1. A coolie is moving on a road with a luggage on his head. Does he perform work against the force of gravity? Give reason for your answer. 2. The moon is revolving around the earth in a circular path. How much work is done by the moon? 3. Give reasons for the following: (a) No work is done if a man is pushing against a wall. (b) Hammer drives a nail into the wood only when it is lifted up and then struck. (c) A horse and a dog are running with the same speed. Which one of them has more kinetic energy than the other? (d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work. 4. Name the form of energy to which potential energy can change. 5. Is it practically possible to convert a form of energy completely into another useful form? Give reason.
Answer (1)
A coolie moving on a road with luggage on his head:
When a coolie walks on a level road with luggage on his head, he does not perform work against the force of gravity because the force he applies (upward force to balance the luggage) is perpendicular to the displacement (horizontal movement). According to the formula for work, W = F ⋅ d ⋅ cos ( θ ) , where θ is the angle between the force and the direction of displacement, no work is done when θ = 9 0 ∘ because cos ( 9 0 ∘ ) = 0 .
The work done by the moon revolving around the Earth:
The moon is revolving in a circular path around the Earth due to gravitational force, which acts as the centripetal force. In circular motion, the gravitational force is always perpendicular to the displacement of the moon, which means no work is done because cos ( 9 0 ∘ ) = 0 .
Reasons for the following scenarios:
(a) No work is done if a man is pushing against a wall:
When a man pushes against a wall, there is no displacement in the direction of the force. Since work is the product of force and displacement, if displacement is zero, then the work done is also zero.
(b) Hammer drives a nail into the wood only when it is lifted up and then struck:
Lifting the hammer gives it potential energy which is converted into kinetic energy as it is brought down. This kinetic energy enables the nail to be pushed into the wood.
(c) A horse and a dog are running with the same speed:
Kinetic energy depends on both mass and speed, defined by the formula K E = 2 1 m v 2 . If a horse and a dog are running with the same speed, the horse, having greater mass, will have more kinetic energy.
(d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work:
Work is done when there is displacement in the direction of force. When the teacher moves around, displacement occurs. In contrast, the child standing still has no displacement in the direction of any exerted force.
Change of potential energy:
Potential energy can change into kinetic energy, such as when an object falls under the influence of gravity.
Complete conversion of energy forms:
It is practically impossible to convert one form of energy completely into another useful form due to the second law of thermodynamics, which states that some energy is always lost as waste heat to the surroundings.
