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WELCOME BACK! • Please set up the next page in your journal for Cornell notes (you may need two pages): –Page: 25 –Date: 1-4-17 –Title: Work, Power, and Energy –Essential Question: How are work, power, and energy related? First, the pertinent vocabulary words… • WORK: the amount of energy transferred from one object to another by a force – measured in Joules (J) or newton-meters (Nm) – only occurs when an object moves in a direction parallel to the force –W = Fd • POWER: the rate at which work is done – measured in Watts (W) or Joules per second (J/s) 𝐖 –P = 𝐭 • ENERGY: the ability to cause change or do work – measured in Joules (J) What is work? • In physics, work has a very specific meaning. • Work represents a measurable change in a system, caused by a force. Work - horizontal • If you push a box with a force (F) of one newton for a distance (d) of one meter, you have done exactly one joule (J) of work. Work • It is important to consider all the forces acting on an object separately. Consider you are pushing a box on a frictionless surface while your friend is trying to prevent you from moving it. • What forces are acting on the box and how much work is being done? F=3N F = 1.5 N Work • The total work done on the box would be: • W = Fnetd = (3 N – 1.5 N)(1 m) = (1.5 N)(1 m) = 1.5 J F=3N F = 1.5 N Work against gravity mass (kg) Work (joules) W = mgh height object raised (m) gravity (m/s2) The path doesn’t matter…why not? A 63.4 g facial tissue box is lifted at constant speed from the ground to height 1.08 m. Find the work done on the box. Fapp & d W=mgh = (0.0634 kg)(10 m/s2)(1.08 m) = Fw = m g 0.685 J A student carries the 63.4 g tissue box 3.2 m at a constant height of 1.08 m above floor. How much work is done on the box? ZERO! F and d are not in the same direction! Fapp d Fw = m g What is power? • The same amount of work done in a shorter time = MORE POWER! •𝐏 = 𝐖 𝐭 = 𝐅𝐝 𝐭 = 𝐦𝐠𝐡 𝐭 • may be in kilowatts (kW) –1 kW = 1000 W • may be in horsepower –1 horsepower = 746 W Power • Consider you and your friend pushing the box. –Remember,1.5 J of work was done in this scenario. How much power was generated if it took 3 seconds to move the box? P= W t = = 1.5 J 3s 0.5 W F=3N F = 1.5 N So, what about energy? The workings of the universe can be viewed as energy flowing from one place to another and changing back and forth from one form to another. • In this unit, we will focus on mechanical energy. • KINETIC ENERGY: energy of motion – depends on mass and speed (velocity) • directly proportional to mass – if speed remains constant and mass doubles, then kinetic energy also doubles • directly proportional to the SQUARE of speed – if mass remains constant and speed doubles, then kinetic energy increases four times Kinetic Energy Kinetic Energy (joules) mass (kg) 𝟏 𝟐 𝐊𝐄 = 𝐦𝐯 𝟐 speed (m/s) What is the kinetic energy of a 650 kg racecar moving at 67 m/s? 𝟏 𝐊𝐄 = 𝐦𝐯𝟐 𝟐 = ½ (650 kg)(67 2 m/s) = 21775 J = 22000 J • POTENTIAL ENERGY: stored energy – GRAVITATIONAL POTENTIAL ENERGY: energy due to an object’s location • location is measured relative to an arbitrary reference line (usually the ground) mass (kg) Potential Energy (joules) PE = mgh height (m) acceleration due to gravity (m/s2) A waiter stands at the top of a 2.3 m high staircase, holding a 3.1 kg platter of food 1.5 m above the landing. What is the gravitational potential energy of the platter? PE = mgh = (3.1 kg)(10 m/s2)(2.3m + 1.5m) = (3.1 kg)(10 m/s2)(3.8 m) = 117.8 J = 120 J