A coolie walks on a level platform carrying a load on his head. The work done by him against gravity is
- A.zero
- B.maximum
- C.equal to the weight of the load
- D.negative
Correct answer
Explanation
The correct answer is A, zero. The force he applies to support the load acts vertically upward, while his displacement along the platform is horizontal, so the angle between force and displacement is ninety degrees and the work done against gravity is zero, however tired he may feel.
Option B, maximum, would apply only if the force and the displacement were in the same direction, as when he climbs a staircase with the load. Option C confuses force with work; the weight of the load is a force in newton, whereas work would be that force multiplied by a vertical displacement. Option D, negative, applies when the force opposes the motion, as friction does. This question is the classic example of the difference between the ordinary and the scientific sense of the word work.
Read the full article: Work, Energy and Power: Formulas, Units and Conversions
Practice Questions
View allWhat is the SI unit of work?
- A.Newton
- B.Joule
- C.Watt
- D.Pascal
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Work is the product of force and displacement, so its unit is the newton metre, which is given the name joule; one joule of work is done when a force of one newton moves a body through one metre in its own direction.
Read the full explanation →The work done by a force is zero when the angle between the force and the displacement is
- A.0 degrees
- B.45 degrees
- C.90 degrees
- D.180 degrees
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Work equals force into displacement into the cosine of the angle between them, and the cosine of ninety degrees is zero, so a force at right angles to the motion does no work at all.
Read the full explanation →The kinetic energy of a body of mass m moving with speed v is given by
- A.mass into g into height
- B.half of mass into the square of the speed
- C.mass into the speed
- D.mass into the square of the speed
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Kinetic energy is the energy a body has because of its motion, and it grows with the square of the speed, so a body moving twice as fast carries four times the kinetic energy.
Read the full explanation →What is the SI unit of power?
- A.Joule
- B.Watt
- C.Newton second
- D.Kilowatt hour
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Power is the rate of doing work, that is work divided by the time taken, so its unit is the joule per second, which is named the watt after James Watt.
Read the full explanation →One horsepower is approximately equal to
- A.100 watt
- B.500 watt
- C.746 watt
- D.1000 watt
Show answer
The horsepower is an older unit of power that survives in the rating of motors and vehicles, and one horsepower is taken as about 746 watt, that is roughly three quarters of a kilowatt.
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