Introduction
CBSE class 9 physics chapter 9 Force and Law of Motion deals with the fundamental concepts of force and laws of motion. This blog article contains a detailed discussion about the types of forces, Newton’s laws of motion, and their application. Force and Laws of motion class 9 notes can be considered as a reliable companion to excel in this chapter, as it simplifies the complex concepts regarding force and illustrates all the topics with a detailed explanation.
What is force and motion?
To move a stationary object from one place to another, we need to apply some effort, and this effort is called force. Force alters an object’s state of rest or uniform straight-line motion. When we pull or push an object, we apply force. This changes the position of an object.
Motion- Motion refers to the change of position of an object with respect to a reference point and time.
Effects of force
Force can do the following with a stationary object:
- Change the direction of a moving object
- Initiate motion in a motionless object
- Force can change the shape and size of an object
- Change the velocity of a moving object
Balanced and unbalanced force
1. Balanced force – An Equal amount of force when applied from different directions, and they cancel each other out. This force is known as a Balanced force.
- A balanced force is unable to change the state of rest or state of motion of an object
- The shape and size of an object may be changed through balanced force.
- FNet ≠ 0
2. Unbalanced force: The net force exerted on an object when it is not zero, then the force applied is called unbalanced force.
- The total force acting on an object is not zero; then this type of force changes the state of uniform motion or rest.
- FNet ≠ 0
3. Frictional force – This type of force opposes motion when two surfaces are in contact with each other, as well as slide against each other. Frictional force always acts in respect of the opposite direction to the motion of the object.
4. Net force- When multiple forces act on one body, they can be resolved into a component called the net force acting on an object. It decides the direction of motion.
Law of motion
Force and Laws of Motion Class 9 Notes include a detailed explanation of the laws of motion. The laws of motion are discussed in the following points:
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Newton’s laws of motion (Laws of inertia)
Newton studied the concept of motion as developed by Galileo and then formulated the three laws of motion. These are called Newton’s laws of motion.
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Newton’s first law of motion
An object will stay in the state of rest or uniform motion until an external unbalanced force is applied. This law is also known as the law of inertia.
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Inertia
The tendency of all objects to resist a change of state of rest or motion is called inertia. Inertia depends on the mass of an object.
Example- We fall backwards when a train starts to move from a rest state. The state of our body opposes the motion of the train.
Inertia can be categorized into these types:
- Inertia of rest- An object that stays at rest will remain at rest unless an external force affects it.
- Inertia of motion – An object will continue to be in motion until an external force is applied to it.
The relation between mass and inertia
- The mass of an object is called the measure of the inertia of the object.
- Inertia depends on the mass of an object.
- Heavy objects possess more resistance due to their large inertia.
- Light objects can change their state of rest or motion because of their lower inertia.
Newton’s second law of motion
According to Newton’s second law of motion
The rate of change of the momentum of an object is proportional to the unbalanced force applied to the object in the direction of the force.
Example: A cricketer catching a ball pulls his hand backward to decrease the velocity of the ball.
Momentum
Momentum is the product of mass and velocity of an object.
- It is a vector quantity.
- Its direction is the same as that of the velocity of the object.
Momentum of a stationary object
SI unit kg m/s
P = mv [v is the velocity and m denotes the mass]
A stationary object has no velocity, so v = 0
Hence, P= m*0= 0
So it has been proved that the momentum of an object is zero.
Momentum and Mass
- As momentum is the product of mass and velocity of an object, it indicates that it is proportional to mass and velocity. So, momentum increases with the increase of mass and velocity.
- According to the concept, if a heavier and a lighter object are moving with the same velocity, the heavier object will have more momentum compared to the lighter one.
- When a small object is moving at great velocity it has tremendous momentum.
Example: A small bullet with little mass can kill a man when it is fired from a gun.
Newton’s third law of motion
- Every action has an equal and opposite reaction.
- When an object applies force to another object, the second object immediately exerts the force back on the first object.
- The forces are opposite in direction but equal in magnitude.
Example – When a bullet is fired from a gun, it moves a little backward. The distance travelled by the bullet depends on the difference between the mass of the bullet and the gun.
Conservation of momentum
If the external force on a system is zero, the momentum will be constant. The total momentum remains conserved in an isolated system.
Conservation of momentum can be expressed by the following formula:
m₁v₁ + m₂v₂ = m₁v₁’ + m₂v₂’
m₁ and m₂ – masses of two objects
V₁ and v₂ – the velocities
V₁’ and v₂’- velocity after interaction
Conclusion
Force and Law of Motion CBSE class 9 physics chapter 9 includes the fundamental principles of the movement of an object. It covers the three laws of motion and the concept of inertia, momentum. Force and Laws of Motion Class 9 Notes simplify the key concepts of this chapter by explaining the real-life examples. These notes help to quickly revise the chapter and memorise the key concepts.
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