Power

•Work has to do with a force causing a displacement.• It has nothing to do with the amount of time that this

force takes to cause the displacement.

•Consider a rock climber… he may ascend a few meters very slowly.

•A hiker can ascend the same height in far less time by taking the easier path.

•They do the same amount of work, but the hiker does it much quicker.

•The amount of work done over time is called power.•The hiker has a better power rating than the rock climber.

P = W/t

P- PowerW- Workt- time

•W= Fd (when force and displacement vectors are in the same direction)

•P= W/t•P= Fd/t•d/t = vSo………

P =Fv

•Unit of measurement for power: the Watt (W).•1 Watt = 1 J/s = 1 kg m2/s3

•Named after James Watt (b. 1736), an engineer whose improvements to the steam engine were very important to the Industrial Revolution.

James Watt (Scottish)

•You often see power measure in horsepower•1 horsepower (hp) = 746 W•The term was adopted in the late 18th century by

James Watt to compare the output of steam engines with the power of draft horses

•Machines, people, animals, etc. all have power ratings.

• I.e., the amount of work over time. More work over less time means a higher power rating.

Example•A 70 kg jogger runs up a long flight of stairs in 4.0 s.

The vertical height of the stairs is 4.5 m. Estimate the jogger’s power output in both watts and horsepower.