Unit 5-1: Work and Power
Work When we were looking at force, we observed that an objects motion is related to how the force acts and how long it acts for. “How Long” does not necessarily refer to time. It can also refer to distance.
Work Work is the transfer of energy into or out of an object. Work is done through many means: Mechanical, Electrical, Magnetic, Etc.
Work In this course, we will be focusing on mechanical work. Work is done mechanically by applying a force across a distance. Work = Force x Distance W = Fd The unit of work, and energy, is the Joule.
Work The Joule (J): Named after James Prescott Joule Joule focused his work on energy and heat movement.
Work The Joule (J): A Joule is derived from multiplying a Newton and a meter. So one Joule is equal to one Newton times one meter. J = N*m This means that when a J is divided by a N, you get a m, and when a J is divided by a m, you get a N.
Work Notice that work includes both force and distance. This means that in order for work to be done, the force has to be applied over a distance. If the object doesn’t move, that means no work is done. The force also must be in the same direction as the distance moved.
Work When there is more than one force present, Such as weight or friction, You do work by using the net force an object feels. If not enough force is applied to lift the object, then no work is done. There was no distance the object moves.
Work Example 1: A mover applies 50N of force to move a desk 2.3m. How much work does he do? Example 2: A crane must lift a 30kg crate at constant velocity. If the crane lifts the crate 24.2m, how much work was done? Example 3: You push the gas pedal, which applies 388N of force against 173N of friction. If you apply the force while the car moves 18.7m, how much work was done?
Power You may have noticed that the definition of work does not say anything about the time it takes for the work to be done. When carrying a backpack full of books upstairs, you do the same amount of work if you walk or run. This seems counter-intuitive, as you would probably be more tired after running.
Power Why the difference? Because of the power. Power is the rate at which work is done. That means power is the rate at which energy is transferred. The faster the rate, the more powerful something is. This is why you are more tired after running, you didn’t expend more energy, but you merely expended the energy faster!
Power Power is calculated by dividing the work done by the time it took. Power = work/time P = W/t The unit of work is the Watt.
Power The Watt (W): Named after James Watt James Watt worked on the steam engine and made many advancements to its efficiency.
Power A watt (W) comes from dividing a joule by seconds: Watt = joules/seconds This means that the watt times a second will equal a joule, and joules divided by watts equals a second.
Power The early unit of power is the horsepower (hp). This was quite literally the amount of work a single horse could do in a day. This is not a great unit of power because it doesn’t relate well to non-engines (like lightbulbs)
Power The kilowatt (kW) is 1000 watts, and a megawatt (mW) is 1,000,000 watts. Electrical systems are usually rated in watts (or kW or mW) while engines use horsepower. Remember both are units of power: 1 hp = 0.75kW
Power Example 1: Determine the power of an engine in horsepower if it can do 18000J of work in 30 minutes. Example 2: Determine how much work can be done by a 750kW oven in 180 seconds. Example 3: Determine the time it would take a 5hp crane to do 3000J of work.