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Work, Power and Energy - Physics - Notes | Study Current Affairs & General Knowledge - CLAT

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Work

  • Whenever a force acting on a body displaces it, work is said to be done.
  • If a force F acting of the body displaces it by a distance s in the direction of force , work done is given by,
    W = F.s (or) 

          W = |F||s| cosθ

  Angle between Force and Displacement Angle between Force and Displacement 

  • If θ < 90°, work done is positive and if θ > 90° it is negative.
  •  Whenever a force acting on a body displaces it, work is said to be done.
  • If a force F acting of the body displaces it by a distance s in the direction of force , work done is given by,
    W = F.s (or) 

           W = |F||s| cosθ

  • If θ < 90°, work done is positive and if θ > 90° it is negative.

 Examples:

  • When body falls freely under gravity, θ = 0, and work done is positive.
  • When a lawn roller is pulled by applying a force along the handle at an acute angle, work done by applied force is (+) ve.
  • When a spring is stretched or when a gas filled in a cylinder filled with moving piston is allowed to expand, work done is positive.
Negative Work Physics: If θ is obtuse (θ > 90°), work done is negative.
  • Examples :
  • When a body is thrown up, its motion is opposed by gravity; θ between F and s is 180°.
  • When a body is moved over a rough horizontal surface, the motion is opposed by the force of friction.
  • Work done by the friction force is negative.
  • When brakes are applied to a moving vehicle.
  • When a positive charge is moved closer to another positive charge.
Zero Work Physics : If θ is right angle (θ = 90°), work done is zero.

Examples :

  • When we fail to move a heavy stone because s > = 0.
  • Coollie carrying some load on his head on a horizontal platform θ = 90°.
  • Person carrying a briefcase moves on a horizontal road. 
  • The total capacity of doing work by a body is called its energy.
  • It is a scalar quantity.
  • Its unit is joule. 
Kinetic energy - the energy possessed by a body by its motion is called its kinetic energy.
  • K=1/2 mvjoule
Potential energy- it is the energy stored in a body by virtue of its position of state.
  • U= mgh 
Law of Conservation of Energy
  • It states that the total energy remains constant in any process i.e., it may neither be created nor destroyed.
  • KE + PE + other forms of energy = constant
  • If conservative forces are acting on a body, then,
  • KE + PE = constant i.e., the loss of PE is equal to the gain in KE.
 Einstein Relation
  • According to Einstein, mass can be converted into energy and energy can be converted into mass. One can be obtained at the cost of the other.
  • The equivalence between mass and energy is expressed in terms of Einstein relation:
  • E = mc²
  • Where m is the mass that disappears, E is the energy that appears and c is the velocity of light in vacuum (c = 3 x 108; m/s).
  • The rate of doing work by a body is called its power.
  • If an agent does work W in time t then average power is,
  • P = W/t = F.s/t = F.v = |F||v|cosθ
  • It is a scalar quantity.
  • SI units of power is watt and is also measured in horse power.
  • 1 HP = 746 W (watt)

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