2018-10-01_Physics_For_You

? Decrease in potential energy

= MghMg

L

=−°

2

(c 16 os 0 )

Kinetic energy of rotation =

1

2

1

23

Iωω^2 =×ML^22

[I=ML

2

3

(Because rod is rotating about an axis

passing through its one end)]

According to law of conservation of energy,

Mg

LMLg

2 L

160

6

3

2

2

(c−°os )=⇒ωω^2 =.

19. (d) : On applying law of conservation of angular
    momentum
    I 1 Z 1 = I 2 Z 2
    For solid sphere,

Im=⇒rmrm= r

2

5

2

5

2

5

2

1

2

12

2

ωω 2

r r

n

(^2) n
2
22

 ^2









 

20. (a) : mv

H

h

2

−









=

+

+  +− 











m 

HH HH

h

()^2222

12 22

ω

H

m

v

r h

ω=

 − 

++ − 

H

hv

HHH

h

2

642

22 2

vr

H

hv

HH

h

Hh

==

 −  +− 

+− 

ω^242

5

12 2

2 2

2

2

.

Η

If h

H

=

4

, we have

v

Hv HH

H

v

′= vv v

+

 + 

=

+

(^4416) =⇒′=
5
12

1

16

4

5

14

15 3

48

35

18

5

6

22

2

..



The Sun the only star that can be seen during the day.

S

tars glow during the day, but they are impossible to see

because of the brightness and closeness of the Sun. Also, the

light coming from the Sun is scattered into the bright blue colour

that we are familiar with as being the colour of the sky. is blue-

tinted atmosphere plays a part in blocking out the stars, which

in fact shine very faintly. On the other hand, if you lived on the

moon, its lack of atmosphere would allow you to see the stars

both day and night.

The radius of the Earth can be calculated by observing the

sunset at the seashore.

I

magine you are located at the equator of the Earth and it is

mid-summer. You note the exact time the Sun sets for you.

You ask a friend living a distance S due west of you to note the

time when the Sun sets for him. If the delay in his observing the

Sunset is y hours then the circumference of the Earth C is given

by C = (24 × S) / y. e justication for this simple equation is:

If a point on the Earth moves through a distance S in y hours,

then the distance it would move in 24 hours, must be equal to the

circumference of the Earth. If you are not on the equator, some

modication of this equation may be required, but the principle

remains the same.

What constitutes the Earth’s magnetic field?

T

here is no bar magnet sitting at the center of the Earth. e

Earth’s magnetic eld arises precisely because the inside of

the Earth is hot enough to melt iron and there is iron in the core

of the Earth. e heat inside the Earth, generated as a result of

decay of radioactive elements at the core produces convection

cells which transport liquid conducting material upward and

then down again, very much like what happens when we put a pot

of water to boil. e motion of the conducting material, in rather

regular loops along with the rotation of the Earth, produces a

dynamo eect - in other words, loops of electric currents. e

Earth’s magnetic eld is supposed to be a result of these currents.

Is there any truth to the saying, “Red skies at night, sailors’

delight; red skies in the morning, sailors take warning”?

T

he saying may have some truth where storms usually

approach from the west and so in what are called storm

systems. If the western sky is red at sunset, then the area west is

free of storm cloud that would block the sunlight skirting Earth’s

curve, and you would probably have good weather for a few day.

However, if the eastern sky is red at sunrise, then the area east

is free of storm clouds, and the next storm may be coming soon

from the west.