# CBSE Class 9 Science Revision Notes Chapter 9 Force and Laws of Motion

1. Force, generally denotes push or pull.

2. Force, is defined as an external agent which changes or tends to change the state of rest or uniform motion of the body or changes the direction of the body or deforms the body.

3. Effects of force :
(i) A force can produce motion in stationary bodies.
(ii) A force can stop moving bodies.
(iii) A force can change the speed and direction of motion of bodies.
(iv) A force can bring about change in dimensions of a body.

4. Balanced forces : Equal forces acting in the opposite directions, which do not change the state of rest or uniform motion of an object are said to be balanced forces.

5. Unbalanced forces : Unequal forces acting in opposite directions, which change the state of rest or uniform motion of an object are said to be unbalanced forces.

6. Newton’s first law of motion : A body will continue in its state of rest or uniform motion in a straight line, unless compelled by some external force to change its state of rest or uniform motion.

7. Inertia : The property of matter, due to which it is incapable of changing its state of rest or uniform motion on its own is called inertia.

8. Law of inertia : The tendency of a body to continue in its state of rest or uniform motion in a straight line, even when some external force is applied, is called law of inertia.

9. Inertia of rest : The tendency of a body to continue in its state of rest, even when some external force is applied on it is called inertia of rest.

10. Inertia of motion : The tendency of a body to continue in its state of uniform motion in a straight line, even when some external force is applied on it is called inertia of motion.

11. Momentum : The force possessed by a body due to the combined effect of mass and velocity is called momentum. Mathematically, it is the product of mass and velocity i.e. p = mv.

12. Units of momentum : Momentum is a vector quantity. Its unit in SI system is newton-second (N-s).                  1 N-s = 1 kg ms–1

13. Law of conservation of momentum : In a given system, the sum total of momenta is a constant quantity and hence cannot be increased or decreased by applying an external force.

14. Newton’s second law of motion : The rate of change of momentum of a body is directly proportional to the impressed force and takes in the same direction in which the force acts.

15. Mathematically, force is the product of mass and acceleration.
F = ma

16. Absolute units of force : In C.G.S. system, absolute unit of force is dyne and in SI system is newton.

17. Newton : A force which produces an acceleration of 1 ms–2 in a body of mass one kilogram is called one newton.
1 N = 1 kg × 1 ms–2

18. Gravitational unit of force is known as kilogram force (kgf).

19. Kilogram force (kgf) : A force which produces 9.8 ms–2 acceleration due to gravity of earth in a mass of one kilogram is called kilogram force.
1 kgf = 1 kg × 9.8 ms–2 = 9.8 N.

20. Impulse or impulsive force : When a force of large magnitude acts on a body for a very short interval of time, then the collective effect of force and time is called impulsive force.

21. Unit of impulsive force : SI unit of impulsive force is newton-second (N-s).

22. Newton’s third law of motion : “To every action, there is an equal and opposite reaction.”

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