Work, Power and Energy - Physics - General Test Preparation for CUET

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 

• 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:

• 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 :

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 -

• K=1/2 mv² joule

Potential energy-

• 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)