Bellwork • A box of San Francisco 49ers jerseys is pushed with 80 Newtons of force for 5 meters up a ramp into a truck with a height of 1 meter to be taken to the thrift store. How much work did the person do on the box? Mechanical Advantage & Efficiency •How do machines do work for us? •We can compare the amount of force put into a machine with the amount of force it produces •This ratio is a machine’s mechanical advantage Mechanical advantage = (output force) (input force) •Can openers are simple machines that multiply the amount of force we input into opening a can •If you use 5 N of force to open a can, and the opener exerts 15 N of force, what is the opener’s mechanical advantage? Output Force Input Force 15N/3N = 3 •Your force is tripled! • A lever is used to lift a heavy rock. The mechanical advantage of the lever is 4 and the lever applies a force of 800 N to the rock. What is the force applied to the lever? Consider this pulley: •If only the direction of force is changed, what is the mechanical advantage? • 1/1 = 1 •No force is multiplied, but the work is easier. Why? In elevators… Counterweight Here is where the people go • When using a pulley with a mechanical advantage of 1.5, a worker applies 250 N to the pulley. The pulley is used to lift a heavy crate. How much force does the pulley apply to the crate? • A 100 N force is applied to a machine to lift an object with a mass of 45 kg. What is the machine’s mechanical advantage? Efficiency • What comes to mind? Efficiency • Efficiency: the percentage of work input used in work output • In other words… Efficiency = (work output) (work input) x 100% The Internal Combustion Engine $$$$ 15%-25% Efficient Where does the rest of the energy go? • • • • • • • HEAT Sound Resisting drag Overcoming friction Braking Idling Powering other parts of the car • A tank of gas contains 970 kJ of energy. If the engine’s work output is 213 kJ, what is the efficiency? If it costs $44.00 to fill up the tank, how much of the cost is lost to inefficiency? • If a compact fluorescent light bulb uses 75 Joules of energy to produce 12 Joules of work, how much more efficient is it than an incandescent light bulb that uses 130 Joules to do the same? • A 100 Newton force is applied over 10 meters, and raises a 500 Newton object 1.5 meters. What is the machine’s efficiency?