Class 9- Force and Laws of Motion

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Forces and Laws of Motion

Sir Isaac Newton, a natural philosopher and one of the greatest scientists of the world listed the fundamentals of Force and Laws of motion in Newtonian Mechanics. Physicists and Astrophysicists have used these concepts along with Modern Physics and explored the untouched parts of the universe. Keeping in mind the importance this topic holds in the modern world, conceptual clarity related to the concepts of Force and Laws of Motion are provided to students right from school level. In India, this starts right from class 9 Science syllabus. Hence, in this blog, we have provided an overview of the essential concepts pertaining to the Force and Laws of Motion. 

What is Force?

Before delving deep into force and laws of motion, let us first understand the terms. Force is the strength or energy applied to move an object to a certain distance. The force is a result of one surface interacting with another. The effect exists until there is an interaction between two surfaces or objects. Force accelerates the objects or a body when and is a vector quantity, having both direction and magnitude. The notation F represents Force, m is mass, and a is acceleration and is measured by the quantity Newton (N).

F=M x a

Force example: Pushing or pulling a table or a drawer by exerting force on it.

Types of Forces 

When it comes to force and laws of motion, it is imperative to note that there are majorly 2 types of forces-

Balanced Forces

When two opposing forces of the same magnitude are applied on an object, the object does not move as the resultant of the forces becomes zero. These forces have the same magnitude but opposite direction. Example: When two players exhibit the same force on the rope, it does not move in any direction.

Characteristics of Balanced Forces 

  • No change is caused at the state of an object by balanced forces
  • Balance forces are always opposite in direction and equal in magnitude
  • Balanced forces 10 to change the size and shape of an object

Unbalanced Forces

When two forces of different magnitude are applied on an object, it moves in the direction of the greater force. This result is due to the differences in the magnitude. A greater force is applied in the opposite direction to move an object in the desired direction.

Characteristics of Unbalanced Forces

  • Unbalanced forces can change the position and speed of an object
  • We may also change the shape and size of an object 

Quick Fact: Some universal forces are Air Resistance, Frictional, and Gravitational forces. 

Newton’s First Law of Motion

“A body or an object remains at a constant motion in a particular direction until a net external force acts to cause a change in its state.” A car remains parked unless we apply some force to change the state of rest. A car remains in motion unless we apply brakes to bring it to rest. This is Newton’s first law of force and laws of motion.

First Newton’s Laws of Motion example: A block at rest remains at rest unless an external force acts on it.

Applications of First Law 

Mow that we have studied the theoretical upfront of Newton’s first law of motion, it is now time to understand its application in the real world. Mentioned below are some of the common applications of first law- 

  • When was moving in a straight line suddenly stops then the passengers tend to fall in the forward direction. This is because, the body of the passenger is in habit of moving in the same way even after the brakes have been pulled, thus, the passengers fall in in the forward direction. 
  • Whenever we hit a carpet by a brush, it loses the inertia of rest and moves from that particular position. The dust particles present on it are still in rest, thus, this process is used to separate dust from the carpet.
  • While shaking a tree you will notice that it only moves to and fro but the fruits on it remain at rest because of its inertia of rest. That is why, fruits fell from tree whenever we shake them.

What do you Understand by Inertia?

Inertia is the tendency of any object or body to repel change in its idle state or uniform force and laws of motion. This is why Newton’s first law of motion is also known as Newton’s Law of Inertia. The inertia depends on the mass of an object. The larger the mass, the more massive inertia it contains. 

Example: Astronauts find it challenging to move because of the absence of an external force called gravity. They remain in the state of motion due to the inertia.

What is Momentum?

Another important concept related to Force and Laws of Motion is Momentum. Momentum is a measure of the mass and time taken to travel one meter per sec when a force acts on it. 

p = m x v

where, p = momentum,
m = mass of the body,
v = velocity of the body.

The resultant between final and initial momentum is known as the change in momentum and is denoted by the equation as given below:

Change in momentum = mv – mu

u = Initial velocity of the body,
v = Final velocity of the body

The rate of change of momentum is the rate at which the total momentum of an object changes.

rate of change of momentum

Newton’s Second Law of Motion

Newton’s second law of force and laws of motion states that “the rate of change of momentum is directly proportional to the forces acting on the object.” I.e: the object moves in the direction in which the force acts. It clearly explains that the force is a product of the mass, accelerated to a certain distance (kg.m/s).

Example: It is easier to push an empty box than a box full of supplies. When we push an empty box, the box accelerates faster due to the force applied by the hands. 

Newton’s Third Law of Motion

Newton’s third law of force and laws of motion states that “there is always a reaction in equal magnitude and the opposite direction for every action caused.”

Example: When a gun fires a bullet, the bullet exerts an equal and opposite reactive force on the gun as it leaves the barrel. The recoil of a gun. 

What is Conservation of Momentum?

The law of conservation of momentum states that “If two objects A & B collide with each other, the total momentum before and after the collision is equal.” One object loses its momentum, and the other object gains the momentum that was lost. 

NCERT In-text Questions

Q1. Which of the following has more inertia: 
(a) a rubber ball and a stone of the same size? 
(b) a bicycle and a train? ?
(c) a five rupees coin and a one-rupee coin? 

Since Inertia depends on the mass of the object, an object with greater mass will hold greater inertia. The following objects hold greater inertia:
a) Stone
b) Train
c) Five-rupee coin

2. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case. 

In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case. 

Ans. The velocity of the football changes four times. 
The player kicks the ball to change velocity from ‘0’ to ‘u’
When the second player kicks the ball towards the goal and velocity changes again
The goalkeeper collects the football and velocity changes back to 0
Goalkeeper kicks it towards a player and velocity changes again

Explain why some of the leaves may get detached from a tree if we vigorously shake its branch. 

When you shake the tree vigorously, the branch attains motion but the leaves stay at rest. Due to inertia of rest, leaves remain the position and hence get detached from the tree.

When you shake the tree vigorously, the branch attains motion but the leaves stay at rest. Due to inertia of rest, leaves remain the position and hence get detached from the tree.

This happens because when the bus moves ahead, the passenger’s body is in the state of rest and the passengers experience a force on them in the backward direction due to their inertia opposing the forward motion. Similarly, when the bus stops, the lower body comes to rest. But the upper part of the body continues to be in motion and hence we fall in forward direction due to inertia in motion.

If action is always equal to the reaction, explain how a horse can pull a cart. 

If a horse uses its legs to exert force on the ground while walking, the ground exerts an equal and opposite force on the horse’s feet to move it forward and the horse pulls the cart.

We believe that through this blog, we have successfully cleared some important concepts related to the Force and Laws of Motion! Want to consult a mentor who has pursued a career path which you want to walk on after 12th? Then reach out to our experts at Leverage Edu who will help you choose a career path that aligns with your interests!

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