defined as that force necessary to provide a mass of one kilogram with an acceleration of one metre per second per second. One newton is equal to a force of 100,000 dynes in the centimetre-gram-second (CGS) system, or a force of about 0.2248 pound in the foot-pound-second (English, or customary) system.

Q10 -is inertia a force?

Answer-Inertia is a force which keeps stationary objects at rest and moving objects in motion at constant velocity. Inertia is a force which brings all objects to a rest position. All objects have inertia. A more massive object has more inertia than a less massive object.

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