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Chapter 9 WORK AND ENERGY Objectives Define work and power Calculate work done Discover what mechanical advantage is and how it makes work easier Calculate mechanical advantage What is Work? Work is done only when force causes a change in the motion of an object. To calculate work you multiply the force by the distance that the force is done over. How to Calculate Work The force is equal to the pushing or pulling along the same line as the direction of motion. We use joules (J) to label work. Practice Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? 1st part of assignment Do pg 285 numbers 1-5 (it’s under practice) Make sure that you have Newtons and meters to calculate work. If you don’t have Newtons you will have to calculate them using F=m*a. If they give you units in kg convert to N using N=kg*9.8 m/s/s Power Power is the rate at which work is done. So, if you do the same amount of work in less time, you exerted more power. To calculate power use the following equation: How to Measure Power Power is measured in watts (W). 1 Watt is the amount of power needed to do 1 J of work in 1 s. Practice It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 2nd part of homework Pg. 287 1-5 Be careful on numbers 4 & 5 – you need to calculate work done before you can calculate power! Machines and Mechanical Advantage Does using a machine decrease the work that you do? Example: Does it take more work for you to lift a car by yourself or to use a jack? Answer: It takes the same amount of work! Machines do not decrease work, they just make it easier by distributing distance or direction. Work Input and Output Work input – the work you put into the machine (W = F x d) Work output – the work you get out of the machine (W = F x d) Example: pg. 288 – the guy lifting a box into a trailer. Work input is equal to work output Mechanical Advantage A quantity that measures how much a machine multiplies force or distance. 𝑚𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑎𝑑𝑣𝑎𝑛𝑡𝑎𝑔𝑒 = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 = 𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 When mechanical advantage > 1 the machine makes the force stronger When mechanical advantage < 1 the machine makes the distance or speed higher Calculating Mechanical Advantage Example: Calculate the mechanical advantage (MA) of a ramp that is 5.0 m long and 1.5 m high. Input (what you do) distance = 5.0 m Output (what the machine does) distance = 1.5 m MA = 5.0 m/1.5 m = 3.3 Assignment due 2/1 Pg 290 1-5 Look at practice hints on left side of page. Also, complete study guide for section 9.1