Question

Certain force acting on a 20kg mass changes its velocity from 5m/s to 2m/s.calculate the work done by the force

Answer 1

m= 20 kg
u= 5m/s
v= 2m/s
K.E(1)= ½mu²= ½ 20× 25 = 250J
K.E(2)=½mv²= ½ 20× 4 = 40 J
Work done= K.E(1)-K.E(2)
= 210 J

Answer 2

Mass, m = 20 kg

Initial velocity, u = 5 m/s

Final velocity, v = 2 m/s

Time, t = 1s

From first equation of motion 

      

      v = u + at

or, a = (v - u)/t 

 

a = (2 ms-1 - 5 ms-1)/15

a = -3 ms-2

From third equation of motion,

 

v² - u² = 2as

⇒ (2ms-1)² - (5ms-1)² = 2 x (-3ms-²) x s

⇒          -21 m²s-² = -6 ms-² x s

⇒ s = 7/2 m

Work done = F x s

Force = m x a

Therefore,

 

W = m x a x s

   = 20 kg x (-3ms-²) x 7/2 m

  = -210 Joules