Question

An apple of mass 250 gram falls from a tree through a vertical distance of 6 m and reaches the ground with a velocity 8 m/s. If it experiences a constant air resistance during its fall, then the magnitude of work done by the air resistance on it is (g = 10 m/s2) A 8J (B) 7 J © J C) 23 J D) 15 J​

Answer 1

Given:

An apple of mass 250 gram falls from a tree through a vertical distance of 6 m and reaches the ground with a velocity 8 m/s.

To find:

Work done by air resistance ?

Calculation:

• In this type of questions, it is best to apply WORK-ENERGY THEOREM: The net work done by all the forces will be equal to the change in kinetic energy of the object

$W = \Delta KE$

$\implies W_{g} + W_{f}= \Delta KE$

$\implies mgh+ W_{f}= \Delta KE$

$\implies mgh+ W_{f}= \frac{1}{2} m {v}^{2} - \frac{1}{2} m {u}^{2}$

$\implies mg(6)+ W_{f}= \frac{1}{2} m {8}^{2} - 0$

$\implies (0.25 \times 10)(6)+ W_{f}= 32(0.25)$

$\implies 15 + W_{f}=8$

$\implies W_{f}= - 7 \: joule$

So, magnitude of work done by air friction is 7 Joule

• OPTION B) IS CORRECT ✔️