TyroCity: Physics XI Notes

Units in different systems

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Generally we can use any convenient unit to measure a physical quantity depending on how much magnitude we are measuring or in which system of units we want to measure it.
What kind of unit we should use?
The unit i) must be accepted internationally.
ii) Should be reproducible.
iii) Should be invariable.
iv) Should be easily available.
v) Should be consistent.
vi) Should be large, if the physical quantity to be measured is a big quantity.
Ex: To measure larger lengths we use units like Km, mt etc, to measure large magnitude of time we use units like hour , day ,week, month , year etc.
vii) Should be small if the physical quantity to be measured is small.
Ex: To measure small time we use units like millisecond, microsecond etc
To measure small lengths we use units like millimeter, centimeter etc.

Types of physical Quantities.:
We can broadly divide the physical quantities in to two types

i)Fundamental Physical quantities
ii)Derived physical quantities.

Fundamental physical quantities: A physical quantity which can exist independently is called Fundamental physical quantity.
Ex: Length, mass and time etc.

Derived physical quantities: A physical quantity which can not exist independently is called derived physical quantity. (Or) A physical quantity which is dependent or derived from any other physical quantity is called derived physical quantity.
Ex : Area, volume, density, speed, acceleration, force, energy etc.
Like the physical quantities we can divide the units in to two types. I)Fundamental units ii)derived units.

Fundamental units : The units of fundamental physical quantities are called fundamental units, (or) The units which are independent or can not derived from any other unit is called fundamental unit.
Ex:Every unit of length is fundamental unit (irrespective of the system to which it belongs);millimeter, centimeter, meter, kilometer etc.
Every unit of time is a fundamental physical quantity ; microsecond, millisecond, second, minute, hour, day etc.

Derived units: The units of derived physical quantities are called derived units. Units of area, volume, speed, density, energy etc are derived units.
Ex: Every unit of speed is a derived unit ; m/sec, cm/sec, km/hr etc.
Every unit of density is a derived unit; kg/m³, gr/cm³ etc.
Every unit of acceleration is a derived unit; m/sec², cm/sec², km/hr² etc.

Systems of units:To measure the fundamental physical quantities Length, Mass and time we have three systems of units, they are i) C.G.S System (Metric system)ii)F.P.S System (British system) and iii)M.K.S System. In all these three systems only three physical quantities length, mass and time are considered to be fundamental quantities.

But, in systems International (S.I) system there are seven fundamental physical quantities. Which are i)Length ii)Mass iii)Time iv)Electric current v)Thermo dynamic temperature vi)Luminous intensity vii)Quantity of substance.
In addition to these two more quantities were added as supplementary physical quantities. They are i)Plane angle ii)Solid angle.

Systems,Fundamental physical quantities and their units:In
C.G.S system: Length (centimeter); Mass (gram); Time (second).
F.P.S system :Length (foot);Mass(pound);Time (second).
M.K.S system: Length (meter); Mass (kilogram); Time (second).
S.I System:Length (meter); Mass (kilogram); Time (second); Electric current (ampere); Thermodynamic temperature (kelvin); Intensity of light (candela); Quantity of matter (mole). The units of suplimentary quantities are Plane angle( radian); Solid angle(Steradian).

Why is a cow more stable than a man?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The C.G. of the cow is lowered to the base of the body and the base of the foot is greater than the man, so the cow is more stable than the man.

What do you mean by geo-stationary satellite? Explain?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The satellite which appears to be at rest from the surface of the earth is called geo-stationary satellite. The time period of geo-stationary satellite is equal to the time period of the earth. The orbit of geo-stationary satellite is called parking orbit.

How does K.E. of an object change if its momentum is doubled?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

We have,

(K.E.)1= P²/2m

Where ‘P’ is momentum and ‘m’ is the mass of the object.
If the momentum is doubled then,

(K.E.)2 = (2P)²/2m

Or, (K.E.)2 = 4(K.E.)1

Therefore, on doubling the momentum, the K.E. of the body increases by four times its initial K.E.

During pregnancy, woman often develop back pains from leaning backward while walking. Why do they have to walk this way?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

To be in stable equilibrium, the vertical line through the C.G. should pass through the base. So, woman leans backward to make the vertical line through the C.G. pass though the base and hence to be stable.

Does a ship sink more in river water or in sea water? Explain.

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

A ship sink more in river water than in sea water because the density of sea water is more than that of river water due to the presence of impurities.

A rigid, lighter than air balloon filled with Helium cannot continue to rise indefinitely. Why?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The density of the atmospheric air is different at different level. The density of the air at upper level is less in comparison to the lower level. When the balloon rises up in air, the weight of the displaced air decreases as the density of the air decreases with height. Hence the upthrust decreases and becomes equal to zero then the balloon stops to rise.

How will you make difference between density and specific gravity of a body?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Density	Specific gravity
The density of the substance is defined as the mass per unit volume of that substance.	Specific gravity of a substance is defined as the ratio of the mass of certain volume of it to the mass of the same volume of water at 4ËšC.
Its unit is kg/m³ or g/c.c.	It has no unit because it is the ratio of same physical quantity.

Why is easier to lift a body in a liquid than in air?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

It is easier to lift a body in a liquid than in air because the upthrust on the body in liquid is greater than in air.

A man carrying a bucket of water on his hand always leans to the opposite side. Explain.

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

To be in stable equilibrium, the vertical line through the C.G. should pass through the base. So, a man carrying a bucket of water on his hand leans to the opposite side to make the vertical line through the C.G. pass though the base and hence to be stable.

Work

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Work is said to be done if a force displaces any object in any direction other than 90˚ to the direction of force applied.
If ‘F’ be the force applied and ‘d’ be the displacement and ‘θ’ be the angle between ‘F’ and ‘d’, work done by the force is,
W = Fcosθd
= Fdcosθ
W= F.d
Work is a scalar quantity. Its SI unit is Joule.

a) When the angle between F and d is acute, the work done by the force is positive.
Eg:- Pushing a duster moves the duster in the direction of force. So θ=0˚ then,
W = Fdcosθ
= Fcos0˚
= Fd [Work is positive]

b) When the angle between F and d is 90˚, the work done is zero.
Eg:- A coolie carry a load and walking on horizontal road does no work because the direction of force is vertical while he moves in horizontal direction. So,
W = Fdcos90˚
= 0

c) When the angle between F and d is 90˚<θ≤180˚, then work done is negative.
Eg:- When a body is pushed, it moves in the direction of force. Work done by the force is positive but the work done by friction is negative.
W = Frdcos180˚
= -Frd

If a cork is released from the bottom of a jar filled with water, what will happen and why?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

If a cork is released from the bottom of a jar filled with water, the cork will rise up and float on the water surface. It is due to the fact that density of cork is less than density of water, and upthrust will be greater than its own weight and the upthrust force it to rise.