JEE Physics/ Lakshmikanta Satapathy/ MCQ On Work Energy Power/ Work-Energy theorem/ Work done by Gravity/ Work done by Air resistance/ Change in Kinetic Energy of body

1. Physics Helpline
L K Satapathy Work Energy Power 3
Work Energy Theorem
1Km
G RW W K  

2. Physics Helpline
L K Satapathy Work Energy Power 3
Consider a drop of rain water , having mass 1 gram ,
falling from a height of 1 km as shown in the figure. It
hits the ground with a speed of 50 m/s. Take ‘g’ as
constant with a value of . The work done by
(i) the gravitational force and (ii) the resistive force of
air respectively are
(a) (i) 1.25 J (ii) – 8.25 J
(b) (i) 100 J (ii) 8.75 J
(c) (i) 10 J (ii) – 8.75 J
(d) (i) – 10 J (ii) – 8.25 J
Question :
1Km
50m s
2
10 /m s

3. Physics Helpline
L K Satapathy Work Energy Power 3
Answer :
Work done by gravitational force :
2 3
10 10 10 ( )GW mgh N m J Ans
   
Let work done by resistive force RW
Gravitational force () = Weight of rain drop
3 2 2
10 10 10GF mg kg m s N 
    
Displacement () = 1 km
3
10S h m  

4. Physics Helpline
L K Satapathy Work Energy Power 3
 Change in kinetic energy 2 21 1
0
2 2
K mv mv   
3 21 2500
10 50 1.25
2 2000
K J
      
1.25 10 8.75 ( )R GW K W J Ans       
Correct option = (c)
Initial KE = 0 & Final KE
21
2
mv
Work energy theorem G RW W K   

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L K Satapathy
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