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Illustrations

Example-1

A force of 30 N acts on a body and displaces it through 10 m. Calculate the work done, if the displacement is

  1. along the force and
  2. at 60° to the direction of the force.
Solution

Given: F = 30 N and s = 10 m.

  1. Displacement is along the force, i.e., θ = 0.
     
    Work done W = Fs = 30 × 10 J = 300 J
  2. Displacement is at 60° to the force, i.e., θ = 60°.
     
    Work done W = Fs cos θ
     
    = 30 × 10 × cos 60° J
     
    =30 × 10 ×  = 150 J
 
Example-2

A boy weighing 350 N runs up a flight of 30 steps each 20 cm high, in 5 s. Calculate the power expended.

Solution

h = Total height of 30 steps

 

= 30 × 20 cm = 6 m

 

W = Work done by the boy

 

= Weight × Height

 

= 350 × 6 = 2100 J

 

Description: 63957.png
 

 
Example-3

A bullet of mass 50 g strikes a wooden plank with a velocity of 200 m s–1 and emerges out with a velocity of 50 m s–1. Calculate the work done by the bullet against the resistive force offered by the plank.

Solution

m = 50, g = 50 × 10–3 kg, u = 200 m s–1v = 50 m s–1

 

Work done = Loss of kinetic energy

 

Description: 63971.png

 

Description: 63980.png

 

= 25 × 10–3 × (3.75)104

 

= 93.75 × 10 J = 937.5 J
 





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