Presentation on theme: "Power. What is Power? Power – The rate at which work is done. Power = zero."— Presentation transcript:
What is Power? Power – The rate at which work is done. Power = zero
For example, a rock climber takes an abnormally long time to elevate her body up a few meters along the side of a cliff. On the other hand, a trail hiker (who selects the easier path up the mountain) might elevate her body a few meters in a short amount of time. The two people might do the same amount of work, yet the hiker does the work in considerably less time than then the rock climber. The hiker has a greater power rating than the rock climber.
Calculating Power Power is the rate at which work is done. It is the work/time ratio. Mathematically, it is computed using the following equation. The standard metric unit of power is the Watt. As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. Thus, a Watt is equivalent to a Joule/second.
For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 746 Watts. Power came from a 236 cc (14.4 cu in), 7.1 kW (9.5 hp) split-single two stroke motorcycle engine. The first prototypes had one wheel at the rear, but having a single rear wheel made the car prone to roll-overs, so the rear wheel layout was changed to two wheels. Top speed= 45 mph. $1000 dollars 2014 Ferrari LaFerrari - 950-hp $4 million Top Speed – 217 mph
James Watt James Watt, 18th century inventor who improved upon the steam engine design is also remembered for measuring the power of his steam engine: his test with a strong horse resulted in his determination that a "horsepower” was 550 foot-pounds per second. The unit of power in the metric system is called the watt; one horsepower equals 746 watts.
Does a horse have 1 hp of power? A horse is stronger than one horse power. The amount of power an actual horse produces is 14.9hp. Over longer periods of time, the average horse produces less than one hp (a continuous time period at a slow haul). However, in a gallop for short periods, that power level shoots up dramatically. This is why a horse can whip say a 5 hp mini motorbike. A person can peak at about 1 hp but their continuous power output is about 1/8th hp.
How is power related to light bulbs? The watt is perhaps most familiar to you from your everyday experience with light bulbs. The power rating (wattage) of a light bulb tells you the rate at which energy is converted by the bulb. (Electrical energy converted to light and heat in an incandescent bulb.)
Sample Work & Power Calculation A man pushing a cart with a force of 40N, moves it 12 m in 10 s. Find the work done and the power exerted. F = 40N d = 12 m t = 10 s W = Fd = (40N)(12m) = 480J P = W/t = 480J/10s = 48J/s = 48W