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.

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.

Refraction through prisms- Old is Gold

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

Under what condition does a prism produce the angle of minimum deviation?
What are the advantages of total reflecting prism over plane mirror?
State the factors on which the deviation produced by a prism depend.

A pendulum clock is taken to moon. Will it gain or lose time?

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

The time period of a simple pendulum is given by

When the pendulum is taken to the moon, the value of ‘g’ will decrease and hence time period will increase. As the pendulum takes more time to complete one vibration, it will lose time.

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.

Why is the bottom of a ship made heavy?

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

We know that for the stability of a floating body, the metacenter should always lie above C.G. of the body. So, the bottom of the ship is made heavy to lower its C.G. below the metacenter to keep it stable.

Explain which one is more elastic- rubber or steel.

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

Steel is more elastic than rubber. We know that Young’s modulus is the ratio of stress to the strain. If the same force is applied to the wire of steel and rubber thread, which are of equal length and cross-section area, we will find that the extension in the rubber thread is much greater than extension in steel wire. Therefore, for a given stress, the strain produced in steel is much smaller than that produced in the rubber. This implies that Young’s modulus for steel is greater than that for rubber. Therefore, steel is more elastic than rubber.

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.

Distinguish between conservative and non-conservative force.

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

Conservative force	Non-conservative force
A force is said to be conservative if the work done by or against force is dependent only on the initial and the final position of the body and not on the path followed by the body.	A force is said to be non-conservative if the work done by or against the force is dependent on the path followed by the body.
Work done by the conservative force in a closed path is zero.	Work done by the non-conservative force in a closed path is not zero.
Eg: Gravitational force, magnetic force, electrostatic force, etc	Eg: friction, viscous force, induction force in cyclotron, etc.

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.

What is meant by the moment of a couple?

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

Moment of a couple is defined as the product of one of magnitude of two equal forces and the distance between these vectors. Two equal & opposite forces acting on the body forms the couple.