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Work and energy:- Physics numerical problems

 

Work and energy:- Physics numerical problems for competition exam preparation


 Q.1.  A 10 kg object is raised to a height of 6 m. How much work is done in lifting the object?  


 A) 425 joules

 B) 378 joules

 C) 500 joules

 D) 588 joules

 ANSWER= D)588 joules

 Explain:-  m= 10kg, h = 6 m

 Hence mgh = 10×6×9.8

 = 588 joules

 Work done in raising an object to a height h = At ​​h height, potential energy of the object


 Q.2.  The mass of an object is 10 kg.  Find the work done in increasing its velocity from 5 m/s to 10 m/s.  


 A) 375 joules

 B) 300 joules

 C) 510 joules

 D) 275 joules

 ANSWER= A)375 joules

 Explain:- work = change in kinetic energy

 ={ (½)×10×10²}-{(½)×10×5²

 = 500-125

 = 375 joules


 Q.3.  A force of 20N is applied on an object placed on a flat surface.  If the object is displaced by a distance of 2 m, how much work is done by the force? 


 A) 75 joules

 B) 40 joules

 C) 30 joules

 D) 57 joules

 ANSWER= B)40 joules

 Explain:- Work = Force × Distance

 = 20 × 2

 = 40 joules


 Q.4.  What will be the velocity of a 100 g mass object having 20 joules of kinetic energy? 


 A) 15 m/s

 B) 10 m/s

 C) 22 m/s

 D) 20 m/s

 ANSWER= D)20 m/s

 Explain:- Mass m = 100g

 = 0.1kg

 Kinetic energy = (1/2)mv²

 20 = (1/2)0.1×v²

 400 = v²

 v =20 m/s


 Q.5.  10HP machine has been started for 1 hour.  What is the total joule of energy consumed by the machine? 


 A) 25,432 kj

 B) 20,110 kj

 C)33,456 kj

 D)26,856 kj

 ANSWER= D) 26,856 kj

 Explain:- 10 HP = 10 × 746 Watt

 = 7460 watts

 1 watt-hour = 3600 joules

 7460 watt hour = 7460 × 3600 joules

 = 26,856000 joules

 = 26,856 kilojoules


 Q.6.  A 15 kg rock is placed at the height of a tower.  If the height of the tower is 100 metres, then how much work is done in raising the rock to the height of the tower?  


 A) 10000 joules

 B) 11200 joules

 C) 14700 joules

 D) 17000 joules

 ANSWER= C)14700 Joules

 Explain:- Potential energy of rock = mgh

 = 15×100×9.8

 = 14700 joules

 Work done in raising the rock to a height of 100 m = Potential energy of the rock at 100 m height


 Q.7.  An object of 0.5 g is moving with a velocity of 8 m/s. What will be the kinetic energy of the object?  


 A) 0.16 joules

 B) 0.016 joules

 C) 0.079 joules

 D) 1.25 joules

 ANSWER= B)0.016 Joule

 Explain:-  0.5 g = 0.0005 kg

 Kinetic energy =  (1/2)mv²

 = (1/2)×0.0005× 8²

 = 0.016 joules


 Q.8.  The momentum of an object is increased by 50%.  How much will the kinetic energy increase? 


 A) 115%

 B) 125%

 C) 120%

 D) 150%

 ANSWER= B)125%

 Explain:- Momentum = mv since the value of m will not change.  Hence the value of v is increased by 50%.  New velocity = v +(v/2)= 3v/2

 Increase in kinetic energy ={(½)m(3v/2)² -(½)mv²}

 = [(½)mv²{(9/4)-1}]

 =(½)mv²×5/4

 Percentage increase = {(5/4)÷1}×100

 = 5×25

 =125%

 {Note:- If 10 is increased by 50%, it will be 15.  (5×2) : On increasing by 50% on either of the numbers 5 or 2, the product will be 15.  If 2 is increased by 50%, it will be 3 (5 × 3 = 15).  If 5 is increased by 50%, it will be 7.5 (7.5 × 2 = 15).}


 Q.9.  A force of 10 N is applied for 4 seconds on an object of 10kg from rest.  Calculate the work done by the force.


 A) 80 joules

 B) 70 joules

 C) 20 joules

 D) 40 joules

 ANSWER= A)80 joules

 Explain:- F = 10N m= 10 kg

 a = F÷m

 = 10÷10

 =1 m/s²

 s = ut + (½)at²

 = 0×1 +(½)×1×4²

 = 0+8

 = 8 m

 Work = F × d

 = 10×8

 =80 joules

 (s=d=distance)


 Q.10.  There is a total of 8000 kg of water on a water tower 20 m high. How much work is done in raising the water? 


 A) 1568000 joules

 B) 1270000 joules

 C) 1720000 joules

 D) 1040000 joules

 ANSWER= A) 1568000 Joules

 Explain:-  potential energy = mgh

 = 20×80000×9.8

 = 1568000 joules

 work = potential energy


 Q.11.  Two objects A and B are moving with the same velocity.  If the weight of A is 10 times more than that of B, then what will be the ratio of the kinetic energies of A and B?


 A) 1:2

 B) 10:1

 C) 2:3

 D) 10:2

 ANSWER= B)10:1

 Explanation:- Let the mass of B be m then the mass of A will be 10m.

 {kinetic energy A}÷{kinetic energy B}

 = {(½)×10m ×v²} {(½)×m×v²}

 = 10÷1

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