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NEET PHYSICSWORK, ENERGY AND POWERMedium

Question

Consider a drop of rainwater having a mass of 1 g1\text{ g} falling from a height of 1 km1\text{ km}. It hits the ground with a speed of 50 m/s50\text{ m/s}. Take gg as constant with a value 10 m/s210\text{ m/s}^2. The work done by the (i) gravitational force and the (ii) resistive force of air is:

A

(i) 1.25 J1.25\text{ J}; (ii) 8.25 J-8.25\text{ J}

B

(i) 100 J100\text{ J}; (ii) 8.75 J8.75\text{ J}

C

(i) 10 J10\text{ J}; (ii) 8.75 J-8.75\text{ J}

D

(i) 10 J-10\text{ J}; (ii) 8.75 J-8.75\text{ J}

Step-by-Step Solution

  1. Calculate Work Done by Gravity (WgW_g): The work done by the gravitational force is Wg=mghW_g = mgh. Given: m=1 g=103 kgm = 1\text{ g} = 10^{-3}\text{ kg}, h=1 km=1000 mh = 1\text{ km} = 1000\text{ m}, g=10 m/s2g = 10\text{ m/s}^2. Wg=103×10×1000=10 JW_g = 10^{-3} \times 10 \times 1000 = 10\text{ J} [Class 11 Physics, Ch 6, Sec 5.2, Example 5.2]
  2. Calculate Change in Kinetic Energy (ΔK\Delta K): The drop starts from rest (vi=0v_i=0) and reaches a final speed vf=50 m/sv_f=50\text{ m/s}. ΔK=KfKi=12mvf20=12×103×(50)2=25002000=1.25 J\Delta K = K_f - K_i = \frac{1}{2}mv_f^2 - 0 = \frac{1}{2} \times 10^{-3} \times (50)^2 = \frac{2500}{2000} = 1.25\text{ J}
  3. Apply Work-Energy Theorem: The change in kinetic energy is equal to the total work done by all forces (gravity + resistive force WrW_r). ΔK=Wg+Wr\Delta K = W_g + W_r 1.25 J=10 J+Wr1.25\text{ J} = 10\text{ J} + W_r Wr=1.2510=8.75 JW_r = 1.25 - 10 = -8.75\text{ J} Thus, the work done by gravity is 10 J10\text{ J} and by the resistive force is 8.75 J-8.75\text{ J}.

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from WORK, ENERGY AND POWER. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSWORK, ENERGY AND POWERconsiderrainwaterhavingfallingheight

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