Physics Times – July 2019

5. 
   

   A train S 1 , moving with a uniform velocity of
   108 km/h, approaches another train S 2 standing
   on a platform. An observer O moves with a
   uniform velocity of 36 km/h towards S 2 , as
   shown in figure. Both the trains are blowing
   whistles of same frequency 120 Hz. When O
   is 600 m away from S 2 and distance between
   S 1 and S 2 is 800 m, the number of beats heard
   by O is __.
   (Speed of the sound = 330 m/s)

   
   


6. 
   

6.6. A block of weight 100 N is suspended by
copper and steel wires of same cross sectional
area 0.5 cm^2 and, length 3 m and 1 m,
respectively. Their other ends are fixed on a
ceiling as shown in figure. The angles
subtended by copper and steel wires with
ceiling are 30° and 60°, respectively. If
elongation in copper wire is lc and

elongation in steel wire is ls, then the ratio

C

S

l

l



 is ___.

(Young’s modulus for copper and steel are 1

× 10^11 N/m^2 and 2 × 10^11 N/m^2 , respectively)

SECTION-

1. a 2. a 3. b 4. d

SECTION-

1. (a,c&d) 2. (b, d) 3. (a,b&d)
   4. (b, c) 5. (b,c,d) 6. (a,c&d)
   7. (a,b&d) 8. (a,b)

SECTION-

1. 1.00 2. 50.00 3. 270.00, 120.


2. 0.75 5. 8.12 or 8.13 6. 2.

SECTION-

1.Sol:

Field at C

As Field by the total shell at C is zero.

So we can assume the hole as a combination

of dq and -dq. So the net Filed is only due to

-dq.

SECTION-

SECTION-

SECTION-

Let Q is the charge on the shell.

Given that Potential at the surface is

0

KQ

V

R



Potential at C

(^0)
( )
C (1 )
KQ K Q
V V
R R
Potential at B

   
0
( )
(1 2 )
B /
KQ K Q
V V
R R

    
1
1 2
C
B
V
V



 

Field at A
0
A (2 ) 2 2 4 2
KQ K Q KQ V
E
R R R R
 
   
So reduced by
SECTION-
1.Sol:
Potential at C
Potential at B
Field at A
Field at C
V 0
R

0
2
( )
C
K Q V
E
R R
 
 