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ICSE-VIII-Physics

04: Energy Class 8 Physics

with Solutions - page 3

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  • Qstn #2
    When does a force perform work?
    Ans : :
    Work is said to be done only when the force applied on a body makes the body move (i.e. there is a displacement of the body).
  • Qstn #3
    State two conditions when no work is done by a force.
    Ans : :
    Two conditions are :
    i) There should be no displacement i.e. S = 0
    ii) The displacement is normal to the direction of force i.e. -θ = 90°
  • Qstn #4
    In which of the following cases is work being done :
    - (a) A boy pushing a rock
    (b) A boy climbing up the stairs
    (c) A coolie standing with a box in his head
    (d) A girl moving on the road.
    digAnsr:   b,d
    Ans : (b) A boy climbing up the stairs
    (d) A girl moving on the road.
  • Qstn #5
    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
    Ans : :
    A coolie carrying a luggage on his head moving on ground does i no work against the force of gravity as displacement is normal to the direction of force of gravity.
  • Qstn #6
    The moon is revolving around the earth in a circular path. How much work is done by the moon ?
    Ans : :
    No work is done, since displacement is normal to the direction of force on the body. The force is centripetal.
  • Qstn #7
    Write the expression for work done by a force.
    Ans : :
    Work done by applying force F is the product of force applied on the body and distance moved by the body in the direction of force , work done = Force × distance moved in the direction of force.
    W = F × d
  • Qstn #8
    State the S.I. unit of work and define it.
    Ans : :
    S.I. unit of work is Joule
    Joule is that much work done when a force of 1 N displaces the body through a distance of m in the direction of force.
  • Qstn #9
    State two factors on which the work done on a body depends.
    Ans : :
    Two factors are:
    1. Magnitude of force applied (F).
    2. Distance moved by the body in the direction of force
      (d) or (s)
  • Qstn #10
    Define the term energy.
    Ans : :
    Capacity of doing work is called energy.
  • Qstn #11
    State the S.I. unit of energy.
    Ans : :
    S.I unit of energy is Joule (J).
  • Qstn #12
    Define 1 joule of energy.
    Ans : :
    Joule is the capacity of a body to work of 1 J irrespective of time taken.
  • Qstn #13
    How is work related to energy ?
    Ans : :
    Relationship between Work and Energy:
    Energy is the capacity of doing work
    Every form of energy is work. i.e. work done on body is stored in the form of energy. Energy is spent when a body does work.
    Thus, to do more amount of work-more energy is needed.
  • Qstn #14
    What are the two kinds of mechanical energy ?
    Ans : :
    Two kinds of mechanical energy:
    1. The Potential energy (P.E.)
    2. The Kinetic energy (K.E.)
  • Qstn #15
    What is potential energy? State its unit.
    Ans : :
    Potential Energy (P.E. or U): The energy possessed by a body due its position above the ground or change in state.
    Unit of P.E. = S.I. unit of energy = Joule (J)
  • Qstn #16
    Give one example of a body that has potential energy, in each of the following :
    Ans : :
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