Question

0.21kg pine cone falls 14 m to the ground, where it lands with a speed of 13 m / s. (a) With what speed would the pine cone have landed if there had been no air resistance? (b) Did air resistance do positive work, negative work, or zero work on the pine cone? Explain

Answer 1

Answe

given,

mass of pine = 0.21 Kg

distance of ground from pine = 14 m

a) Speed of pine before it land on the ground

  Using energy conservation

   Potential energy is converted into kinetic energy

     $m g h = \dfrac{1}{2}mv^2$

     $v^2 = 2 g h$

     $v = \sqrt{2 g h}$

     $v = \sqrt{2\times 9.8 \times 14}$

     $v = \sqrt{274.4}$

            v = 16.56 m/s

b)     using work energy theorem.

      W = change in kinetic energy

      $W = \dfrac{1}{2}m(v_f^2 - v_i^2)$

      $W = \dfrac{1}{2}\times 0.21 (13^2 - 16.56^2)$

                  W = -11.04

hence, work done by air resistance is negative because velocity is decreased due to air resistance.