UNIT.1.LAWS OF MOTION
1. The three laws of force proposed by Sir Isaac Newton.
2. Mechanics is the branch of physics that deals with the effect of force on bodies.
3. Mechanics is divided into two branches, namely, statics and dynamics.
4. Statics deals with the bodies, which are at rest under the action of forces.
5. Dynamics is the study of moving bodies under the action of forces.
6. Dynamics is further divided as Kinetics, Kinematics.
7. A moving body naturally comes to rest without any external infl uence of the force. Such motions are termed as ‘natural motion’ (Force independent)
8. Any body would like to continue to be in its state of rest or the state of motion. This is known as ‘inertia’.
9. The resistance of a body to change its state of rest is called inertia of rest
10. The resistance of a body to change its state of motion is called inertia of motion.
11. The resistance of a body to change its direction of motion is called inertia of direction.
12. An athlete runs some distance before jumping. Because, this will help him jump longer and higher is an example for Inertia of motion.
13. When you make a sharp turn while driving a car, you tend to lean sideways, is an example for Inertia of direction.
14. When you vigorously shake the branches of a tree, some of the leaves and fruits are detached and they fall down, is an example for Inertia of rest.
15. The product of mass and velocity of a moving body gives the magnitude of linear momentum
16. Linear Momentum = mass × velocity : p = m v
17. Unit of momentum in SI system is kg m s–1 and in C.G.S system its unit is g cm s-1
18. Force has both magnitude and direction. So, it is a vector quantity.
19. If two parallel forces F1,F2 are acting in same direction then the resultant is equal to F= F1+F2.
20. If two parallel unequal forces F1,F2 are acting in opposite direction then the resultant is equal to (i) F1 - F2.If F1>F2. And (ii) F2 – F1.If F2>F1.
21. If two Parallel equal forces F1,F2 acting in opposite directions in the same line of action then the resultant is equal to (ie) ) F1 - F2 = 0.
22. If the resultant force of all the forces acting on a body is equal to zero, then the body
will be in equilibrium. Such forces are called balanced forces.
will be in equilibrium. Such forces are called balanced forces.
23. If the resultant force is not equal to zero, then it causes the motion of the body due to unbalanced forces.
24. ‘Equilibrant’ which is equal to the resultant force in magnitude, but opposite in direction.
25. Tug of war is an example for unlike parallel forces whereas action of a lever is example for unbalanced forces.
26. The rotating or turning effect of a force about a fixed point or fixed axis is called
moment of the force about that point or torque (τ).
moment of the force about that point or torque (τ).
27. Torque is a vector quantity, Its SI unit is N m.
28. Two equal and unlike parallel forces applied simultaneously at two distinct points
constitute a couple.
constitute a couple.
29. Rotating effect of a couple is known as moment of a couple.
30. Turning a tap, winding or unwinding a screw, spinning of a top, etc are examples for moment of couple..
31. Moment of a couple M = F × S.
32. The unit of moment of a couple is newton metre (N m) in SI system and dyne cm in
CGS system.
CGS system.
33. Couple is taken as positive if the body is rotated in the anti-clockwise direction and negative if it is rotate in the clockwise direction.
34. As per principle of moments moments in clockwise direction is equal to moments in anti clockwise direction.
35. Newton’s first law otherwise known as “law of inertia” and Newton’s second law otherwise known as “law of force”.
36. The acceleration is produced along the radius called as centripetal acceleration.
37. SI unit of force is newton (N) and in C.G.S system its unit is dyne.
38. In the SI system of units, gravitational unit of force is kilogram force, represented by kg f.
39. A large force acting for a very short interval of time is called as impulsive force.
40. Unit of impulse is Kg m s-1 or Ns.
41. In cricket, a fielder pulls back his hands while catching the ball resulting in a lesser impulse on his hands.
42. The velocity which is just sufficient to just escape from the gravitational pull of the Earth. This velocity is called escape velocity.
43. Propulsion of rockets is based on the law of conservation of linear momentum as well as Newton’s III law of motion.
44. G is the universal gravitational constant and its value in is 6.674 × 10–11 N m2 kg–2.
45. Unit of acceleration due to gravity is m s–2.
46. Mean value of the acceleration due to gravity is taken as 9.8 m s–2.
47. The velocity of a body during the downward free fall motion varies by 9.8 m s–1 for every 1 second.
48. The radius of the Earth is R = 6378 km.
49. Relation between g and G is g =GM/R2.
50. Mass of the Earth M = g R2/G and its value is 5.972 × 1024 kg.
51. The value of acceleration due to gravity(g) is maximum in the polar region and minimum at the equatorial region.
52. Relation between radius of earth and acceleration due to gravity is g ∝ 1/R2.
53. Mass of a body is defined as the quantity of matter contained in the body. Its SI unit is
kilogram (kg).
kilogram (kg).
54. Weight of a body is defined as the gravitational force exerted on the body due to the Earth’s gravity alone.
55. Weight is a vector quantity.
56. SI unit of weight is newton (N).
57. The value of acceleration due to gravity on the surface of the moon is 1.625 ms–2
58. The value of acceleration due to gravity on the surface of the moon is about 0.1654 times the acceleration due to gravity of the Earth.
59. If a person whose mass is 60 kg stands on the surface of Earth and the same person goes to the surface of the Moon.His mass will be 60 kg or same.
60. Lift is moving upward with an acceleration”a” apparent weight is greater than the actual weight.
61. Lift is moving downward with an acceleration”a” apparent weight is lesser than the actual weight.
62. When the lift is at rest the apparent weight is equal to the actual weight.
63. When the lift is falling down freely the apparent weight is equal to zero.
64. When we move to a higher altitude from the surface of the Earth, the value of g reduces.
65. when you move deep below the surface of the Earth, the value of g reduces.
66. Value of acceleration due to gravity is zero at the centre of the Earth.
67. If a person whose mass is 60 kg stands on the surface of Earth, his weight would be 588 N.
68. If a person whose mass is 80 kg stands on the surface of Earth, his weight would be 784 N.
69. If a person whose mass is 60 kg stands on the surface of moon, his weight would be 97.5 N .
70. If a person whose mass is 90 kg stands on the surface of moon, his weight would be 146.25 N
71. Mass of the Earth M = g R2/G.Therefore the expression for “g”= GM/R2.
72. As per Newton’s universal law of gravitation F=Gm1m2/r2.
73. When birds fly they push the air downwards with their wings is action and the air pushes the bird upwards is reaction.
74. When a person swim he pushes the water using the hands backwards is action and the water pushes the swimmer in theforward direction is reaction.
75. When you fire a bullet, the gun recoils backward and the bullet is moving forward is action and the gun equalises this forward action by moving backward is reaction.