Here we bring you the CBSE Class 9 Science notes on chapter 9 ‘Force and Laws of Force and Laws of Motion’ (Part-I). These chapter notes are prepared by the subject experts and cover every important topic from the chapter. You get a crisp and brief explanation of all these topics so as to make your learning easy and organized. At the end of these notes, you can try the questions asked from the discussed set of topics. These questions will help you to track your preparation level and get a hold on the subject.
Main topics covered in this part of CBSE Class 9 Science, Force and Laws of Motion: Chapter Notes, are:
- Force and its effects
- Balanced Force
- Unbalanced Force
- Newton's First Law of Motion
Key notes for Chapter - Force and Laws of Motion, are:
A push or pull on a body is called force.
Characteristics of force:
- Force has both magnitude and direction, making it a vector quantity.
- It is measured in the SI unit of Newton
- It is represented by the symbol F.
Effects of Force:
- It can change the speed of a body.
- It can change the direction of Force and Laws of Motion of a body.
- It can change the shape of a body.
Balanced and Unbalanced Forces
(i) Balanced Forces: If the resultant of applied forces is equal to zero, it is called balanced forces.
For example: In the tug of war game when the force applied by both teams is equal in magnitude then the rope does not move in either side. This is due to the balanced forces in which resultant of applied forces comes out to be zero.
- Balanced forces do not cause any change of state of an object.
- Balanced forces are equal in magnitude and opposite in direction.
- Balanced forces can change the shape and size of an object. For example: When we press a balloon from opposite sides, the size and shape of balloon is changed.
(i) Unbalanced Forces: If the resultant of applied forces are greater than zero, the forces are called unbalanced forces.
To move an object unbalanced forces are to be applied from the opposite directions. In case of unbalanced forces acting on a body, it moves a in the direction of the greater force.
Unbalanced forces can:
- Change the speed and position of an object.
- Change the shape and size of an object.
Some Common Forces
- Muscular Force: The force exerted by the human body muscles is called muscular force.
- Gravitational Force: The attractional force applied by earth on an object in downward direction is called gravitational force.
- Frictional Force: The force which opposes the Force and Laws of Motion of an object while being in contact with the other object, is known as frictional force.
- Air Resistance: Force which is exerted on the objects while flying in air is named as air resistance. It acts in a direction opposite to the velocity of the object.
Newton’s Laws of Force and Laws of Motion:
There are three laws of Force and Laws of Motion those formed by Newton. They are explained below:
(i) Newton’s First Law of Force and Laws of Motion or Law of Inertia
It states that any object will remain in the state of rest or in uniform Force and Laws of Motion along a straight line until it is compelled to change the state by applying external force.
Defintion: Inertia is a property or tendency of every object to resist any change in its state of rest or of uniform Force and Laws of Motion.
It is measured by the mass of an object. The heavier the object, the greater will be its inertia.
Application of Newton's first law of Force and Laws of Motion:
- When a straight moving bus suddenly stops down, the passengers sitting inside fall in the forward direction. This is because the body of the passenger initially moving in a straight line tends to move the same way even after the brakes are applied, making the passenger fall in the forward direction.
- When we hit a carpet it loses inertia of rest and moves. But the dust in it retains inertia of rest and is left behind. Thus dust and carpet are separated.
- When a tree is shaken, it moves to and fro. But fruit remains at rest due to its inertia of rest. Due to this fruit breaks off the tree.
Momentum: The momentum of a body is defined as the product of its mass and velocity.
Thus, momentum = mass × velocity
Or, p = m x v
where, p = momentum
m = mass of the body
v = velocity of the body
The Sl unit of momentum is kilograms meters per second (kg.m/s)
Change in momentum: It is defined as the difference between final momentum and initial momentum. Let u be the initial momentum of a body and v be its final momentum, then
Change in momentum = mv - mu
Rate of change of momentum: The rate at which the momentum of an object is changing is known as the rate of change of momentum.
Applications of Momentum:
- A small bullet can penetrate into the body of a man and kill him when fired from a gun because it has a large momentum due to its great velocity.
- A person gets severely injured when hit by a fast moving vehicle which is due to the momentum of vehicle due to large mass and high velocity.
Try the following questions:
Q1. State and define the SI unit of force.
Q2. When balanced forces act on a stationary body, then the body is deformed True or False
Q3. Why people sitting in bus fall backward when bus suddenly starts?
Q4. Why leaves fall when we are shacking branch of tree?
Q5. What will be the momentum of a stone having mass of 10 kg when it is thrown with a velocity of 2 m/s?