Work, Power and Energy

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GREEBY K.VARGHEESE

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Q1.  A force F= 5i +6j – 4k acting on a body produces a displacement  S = 6i + 5k . The work done by the force is

 Answer: 10 units 

Hint: W= F.S = ( 5i +6j – 4k) . (6i + 5k ) = 30+0-20 = 10 units


Q2.  1 HP = ......

 Answer: 746 W

 

Q3. If the velocity of a body  doubles , then its kinetic energy will become...

 Answer: FOUR TIMES 

Hint:  

Kinetic energy, KE α  V2 , 

New kinetic energy, KE1 α  (2V)2 

KE1  α  4 x KE

 

Q4.  Work done by gravitational force in one revolution of  Satellite 
         around the Earth in a circular orbit 

 Answer:  Is equal to Zero

Hint: Gravitational  Force due to Earth(Radially inwards) is perpendicular
         to the direction of motion  of satellite(tangential) in circular orbit 
         around the Earth. W = F S cos( 90 ) = 0 

Q5.  During inelastic collision between two bodies which of the following quantities always  remain conserved.

 Answer: TOTAL LINEAR MOMENTUM 
 

Q6.  Two masses of 1kg and  4kg are moving with equal Kinetic energy. The ratio of the magnitudes of their momentum is...

 Answer :  1 : 2
Hint :

Momentum p = ( 2 m  E )1/2

 Where E  is the Kinetic Energy, which is constant.

So, p  α  m1/2

p1 / p2 = (m1 /m2 )1/2

p1 / p2 = ( ¼ )1/2

p1 / p2 =  ½  

p1 : p2  = 1 : 2

Q7.  A Spring of force constant 800N/m  has an extension of  5cm .
        The Work done in extending from 5cm to 15cm is...

 Answer: 8J
 

Hint :  W = 1/2Kx² = 1/2 x 800(0.15² - 0.05²) = 8J

 

Q8. A man pushes a wall and fails to displace it. He does

 Answer : NOT WORK AT ALL

Hint: Since there is no displacement, no work
 

Q9. The work performed on an object does not depend upon

 Answer: INITIAL VELOCITY OF THE OBJECT

Hint :  W = F S Cos (θ) 
 

Q10.  A ball of mass M moving with velocity V collides (elastically) 
             with an identical ball at rest. After Collision the first ball 
             comes to rest and the speed of other ball is...


 Answer:  V

Hint :
       If a body suffers collision with other body of same mass
       at rest , the velocities of two bodies are interchanged
       after collision.  

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