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Topic 5: Work, Energy and PowerWork done by forcePositive work is considered to be done when the obiect moves in the direction ofthe force.

Direction of force Work done

work done = F cosa s

Work done by gasPositive work is done by a gas when it expands against the external pressure.{When the gas contracts, work done is neqative. We say that "wo.k is done on the gas,,.)

external pressure (p) externalpressure {p)

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Enelgy conServation and convercionEnergy is q!]!Ay! conserved in a closed system {no external energy source present) i.e. totalenergy is a constant.When we say "energy loss", it is not really lost - it is simply converted into other non usefulforms e.g. sound, heat etc.

Mechanical Energy

Kinetic Eneray. Energy possessed by virtue ofits Eg!.g-[.1_

Potential Enerev. Energy stored in a body as aconsequence of its position,state or shape.

GravitaiionalPE E = nr.ghElastic PF

Note: the concept of conversion between the different formswhen solving quantitative questions. Keep this method in mind,{a) yglgglly is involved;(b)I9!g!! is involved.Problem solvins Methodiotal lnitial Energy = Total Final Energy +F erqy losslf no Energy LostTotal lnitial Energy = total Final Energy

of energy is a very useful oneespecially when:

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EfficiencyDue to energy being converted to non-useful forms, efficiency is less than 100% in real systems.

useful energy outputEfficiency ? = x 100%totalenergy input

Rate of work done (tJ with respect to time {1); also defined as energy {_E) converted withrespect to time.WEit

When solving questions that involve motion of body another useful form of power is:where v : velocity of body - must beconstant if you want to use thisequation

Efficiency can also be calculated using useful power output and total power input:

Efficiency rl = useful power output x TOOo/"total power input

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