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.

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.

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

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.

Viscosity-Old is Gold

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

Machine parts are jammed in cold days. Why?
During certain windstorm, light roofs are blown off, why?
Define viscosity. Does it depend on temperature?
When a smooth-flowing stream of water comes out of a faucet, it narrows as it falls. Explain why this happens.
Explain why a suction effect is experienced by a person standing close to the platform at a station when a fast train passes.
Why does a flag flutter on strong wind?

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.

If the earth is struck by meteorites, the earth will slow down slightly. Why?

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

We have,
Iω = constant
When the earth is struck by meteorites, then the moment of inertia of the earth is increased, consequently the angular velocity is decreased. So, the earth will slow down if it is struck by meteorites.

Dispersion- Old is Gold

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

When white light is dispersed by a prism, red light appears at the top of the spectrum whereas violet at the bottom. Why?
How is dispersive power related to refractive index of the material?
Explain why a mirror cannot give rise to chromatic aberration.
Why a number of dark lines seen in the spectrum of light from the sun?
Why there is no dispersion of monochromatic light?
What do you mean by dispersive power?
Explain the meaning of achromatism in a lens.
What is the cause of dispersion of light?
Why is sky blue? Explain.
The sun looks red at sun-rise and sun-set. Why?
Distinguish between chromatic aberration and spherical aberration.

Why does ice float in water?

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

The density of the ice is less than the density of the water. The upthrust on the ice is greater than or equal to the weight of the liquid displaced. So the ice floats in water.

1. State Newton’s Second Law of Motion.

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

Newton’s Second Law of Motion states that ‘the rate of change of momentum is directly proportional to the net external force applied on the body.’ i.e. F ἀ (dP/dt)

What is meant by elastic and inelastic collision?

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

The collision in which total linear momentum and total K.E. is conserved is called elastic collision.

i.e. In elastic collision,

m1u1 + m2u2 = m1v1 + m2v2

½(m1u1²+ m2u2²) = ½(m1v1² + m2v2²)

Eg: Collision between atomic or sub-atomic particles, collision between gas molecules, etc.
The collision in the total linear momentum is conserved but the total K.E. is not conserved is called inelastic collision.
i.e. In inelastic collision,

m1u1 + m2u2 = m1v1 + m2v2
But,
½(m1u1² + m2u2²) ≠ ½(m1v1² + m2v2²)

Eg: A bullet embedded in a target is one example of inelastic collision.