“Work and Energy: Energy Method Workshop” is available to Standard subscribers.
A ball is dropped, a car brakes, a pump fills a tank. You can solve each one with forces and time, but it is long. With energy it takes three lines. In this course you learn when to use the energy method and how to use it, solve ten harder problems on speed, braking, friction, power and efficiency, and then take a timed test.
Strategy: let energy do the work: For a speed or a distance, compare the energy at two moments. The potential energy lost becomes kinetic energy, so after a fall v=2gh, whatever the mass or the path. With friction, the missing energy is heat: the friction force times the distance.
Strategy: power, efficiency and energy chains: Efficiency is the useful energy out divided by the energy in, and it is always less than 100%
The chapter itself stays free
This is an extra course for Work and Energy. Its main courses are open to everyone:
. In a chain of machines the efficiencies multiply. The input power is the useful power divided by the efficiency, and the cost needs kilowatt hours.
Strategy: bounces, brakes and many steps: Long problems become short when you tell their energy story: what the energy is at each moment, and where some of it goes. Bounces keep a fraction of the energy each time, braking distances grow with the square of the speed, and a roller coaster's speed depends only on how far below the start it is.