Power And Velocity
A car engine delivers a constant power of 50 kW while the car moves at a steady speed of 25 m/s along a level road against resistive forces. What is the total resistive force opposing the car's motion?
Select the correct option:
Solution
2000 N
As stated in NCERT Class 11, Chapter 6 (Work, Energy and Power), instantaneous power for a force acting along the direction of motion is P = F·v. Because the car moves at steady speed, its kinetic energy is constant, so the driving force exactly balances the resistive force. Rearranging, F = P/v = 50000 / 25 = 2000 N. This is the magnitude of the resistive force the engine must overcome. The option 1250 N divides by an incorrect speed. The option 5000 N misplaces a factor of ten in the power. The option 1000 N halves the correct result without justification. A unit and magnitude check: watts divided by metres per second gives newtons, and a resistive force of 2000 N for a car cruising at 25 m/s with 50 kW output is physically reasonable for highway driving.
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About This Question
- Subject
- physics
- Chapter
- work, energy and power
- Topic
- power and velocity
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
2000 N
As stated in NCERT Class 11, Chapter 6 (Work, Energy and Power), instantaneous power for a force acting along the direction of motion is P = F·v. Because the car moves at steady speed, its kinetic energy is constant, so the driving force exactly balances the resistive force. Rearranging, F = P/v = 50000 / 25 = 2000 N. This is the magnitude of the resistive force the engine must overcome. The option 1250 N divides by an incorrect speed. The option 5000 N misplaces a factor of ten in the power. The option 1000 N halves the correct result without justification. A unit and magnitude check: watts divided by metres per second gives newtons, and a resistive force of 2000 N for a car cruising at 25 m/s with 50 kW output is physically reasonable for highway driving.
This medium difficulty physics question is from the chapter work, energy and power, covering the topic of power and velocity. It appeared in the 2025 exam.
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