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## Chapter 14 Work, Power and Simple Machines

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**Questions to think about before…**• What does work mean to you??? • List some examples of work:**Work & Science**• Now...think about work in terms of science...it probably means something very different than what you listed above.**14.1: Work and Power**• What is work? • Recall...From Chapter 12 • Question: How does an stationary object begin moving?**Answer…**• Answer: When an unbalanced force acts on it. • Work: the product of force and distance • Work is done when a force acts on an object in the direction the object moves.**Work Requires Motion**Question: Does a weight lifter do work on the barbell to lift it over his head?**Stationary Objects**• Question: Is the weight lifter doing work while he holds the barbell stationary over his head?**ANSWER**• Answer: NO, the barbell is stationary • For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is NO movement, NO work is done!!!**Work Depends on Direction**• The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement. • A force does not have to act entirely in the direction of movement to do work.**Is work being done????**• The force acts upward and to the right. • The suitcase only moves to the right. • Any part of a force that does not act in the direction of motion does NO work on an object**Calculating Work**• Work = Force x Distance • Units of Work • SI unit for force is newtons • SI unit for distance is meters**JOULE**• The SI unit for work is newton-meter or the JOULE (J) • When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.**Practice Problem**• Imagine the weight lifter. The weight lifter lifts a 1600 newton barbell over his head. Assume the barbell is lifted to a height of 2.0 meters. What is the work done? • Work = Force x Distance**Practice Problem Answered**Work = 1600 N x 2.0 m Work = 3200 N m = 3200 J**What is Power?**• Power: the RATE of doing work • Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time**Answer: YES, because the shovel is moving in the same**direction as the force being applied**Answer: YES, but because the snow blower does the work in**less timeit has more POWER!!!**Calculating Power**• Power = Work / Time • Work is in joules (J) • Time is in seconds (s) • The SI unit for POWER is the watt (W) = one joule per second • Thus, a 40-watt light bulb requires 40 joules each second that it is lit.**Practice Problem**• You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?**Practice Problem Answered**Power = work / time OR can be written as: Power = (Force x Distance) / Time (72 N x 1.0 m)/ 2.0 s = 36 J/s = 36 Watts**Horsepower**• Horsepower (hp): common unit for power. One horsepower is equal to about 746 watts. • FYI...Interesting side note: Horsepower is literally based on the power output of a very strong horse!!!**14.2 Work and Machines**• Machine = a device that changes a force • Machines make work easier to do. They can: • Change the size of the force needed • The direction of a force • The distance over which the force acts • However… They can’t do work for us!**Ex: a car jack**• Each rotation of the jack applies a small force over a large distance and the car is lifted a small distance • Tradeoff = total distance traveled is much greater**Ex: oars of a boat**• You move oars a small distance and the end in the water moves a large distance • Tradeoff = increased travel of the oar requires you to exert a greater force**Ex: pulley**• You pull down on the rope and the load moves up**Work Input**• Because of friction, the work done by a machine is always less than the work done on the machine! • The force you exert on the machine is called input force. The distance the input force acts through is called the input distance. • Work input = IF X ID**Work Output**• The force exerted by the machine is called the output force. The distance the output force is exerted through is the output distance. • Work output = OF X OD**Mechanical Advantage = the number of times that the machine**increase an input force • AMA = load force/effort force • Q: Using a lever, a person is able to lift a 100N object using only 20N of force. Calculate the MA of this machine**A: AMA = 100N/20N = 5**• In other words, this machine has multiplied the effort force 5 times.**Ideal Mechanical Advantage = MA without friction**• IMA = Input Distance/Output Distance • Q: A woman drives her car onto a ramp. She drives 1.8 meters along the ramp to raise it 0.3m off the ground. Calculate IMA**Efficiency**• Because some of the work input to a machine is always used to overcome friction, the work output is always less. • Efficiency = the percentage of the work input that becomes work output • Always less than 100% due to friction • Efficiency = W.output/W. input X 100%**14.4 Simple Machines**• The six types of simple machines are: • Lever • Wheel and axle • Inclined plane • Wedge • Screw • Pulley