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Transcript
Work Notes Ch 6 Work, Power, Energy What is Force? Force = a push or pull on an object by another object. Forces cause changes in motion ◦ Cause acceleration/ Change in direction/ Start or Stop SI unit of force = Newton (N) Force is a vector ◦ Vectors need both magnitude and direction ◦ Use arrow to indicate direction of force The 9 Common Forces Gravity Friction Applied Normal Spring Magnetic NET Tension Buoyant Electrical Gravitational Force, Fg Symbol: Fg Definition: Pull between 2 objects with mass ◦ gravitational attraction between an object and earth (Weight) Fg=m g g = 9.8 Direction: Pull towards center of earth Ex. A roll of tape rests on a table ◦ gravity pulls down on roll of tape Fg Mass vs Weight MASS WEIGHT Always remains constant Depends on gravity Does not depend on gravity Weight(N)=mass(kg) x gravity(m/s2) Unit = kg Changes if gravity changes (ex. You are on the moon) Use an electronic balance to measure Unit = N Eureka http://www.youtube.com/watch?v=xfGll1REu2A Work ◦ transfer of energy through motion ◦ force exerted through a distance Distance must be in direction of force! W = Fd W: F: d: work (J) force (N) distance (m) 1 J = 1 N·m One Joule = amount of work needed to move any mass 1 meter with force of 1 Newton #1 Example Problem Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W=? WORK: W = F·d W = (30 N)(1.5 m) W = 45 J W F d What is work???? continued 2 things are needed for there to be work ◦ Application of force ◦ Movement of object by force Forces parallel to direction of motion do work Forces perpendicular to direction of motion do no work Work is done when a force “helps or hinders” the motion of an object No motion or displacement means no work done. If displacement = 0, work = 0 #2 Example Problem A dancer lifts a 40 kg ballerina 1.4 m in the air and then walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W=? W F d WORK: W = F·d F = m·a F =(40kg)(9.8m/s2)=392 N W = (392 N)(1.4 m) W = 548.8 J during lift No work after lift. “d” is not in the direction of the force. #3 Example Problems A teacher applies a force to a wall and becomes exhausted. ◦ No. The wall is not displaced. A book falls off a table and free falls to the ground. ◦ Yes. A force (gravity) acts on the book and causes book to be displaced A rocket accelerates through space. ◦ Yes. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space. #3 Example Problems – cont. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. ◦ No. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room).Yet the force does not cause the displacement. Recall… • Forces parallel to direction of motion do work • Forces perpendicular to direction of motion do no work • a vertical force does not do work on a horizontally displaced object. Power Power is the rate at which work is done Fast or slow Equation Power = work / change in time P=W/Δt •Unit is watt (W) or 1J/s •Or horsepower (hp) 1hp=746W P =W = F d = m g d = F v Δt Δt Δt #5 Example Problem If two people mow two lawns of equal size and one does the job in half the time, who did more work? Same work. Different power exerted. #6 Example Problem A 60W light bulb does Work to produce light and heat energy (electric energy that is transferred into light energy). If a 60W light bulb is on for 30s, how much work is done? P= W/ Δt 60W=W/30s 1800J=W Work-Energy Relationship Ex. A sled, from rest, is kicked across a frozen pond. Amount of work done on sled = change in energy of sled. ◦ Wnet = Δ E ◦ Net work = change in energy Wfriction = ΔME Video Eureka – Work http://www.youtube.com/watch?v=xBnS23U_ao4