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Ch. 5-1 Work Ch 5-1, page 168 - 171 Journal page 39 You (have/have not) done work on a chair you hold at arm’s length all day. A force that causes ______ of an object in the direction (parallel) of the force does ____ on the object. Write the formula for work. Work is not done on an object unless… Work is done only when components… Why is no work done on the chair in the above example? If you are pushing a crate along the ground, some of the force is directed downward. Only the ______ component of your applied force does work on the crate. [Sketch Fig 5-2, page 169.] Write the formula for work when force is applied at an angle to the direction of motion. If Θ = 0o, cos 0o = … and all of the force is directed… If Θ = 90o, all force is applied straight up or down, and then cos 90o = 0 and Wx = _____. Write the units for work expressed two ways. The sign of work is important Journal pg ___, book pg. 170 - 171 Work is a ___________ quantity though it can be positive or negative. Work is positive when… Work is negative when… Give examples of both + and – work. If the work done changes the object’s speed, a + sign tells you… A – sign means… Energy Journal page __, cont, book page 172 - … Kinetic Energy is … – If a constant net force is applied to an object we know the object is… – This is Newton’s _______ ______. Kinetic energy depends on… Write the formula for kinetic energy. KE is a (vector or scalar) quantity. What are the units for KE? Work Sample Problem 5B, page 173 Work-Energy Theorem Practice 5C page 174-176, J. p 42 Define the work-kinetic energy theorem and write the formula for the work-kinetic energy theorem in words and variables. Work Practice 5C problem #1, 2, 4 on page 176. Potential Energy Journal p. 43, book page 177 - 180 Give two examples of objects with PE. Potential energy is _________ energy. potential energy—(define) gravitational potential energy—(define) Write the formula (variables & words) for GPE. The unit for PE is… Since GPE is a result of an object’s _____, it must be measured relative to some _____ level. Elastic potential energy Journal pg 43, cont. book page 178 Define elastic PE… Write the formula (variables & words) for elastic PE. Conservation of Energy Journal pg 47, book page 181— When we say energy is conserved, we mean… Saying energy is conserved does NOT mean… An example of a conserved quantity is… Describe three types of energy involved in the swing of a pendulum of a clock. In this animation, you see a mass attached to the end of a string which forms a pendulum. The pendulum begins with only gravitational potential energy (GPE) since it is not moving yet. After being released, GPE is turned into kinetic energy (KE). Notice that no matter where the pendulum is, the sum of the GPE and KE is always equal to the original amount of energy the system started with. This demonstrates the Law of Conservation of Energy. Questions: Where does the pendulum have the highest velocity? How does the original height of the pendulum compare to its final height? This simulation is assuming that there is no air friction and no friction in the pivot point of the rope. from http://regentsprep.org/Regents/physics/phys02/pend/default.htm Mechanical Energy 1 Journal pg 48, cont., book page 182— Mechanical energy is often… Write the formula (both words & variables) for mechanical energy. Write an expanded formula for ME when KE and PE are involved (under yellow box). “Practice 5E,” #2, page 185. Energy conservation occurs even when acceleration varies The diagram below depicts the motion of Li Ping Phar (esteemed Chinese ski jumper) as she glides down the hill and makes one of her record-setting jumps. The total mechanical energy of Li Ping Phar is the sum of the potential and kinetic energies. The two forms of energy sum up to 50 000 Joules. Notice also that the total mechanical energy of Li Ping Phar is a constant value throughout her motion. There are conditions under which the total mechanical energy will be a constant value and conditions under which it will be a changing value. This is the subject of Lesson 2 - the work-energy relationship. For now, merely remember that total mechanical energy is the energy possessed by an object due to either its motion or its stored energy of position. The total amount of mechanical energy is merely the sum of these two forms of energy. And finally, an object with mechanical energy is able to do work on another object. From the Physics Classroom, KE Mechanical energy (is/is not) conserved in the presence of friction because… Rate of Energy Transfer Journal page 49, book page 187 - 188 The rate at which work is done is called _____. Power is (define)… Write the formula in words and variables. Write an alternative formula for power and show how it is derived. The SI unit of power is the _____ which is equal to… – One horsepower is equal to _______ W which = _______ kW (about (what fraction) of a kW) Machines with different power ratings do… – so what about that stage curtain?