Class 9 Chapter 9 – Force and laws of motion
 The concept of force is based on push, hit or pull.
 Pushing, hitting and pulling of objects are all ways of bringing objects in motion.
 They move because we make a force act on them.
 We also know that a force can change the shape and size of objects.
 When an object is pulled from both the sides with equal forces this forces are called balanced forces and do not change the state of rest or of motion of an object.
 If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion.
 Thus, to accelerate the motion of an object, an unbalanced force is required.
FIRST LAW OF MOTION
 An object remains in a state of rest in a straight line unless compelled to change that state by an applied force.
 In other words, all objects resist a change in their state of motion.
 In a qualitative way, the tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.
 This is why, the first law of motion is also known as the law of inertia.
INERTIA AND MASS
 Heavier or more massive objects offer larger inertia.
 Quantitatively, the inertia of an object is measured by its mass.
 Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
 The mass of an object is a measure of its inertia.
SECOND LAW OF MOTION
 In other words, there appears to exist some quantity of importance that combines the object’s mass and its velocity.
 One such property called momentum was introduced by Newton.
 The momentum, p of an object is defined as the product of its mass, m and velocity, v. That is, p = mv
 Momentum has both direction and magnitude.
 Its direction is the same as that of velocity, v.
 The SI unit of momentum is kilogrammeter per second (kg m s^{1}).
 The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
 The unit of force is kg m s^{2} or Newton, which has the symbol N.
 The second law of motion gives us a method to measure the force acting on an object as a product of its mass and acceleration.
MATHEMATICAL FORMULATION OF SECOND LAW OF MOTION
THIRD LAW OF MOTION
 The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
 These two forces are always equal in magnitude but opposite in direction.
 These forces act on different objects and never on the same object.
 In other words, there is a pair of forces and not just one force.
 The two opposing forces are also known as action and reaction forces.
 When a gun is fired, it exerts a forward force on the bullet.
 The bullet exerts an equal and opposite reaction force on the gun. This results in the recoil of the gun.
LAW OF CONSERVATION OF MOMENTUM
 The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them.
 This statement can alternatively be given as the total momentum of the two objects is unchanged or conserved by the collision.
CONSERVATION LAWS
 All conservation laws such as conservation of momentum, energy, angular momentum,
 charge etc. are considered to be fundamental laws in physics.
 These are based on observations and experiments.It is important to remember that a conservation law cannot be proved.
 It can be verified, or disproved, by experiments.
 An experiment whose result is in conformity with the law verifies or substantiates the law; it does not prove the law.
 On the other hand, a single experiment whose result goes against the law is enough to disprove it.
CBSE 9th Standard Science
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CBSE 10th Standard Science Syllabus and Revision Notes
 Chapter 1 – Matter in our surroundings
 Chapter 2 – Is matter around us pure
 Class 9 chapter 3 – Atoms & Molecules
 Class 9 Chapter 4 – Structure of the Atom
 Class 9 Chapter 5 – The fundamental unit of life
 Class 9 Chapter 6 – Tissues
 Class 9 Chapter 7 – Diversity in living organisms
 Class 9 Chapter 8 – Motion
 Class 9 Chapter 9 – Force and laws of motion
 Class 9 Chapter 10 – Gravitation
 Class 9 Chapter 11 – Work & Energy
 Class 9 Chapter 12 – Sound
 Class 9 Chapter 13 – Why do we fall ill
 Class 9 Chapter 14 – Natural Resources

Lessons
 Chapter 1 – Matter in our surroundings
 Chapter 2 – Is matter around us pure
 Class 9 chapter 3 – Atoms & Molecules
 Class 9 Chapter 4 – Structure of the Atom
 Class 9 Chapter 5 – The fundamental unit of life
 Class 9 Chapter 6 – Tissues
 Class 9 Chapter 7 – Diversity in living organisms
 Class 9 Chapter 8 – Motion
 Class 9 Chapter 9 – Force and laws of motion
 Class 9 Chapter 10 – Gravitation
 Class 9 Chapter 11 – Work & Energy
 Class 9 Chapter 12 – Sound
 Class 9 Chapter 13 – Why do we fall ill
 Class 9 Chapter 14 – Natural Resources

Question Bank
 Class 9 Question Bank Chapter 1 – Matter in our surrounding
 Class 9 Question Bank Chapter 2 – Is matter around us pure
 Class 9 Question Bank Chapter 3 – Atoms and molecules
 Class 9 Question Bank Chapter 4 – Structure of the Atom
 Class 9 Question Bank Chapter 5 – The fundamental unit of life
 Class 9 Question Bank Chapter 6 – Tissues
 Class 9 Question Bank Chapter 7 – Diversity in living Organism
 Class 9 Question Bank Chapter 8 – Motion
 Class 9 Question Bank Chapter 9 – Force and laws of Motion
 Class 9 Question Bank Chapter 10 – Gravitation
 Class 9 Question Bank Chapter 11 – Work and energy
 Class 9 Question Bank Chapter 12 – Sound
 Class 9 Question Bank Chapter 13 – Why do we fall ill
 Class 9 Question Bank Chapter 14 – Natural Resources