CBSE Science revision notes for Class 9 Chapter 9 Force and Laws of Motion are given on this page. These notes are created by Science experts at askIITians, based on the latest CBSE syllabus for Class 9 Science. You can read these free revision notes online and revise all the concepts of the chapter in less than 30 minutes. However, if you are studying this chapter for the first time, we recommend that you study the notes at your own pace and memorise the concepts first.
Chapter 9 of Class 9 Science Force and Laws of Motion includes important fundamental concepts of Physics such as force, types of forces and the laws of motion. All these topics are important from the exam point of view and competitive exams as well. Let us see what is included in CBSE Class 9 revision notes for Force and Laws of Motion.
To move a stationary object from one place to another, we need to put some effort. This effort is known as force. For Example, a push, a hit or a pull.
Fig.1- How does an object move
Whenever we push or pull an object a force acts upon them and makes them move from one place to another. Hence, the force can do the following to stationary objects –
Fig.2 - Effects of Force
Balanced Forces – When equal amounts of forces are applied to an object from different directions such that they cancel out each other, such forces are known as balanced forces.
Fig.3- Balanced Forces
Unbalanced Force – When forces applied to an object are of different magnitude (or not in opposite directions to cancel out each other), such forces are unbalanced forces.
Fig.4 – Unbalanced Forces
It is a force extended when two surfaces are in contact with each other. It always acts in a direction opposite to the direction of motion of the object.
Fig. 5 – The force of Friction
Galileo’s Observation
Galileo’s Arguments
Fig 6 - Galileo’s Arguments
Fig 7 - Galileo’s Arguments
Fig 8 - Galileo’s Arguments
Fig 9 - Galileo’s Arguments
Galileo's Inference
Whether an object is moving uniformly on a straight path or is at rest, its state would not change until and unless an external force is applied to it.
Fig 10 - Newton’s First Law of Motion
Hence, we can say that objects oppose a change in their state of motion or rest. This tendency of objects to remain in the state of rest or to keep moving uniformly is called Inertia.
Examples of Inertia
Fig 11 - Example of Inertia
Fig 12 - Example of Inertia
Inertia and Mass
Consider the image given below; it is easier for a person to push the empty bucket rather than the one that is filled with sand. This is because the mass of an empty bucket is less than that of the bucket filled with sand.
Fig 13 - Inertia and Mass
The momentum of a stationary object –
Let the mass of a stationary object be ‘m’,
Let the velocity of a stationary object be ‘v’,
The stationary object has no velocity, so v = 0,
Therefore, p = m*v = m*0 = 0
So, the momentum of a stationary object is zero.
For Example –
A cricketer when catches a ball pulls his hands in the backward direction to give some time to decrease the velocity of the ball. As the acceleration of the ball decreases the force exerted on catching the moving ball also decreases. If the cricketer would try to stop a moving ball suddenly he would have to apply a larger force.
Fig 14 - Application of the Second Law of Motion
Mathematical Formulation of the Second Law of Motion
Based on the definition of the second law of motion, we can infer that -
Fig 15 - Mathematical Formula of the Second Law of Motion
Therefore, with help of the second law of motion, we can evaluate the amount of force that is being exerted on any object. From the formula stated above, we can see that the force is directly proportional to acceleration. So the acceleration of an object can change depending upon the change in force applied.
Force = ma
SI Unit: kg-ms-2 or N (Newton)
Action and Reaction Forces
Two forces acting from opposite directions are called Action and Reaction Forces.
For Example, a ball when it hits the ground (action) bounces back with a certain force reaction.
Fig. 16 - Action and Reaction Forces
The Third Law of Motion states that –
Fig 17 - Application of the Third Law of Motion
As per the law of conservation of momentum, the sum of momenta of two objects before the collision and after collision remains the same given that no external unbalanced force acts upon them. In another way, collision conserves the total momentum of two objects.
Fig 18 - Conservation of Momentum
Consider the figure given above. Two balls A and B having certain initial velocities collide with each other. Conditions before the collision-
The figure below explains how the momentum of the balls is conserved after the collision.
Fig.19 – Conservation of Momentum
Facts about Conservation Laws
CBSE free revision notes for Class 9 Science Chapter 9 Force and Laws of Motion by askIITians include all the topics of the chapter such as the concept of force, laws of motion, inertia, momentum, acceleration, conservation of momentum.
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