A stone is held at the top of a tower. Its energy at that point is entirely
- A.kinetic energy
- B.potential energy
- C.half kinetic and half potential
- D.heat energy
Correct answer
Explanation
The correct answer is B, potential energy. A stone at rest at a height has no motion and therefore no kinetic energy, but it possesses gravitational potential energy equal to its mass into the acceleration due to gravity into the height of the tower.
Option A, kinetic energy, is what that potential energy becomes as the stone falls, and it is greatest at the instant the stone reaches the ground. Option C, an equal share of both, holds only at the point where it has fallen through half its height, ignoring air resistance. Option D, heat energy, appears only after the impact, when the mechanical energy is dissipated. Through the fall the sum of the two mechanical forms stays constant, which is the law of conservation of energy in its simplest illustration.
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
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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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