Laws of Motion

Dhristi JEE 2022-24

Laws of Motion – Explanation in Detail

Isaac Newton discovered the Laws of Motion. Newton’s laws of motion explain the connection between a physical object and the forces acting upon it. By understanding the concept, it provides all the information about the basis of modern physics. After the development of the three laws of motion, Newton revolutionized science. Newton’s three laws together with Kepler’s Laws explained the concept of planets moving in elliptical orbits rather than in circles. Although Newton’s laws of motion may seem so general today, they were considered revolutionary centuries ago. Newton’s three laws of motion help someone understand how objects behave when standing still, when moving and when forces act upon them. Sir Newton’s three laws are Newton’s First Law: Inertia, Newton’s Second Law: Force, Newton’s Third Law: Action & Reaction.


Newton’s First Law

Newton’s First Law states that ‘An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force’. This law describes that every object will remain at rest or in uniform motion in a straight line unless forced to change its state by the action of an external force. The tendency of an object to resist changes in a state of motion is inertia. There will be a net force on an object if all the external forces cancel each other out. If this happens, then the object will maintain a constant velocity. If the Velocity remains at zero, then the object remains at rest. If any external force acts on an object, the velocity will change because of the force applied. An example of the First law of motion is : The motion of a round ball falling down through the atmosphere, A rocket being launched up into the atmosphere.


Newton’s Second Law

Newton’s Second Law states that ‘The acceleration of an object depends on the mass of the object and the amount of force applied’. This law defines a force to be equal to a change in momentum (mass times velocity) per change in time. Momentum is described as the mass m of an object times its velocity V. Example of the second law of motion is: An aircraft’s motion resulting from the aerodynamic force, aircraft thrust and weight.


Newton’s Third Law

Newton’s Third Law states that ‘Whenever one object implies a force on a second object, the second object implies an equal and opposite force on the first’. This law defines that for every action in nature there is an equal and opposite reaction. If object A applied a force on object B, object B will also apply an equal and opposite force on object A. In other words, it can be said that forces result from interactions. An example of the third law of motion is: The motion of a jet engine produces thrust and hot exhaust gasses flow out the back of the engine, and a thrusting force is produced in the opposite direction.


Derivation of First Equation of Motion

Let’s Consider a body of mass m having initial velocity u.

Let after time be t its final velocity becomes v due to uniform acceleration a.

Now it is defined as:

Acceleration = Change in velocity / Time taken

Acceleration = (Final velocity - Initial velocity) / Time taken

 a = (v - u) / t

 a t = v - u

or  v = u + at

This describes the first equation of motion.


Derivation of Second Equation of Motion

As it is defined, the Second equation of motion: s = ut + (1/2) at2

Let’s take the distance traveled by the body be s.

Now:

Distance = Average velocity x Time

Also, Average velocity = (u + v) / 2

Therefore, Distance (t) = (u + v) / 2t       ......eq.(1)

Again from first equation of motion:

v = u + at

Substituting this value of v in eq.(1), we get

s = (u + u + at) / 2t

s = (2u + at) / 2t

s = (2ut + at2) / 2

s = (2ut / 2) + (at2 / 2)

or  s = ut + (1/2) at2

This describes the second equation of motion.


Derivation of Third Equation of Motion

As it is defined that the third equation of Motion:  v2 = u2 + 2as

Now,

v = u + at

v - u = at

or  t = (v - u) / a            ........ eq.(2)

Also ,

Distance = average velocity x Time

Therefore, s = ((v + u) / 2) x ((v - u) / a)

s = (v2 - u2) / 2a

2as = v2 - u2

or  v2 = u2 + 2as

This describes the third equation of motion.

FAQs on Laws of Motion

1. Define Newton’s First Law of Motion?

Newton’s first law of motion states that ‘An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force’. First law of motion describes that things cannot start, stop, or change direction all by themselves, and it requires some force from the outside to cause such a change. This property of an object or a body to resist changes in its state of motion is called inertia. Thus the first law of motion is also known as the law of inertia.

2. Define Newton's Second Law of Motion?

Newton’s second law of motion states that ‘The acceleration of an object depends on the mass of the object and the amount of force applied’. Second law of motion defines what will happen to the massive body when acted upon by an external force. This law of motion states that the force acting on the body is equal to the product of its mass and acceleration. This law also describes precisely how much an object will accelerate for a given net force.

3. Define Newton’s Third Law of Motion?

Newton’s Third Law of motion states that ‘Whenever one object implies a force on a second object, the second object implies an equal and opposite force on the first’. Third law describes that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The law is also known as the law of action and reaction. Third law is essential in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion.

4. Describe the real life example for three laws of motion?

Example of First law of Motion is: The motion of a round ball falling down through the atmosphere. 


Example of the Second Law of Motion is: An aircraft’s motion resulting from the aerodynamic force, aircraft thrust and weight.

 

Example of the Third Law of Motion is: The motion of a jet engine produces thrust and hot exhaust gasses flow out the back of the engine, and a thrusting force is produced in the opposite direction.

5. From where can a student find the study materials for the concept Laws of Motion?

Students can find everything they need on the vedantu app. The study materials are created by professionals in the field and the content is accurate and reliable. These study materials are completely free and there are no charges at all. All students just have to sign in and then they will be able to download what they want in pdf format. Students can find various study materials related to Laws of motion. Every student can take advantage of these free resources that will surely help you ace your exams. 

6. What are the real-life applications of newton’s three laws of motion?

Let’s discuss the real-life application of three laws of motion:

  • First Law of Motion: A ball falling from the atmosphere.

  • Second Law of Motion: If you push the car and the truck with the same force, the car will have greater acceleration.

  • Third Law of Motion: While doing Bungee Jumping, the earth exerts a gravitational force on the mass of the jumper, while at the same time the bungee reacts by exerting an opposite but equal force.

7. What three physical quantities are used in equations of motion?

The following quantities are used in the equation of motion:

  1. Displacement

  1. Velocity: Initial and final

  1. Acceleration

  1. Time

8. A ball is thrown vertically upwards with a speed of 19.8 m/s from the roof of the apartment building and returns to the ground in 6 seconds. Find the height of the building.

Given t = 6 s, a = 9.8ms-2, u = - 19.8 m/s


Here, u = - ve because we are taking downward vertical motion of the ball from the point of projection up to ground.


We know that s = ut + 1/2at2, now putting the above values in this equation, we get:


s = - 19.8 x 6 + ½ * 9.8 * 62


Height of the roof = 57.6 m

9. Who Discovered Gravity?

The gravity was discovered by Sir Isaac Newton.

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