Question

certain force acting on a 20 kg mass changes into velocity from 5 ms^-1 to 2 ms ^-1 . calculate the work done by the force

Answer 1

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