Work. Energy. Power

Physical work

• Work (W) is defined as the force (F) times the distance (s) moved in the direction of the force (cosΘ)

• NB! Θ is angle between the applied force and tirection of motion

• If there are no movement in the direction of force, then no work is done

• Work is scalar quantity• Work is measured in joules (1J = 1Nm=1kgm2s-2)

Work vs Energy• Work done is equal to converted energy

• When work is done, energy is converted to different types.• Kinetic energy (Ek or KE) is the energy a body because it is

moving• Potential energy (Ep or PE) is the energy stored in a body

or in a system due to its position in a force field or due to its configuration– Gravitational potential energy is the energy a body has because

of its position in vertical height relative to a given point (height from surface of earth)

Kinetic energy

• m – mass of body• v – velocity (speed) of body• Kinetic energy is scalar quantity measured in

joules (1J)

Gravitational potential energy

• m – mass of body• g=9.81 ms-2 – gravitational acceleration• Δh – change in height (vertical)• Potential energy is scalar quantity measured in

joules (1J)

Conservation of energy

• Energy cannot be created or destroyed; only changed form one form to another

• There are many different forms of energy:– kinetic energy;– potential energy;– elastic potential energy;– heat energy;– light energy;– sound energy;– chemical energy;– electrical energy;– magnetic energy;– nuclear energy;– etc

Power

• Power (P) is measure how fast work (W) is done or how quickly energy is converted (ΔE):

• Δt – is time taken in seconds• Power is scalar quantity measured in watts

(1W=1Js-1=1kgm2s-3)

Efficiency

• Efficiency is a ratio of how much work, energy or power we get out of a system compared to how much is put in

• Efficiency is scalar quantity and has no unit! It is measured usually in percentages.

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