Question

A coconut of mass m falls from the tree through a vertical distance of s and could reach ground with a velocity of v ms^-1 due to air resistance. Work done by air resistance is

• A.
  • 
• B. – $\frac { 1 } { 2 }$ mv²
• C. mgs
• D. mv² + 2mgs

Answer 1

Answer: (A)

• 

Answer 2

Work done = Change in K.E.

i.e. W_g + W_a = $\frac { 1 } { 2 }$ mv²

where W_g = mgs and W_a is the work done by air resistance

∴ W_a = -mgs + $\frac { 1 } { 2 }$ mv²

= - $\frac { \mathrm { m } } { 2 }$ (- v² + 2gs)

= - $\frac { \mathrm { m } } { 2 }$ (2gs – v²)