class 11 cbse science physics parallax length mass

1. UNIT AND MEASUREMENT
Class – 11th
Aditya Mishra
Email: adityakumar2009@gmail.com
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2. MEASUREMENT OF LENGTH
Measurement of length
Direct method
1 2
Indirect method
- Measurement of length using
measuring instruments such as
meter scale, Vernier caliper ,
Micro-meter
- Absolute measurement
- Measurement of length using
comparison with the standard
object E.g Dial gauge
- Comparative measurement
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3. 1. Parallax method :
- Indirect method
- Used to measure large distances
e.g. distance between planet and star from earth
MEASUREMENT OF LARGE DISTANCES
o
1 = 1 = 1 degree

2 = 1 = 1 minute
 
3
-6
= 1 = 1 second
= 4.85 × 10 rad
 
o
1 = 60
1 = 60
 
-6
-4
= 60 1
= 60 4.85 x 10 rad
= 2.908 10 rad




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4. WHAT IS PARALLAX?
• Change in the position of the object with respect to the background
when an object is seen from two different positions is called parallax
L L
R
Seen from left eye Seen from right eye
(a) (b)
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5. MEASUREMENT OF SMALL DISTANCES


O
P
Q
b (basis)

D
D
• Assuming the object is far away i.e. D >>>> b , b/D << 1 ,
• is very small
• arc PQ = straight line PQ s = b
S r 
 
b D 
 
b
D
 

Parallax
Angle
PQ = b
OP = OQ = D
POQ = 

s
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6. PROBLEM
Q.1 A moon is observed from two diametrically opposite points A and B on
Earth. The angle theta subtended at the moon on the two directions of
observation is 1o54/. The diameter of Earth is 1.267 x 107m. Compute the
distance of moon from earth ?
Ans : Given :
o
= 1 54
 
= (60 +54 )
= 114
 

-6
= 114 1
= 114 4.85 10 rad


 
-2
= 3.32 10 rad

7
d = 1.267 10 m

To Find :
Solution :
We know from the parallax method,
Paralytic angle
b
=
D

Distance between sun
and earth (D)
D θ = b
 
D =
b

7
2
1.276 10
=
3.32 10


7 2
1.276 10
=
3.32


8
D = 3.84 10 m

 
o
1 = 60
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7. MEASUREMENT OF SMALL DISTANCES
Mono-layer : single layer of molecules
t
Volume of the solution
t =
Area of the solution
n V
=
A
t = dimaeter of one molecule (d)
n = number of atoms present in
mono-layer
V = total volume of solution
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8. MEASUREMENT OF MASS
Measurement of very small masses:
• Mass of atoms and molecules cannot be measured in kg
• Unified atomic mass unit (u)
1
1 = of mass of carbon -12 ( including mass of electrons)
12
u
-27
1 = 1.66 10 kg
u 
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