Physics: Measurements

Physics: Measurements

 

• The International System of Units (SI) based on seven base units.

• The SI Unit of length is metre (m).
• SI Unit of amount of substance is mole
• SI Unit of temperature is Kelvin
• Unit of Power is Joule/sec or Watt.
• SI Unit of electric current is Ampere
• For measuring large distances, the light year is the used.
• 1 light year = 9.46 × 1015m
• 1 tonne (t) = 1000 (or 103) kg
• Unit of Time: The SI unit of time is the second(s)
• The ordinary mile is more precisely known as the statute mile
• SI Unit of luminous intensity is Candela
• Radius of the earth: 6.4 × 106 m.
• The planet Mars has 1/2 the radius of the earth, the size of a geosynchronous orbit is 6.5 earth radii.
• The earth-moon separation is about 60 earth radii.
• The mean distance from the earth to the sun is called an astronomical unit.
• An astronomical unit approximately 1.5 × 1011 m.
• The star nearest the Sun, Proxima Centauri, is about 270,000 AU away.
• To measure small lengths, we use Vernier callipers or screw gauge.
• The unit ‘litre’ is also used to measure the volume of liquids.
• Standard measuring vessels are used to measure volumes of liquids like milk, kerosene oil, mobile oil at petrol pumps, etc.
• Distance is a scalar quantity
• SI Unit of Frequency is Hertz
• The weight may vary from place to place but the mass stays constant.
• SI Unit of Energy is Joule
• SI Unit of Pressure is Pascal
• 1 parsec = 3.08 x 1016 = 3.26 Light year
• 1 Horse power = 746 W
• Our sense of touch is not always a reliable guide to the degree of hotness of an object.
• Temperature is a measure of the degree of hotness of an object.
• Thermometer is a device used for measuring temperatures.
• Clinical thermometer is used to measure our body temperature.
• The range of this thermometer is from 35°C to 42°C.
• The range of laboratory thermometers is usually from –10°C to 110°C.
• The normal temperature of the human body is 37°C.
• The distance moved by an object in a unit time is called its speed.
• Periodic motion of a pendulum has been used to make clocks and watches.
• Motion of objects can be presented in pictorial form by their distance - time graphs.
• The distance-time graph for the motion of an object moving with a constant speed is a straight line.
• The shortest distance measured from the initial to the final position of an object is known as the displacement.
• Automobiles are fitted with a device that shows the distance travelled. Such a device is known as - odometer.
• As the object covers equal distances in equal intervals of time, it is said to be in uniform motion.
• The tendency of undisturbed objects to stay at rest or to keep

• moving with the same velocity is called inertia.

• The first law of motion is also known as the law of inertia.
• 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.
• In an isolated system (where there is no external force), the total momentum remains conserved.
• Heavier or more massive objects offer larger inertia.
• Law of conservation of momentum is based on Newtons 3^rd law of motion.
• Mechanical motion is of two types, transitional (linear) and rotational (spin).
• The instantaneous velocity shows the velocity of an object at one point.
• Acceleration shows the change in velocity in a unit time.
• Force acting on an object may cause a change in its state of motion or a change in its shape.
• When the velocity decreases the body is said to undergo retardation or deceleration.
• Galileo was the first to find out that all objects falling to Earth have a constant acceleration of 9.80 m/s2 regardless of their mass.
• The weight of a body is maximum at the poles and minimum at equator.
• On the surface of the moon, the value of the acceleration due to gravity is nearly one-sixth of that on earth.
• An object on the moon would weigh only one-sixth its weight on earth.
• Force acts in the opposite direction to the way an object wants to slide.
• Measures of friction are based on the type of materials that are in contact.

• Concrete on concrete has a very high coefficient of friction