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Newton’s Laws of Motion- Examples for Kids of Class 9 & 11

Newton’s law of Motion

In the field of physics, Newton’s law of motion is a foundation of classic mechanics.  It gives us the idea of how an object behaves when it stands, moves or any force act on it and its response. There is a total of 3 laws of motion.  In 1687, these three laws were first compiled by  Sir Isaac Newton in his Philosophiae  Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Basically, these laws describe the relationship between force (which acts on an object or body) and the motion ( Response of the object or body to that acting force). In this article, we will understand Newton’s laws and their explanations with examples and solve problems based on Newton’s law of motion.

 

Newton’s first law Motion

A stationary object will remain stationary forever and a moving object will forever move in a straight line with constant momentum unless prevented from changing state by an externally applied force.

• From Newton’s first law we derive two very important concepts – (i) the inertia of matter and (ii) the definition of force.

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Example of Newton’s First Law of Motion

Newton’s first law Motion- The inertia of matter

The Characteristic for which a stationary object or body wants a stationary state or a moving object to maintain its state of motion or prevent it from changing its state is called inertia. The mass of matter is the measure of inertia.

•Interior is two types-
1. Interia of rest – A stationary object always remains in a stationary state. The tendency of matter to remain in a stationary state forever is called Interia of rest.

Example- When a car starts moving suddenly, the passenger sitting in the car is thrown backwards. This happens for stability. When the vehicle is stationary the passenger is also stationary i.e. all parts of the passenger’s body are stationary. When the car starts moving suddenly, the lower part of the passenger’s body moves with the car because it is adjacent to the car. But the upper body wants to stay still under the influence of static. So the passenger leans back.

2. Interia of the motion-The tendency of a moving object to move along a straight line with constant velocity is called the Interia of motion.

Example- When a moving car stops suddenly, the passenger of the car leans forward. This happens for speed. As the vehicle is in motion, the entire body of the rider is in motion with the vehicle. As the vehicle comes to a stop, the passenger’s lower body comes to rest, but the upper body still leans forward to maintain momentum. The body wants to go As a result the passenger leans forward.

Newton’s first law: Definition of force

According to Newton’s first law of motion, a stationary object will always remain at rest and an object in motion will always move in a straight line with constant momentum unless forced to change state by an externally applied force. That is, what is applied from outside to change or try to change the static or dynamic state of an object is called force.

Or, a physical cause that changes or attempts to change the state of motion or the state of the rest of an object, known as force.

Newton’s Second Law of Motion

The rate of change of momentum of an object is proportional to the force applied to the object and the change of momentum also occurs in the direction in which the applied force acts.

•Newton’s second law explains (i) the concept of momentum, (ii) the measure of force, (iii) the unit of force and (iv) the neutral principle of force.

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Example of Newton’s Second Law of Motion

Newton’s second law of Motion: What is Momentum?

The amount of motion created by the combination of mass and velocity in a moving object is called the momentum of the object. The value of the momentum of an object is equal to the product of the object’s mass and velocity. If an object has mass m and velocity then the momentum of the object = m ×  v

Unit of Momentum- In C.G.S  Method The unit of momentum is – g. cm./s¹
In SI Method The unit of momentum is – Kg. m./s¹

Newton’s second law of Motion: Measurement of force

Let’s assume, an object having mass m, moving with a velocity of u. Force F is applied on the object for t time. Now, the formed acceleration in the object is a and the final velocity is v.

Initial momentum is = mu
After t time ,momentum is =mv
So, change in momentum = mv-mu [Final- Initial]

The Rate of changed  momentum = (mv-mu )/t
= m(v-u) /t = ma [ a= (v-u) /t ]
Acceleration of motion is derived by, Final velocity -Initial velocity /time

Newton’s second law states that the momentum change rate is proportional to the applied force on the object.  so,
F∝ ma
or, F = K×  ma, Here K is a constant. Its value depends on the unit of force. Now, if the force acting on an object of unit mass causes unit acceleration to be a unit force, i.e., when m = 1; If a = 1, then F = 1

1 = K .1.1
or, K = 1

Therefore, according to the above definition of unit force,
F = ma

i.e., applied force = mass of object x acceleration of the object.

Unit of force

Method  Unit
C.G.S, Dyne
S.I, Newton

Newton’s Third Law of Motion

Every action has an equal and opposite reaction.

Explanation – When one object applies a force on another object, the second object also applies an equal and opposite force on the first object. The force exerted by the first object on the second object is called action and the opposite force exerted by the second object on the first object is called reaction. [ According to Newton’s third law, two forces are equal and opposite]

Newton’s Third law of Motion: Examples

1. When a gun is fired, the gun exerts force on the bullet. This force causes the bullet to accelerate forward. Bullets also exert an equal and opposite reaction force on the gun. As a result, the gun moves backwards. So the person who fires the gun feels shocked.

2. When a rider jumps from a boat to the shore, the boat moves backwards. When launching from the boat, the rider exerts a force on the boat. As a result, the boat moves backwards. Immediately the boat exerts an equal and opposite reaction force on the rider, causing the rider to reach the shore.

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Example of Newton’s Third law of Motion

Newton’s law of Motion- Examples for Class 9 and 11 kids

Q. If a force of 1N is applied to an object of mass 1.250g, what will be its acceleration?

Mass of object m = 250 g = 250 /1000 kg=1/4 Kg.

Force applied on the object, F = 1 N; Acceleration, a =?

According to the formula, F = ma
1 × 1/4 × a
Or,a = 4 m/s²

Q. What is the acceleration of a force of 1 dyne acting on a mass of 1 mg?
Applied force, F = 1 Dyn
Mass of object, 1 = 1 1000 = 0.001 g;
Acceleration of object, a =?
According to the formula, F = ma

Or, 1 = 0.001 x a

Or, a = 1/ 0.001 = 1000 cm/s²

Q. How much force must be exerted on a moving object of mass 50 g to cause a deceleration of 5 cm s-2?

|Ans Mass of object, m = 50 g; Attenuation, a = 5cms-2 . Force applied to an object,

F = ma = 50 x 5 = 250 dyn |

Q.A certain force acting on an object of mass 0.5 kg produces an acceleration of 2 ms-2. Express the value of the applied force in newtons.
Mass of object, m=0. 5 kg;
Acceleration, a = 2-m/s²

. Force applied to an object,

F = ma

F= 0.5x 2 = 1 N

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Newton’s Law of Motions: FAQs

Q. What is the second law of newton?
The second law of newton states that The rate of change of momentum of an object is proportional to the force applied to the object and the change of momentum also occurs in the direction in which the applied force acts.

Q. What do you mean by the inertia of matter?

The Characteristic for which a stationary object or body wants a stationary state or a moving object to maintain its state of motion or prevent it from changing its state is called inertia. The mass of matter is the measure of inertia.

Q.Write down the units of force.
In C.G.S, the unit of force is Dyne and In SI, the unit of force is Newton.

Q.What is the mathematical expression of newton’s second law?
The mathematical expression of newton’s second law is F = ma i.e., applied force = mass of object x acceleration of the object. [F= Applied force, m = mass if the object and a stands for acceleration]

Q.Why is Newton’s first law of motion called the law of inertia?
The Characteristic for which a stationary object or body wants a stationary state or a moving object to maintain its state of motion or prevents it from changing its state is called inertia. Newton’s first law describes this characteristic of the object. Hence Newton’s first law of motion is also called the law of inertia.

 

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