VIEWS: 3 PAGES: 33 POSTED ON: 2/18/2012
Objectives Define work and identify the units Describe the conditions that must exist for a force to do work on an object Calculate the work done on an object Describe and calculate power Compare the units of watts and horsepower as they relate to power Work Definition - quantity of energy transferred by a ______ when it is applied to a body and causes that body to _________ in the ___________ of force Formula - ___________ Units - Newton meter (____) or a joule (__) Work Continued Two factors 1. size of _______, and ________ it is applied ex: pulling a suitcase * any part of a force that does not act in the direction of motion does no work on an object 2. ___________ of something by that force Work Cont. - weight lifter who holds a barbell weighing 1000 N does NO ________ - Why? - he may get very tired, but if the barbell is not moved by the force he exerts, he does no work on the barbell - work is done on his ________ when he raises the barbell Work Problems Q: A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? Q: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes? Work Problems Cont. Q: Jake, a 235 N track athlete completes his race, which totals 15750 J, what is the total distance Jake ran? A: Q: Joey, performed 900 J of work, while lifting a box 12 meters. What force did Joey exert on the box? A: Power Definition - a quantity that measures the ____ at which ______ is done Formula - P = ______ Unit - Joule per second (____) or _______ (W) - Horsepower (___) = 746 W Power Problems Q: Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output? Q: Anna walks up the stairs on her way to class, She weighs 565 N and the stairs go up 3.25 m vertically. Calculate he power output if she climbs the stairs in 12.6 s. What is her power output if she climbs the stairs in 10.5 s? Objectives Describe what a machine is and how it makes work easier Relate the work input to a machine to the work output of the machine Compare a machines actual mechanical advantage to its ideal mechanical advantage Calculate the ideal and actual mechanical and actual mechanical advantages of various machines Explain why efficiency of a machine is always less than 100% Calculate the machines efficiency Machines Definition - a device that changes __________ ex. How does a machine change force - 3 ways a machine makes work easier to perform - change the _____ of _________ needed - the ________ of a force - the __________ over which a force acts Changing the Force Increasing the force you applied ex. - small force exerted over a large distance becomes a ________ _______ exerted over a ________ _________ Changing the Distance Increasing distance ex. - small movement of oar at the hands makes a large distance the oar in the water will move. *remember the trade off ________ distance _________ force* Changing the Direction Changing the direction of the applied force ex. - pulling back on the handle of the oars causes its other end to move in the opposite direction. Work Input Work Input - the _______ _______ on a machine as the _______ ________ acts through the ________ ____________ - input force: ________ exerted on a machine - input distance: _________ the input force acts through - equals the input force multiplied by the input distance ex. - input __________ < output ___________ - input __________ > output ___________ Work Output Work Output - the ________ done by a machine as the output force acts through the output __________ - output force: force exerted by a machine - output distance: distance the output force is exerted through *due to friction the work done by a machine is always _______ than the work done on the machine Mechanical Advantage Definition - a quantity that measures how much a machine multiplies __________ or ___________ Two types __________: measures the actual forces action on a machine - AMA = __________: measures the mechanical advantage in the absence of _________ - IMA = Mechanical Advantage Problems Q: Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a actual mechanical advantage of 5.2, how much force is exerted on a nail in the claw? A: Q: If you exert 100 N on a jack to lift a 10,000 N car, what would be the jack’s actual mechanical advantage (AMA) A: Mechanical Advantage Problems Q: Calculate the ideal mechanical advantage (IMA) of a ramp that is 6.0 m long and 1.5 m high? A: Q: The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance? A: Efficiency of Machines Definition - a quantity, usually expressed as a ___________, that measures the ratio of _________ work input Formula - Efficiency = - % of work input that becomes work output - due to friction, efficiency of any machine is always less than _________ Efficiency Problems Q: Alice and Jim calculate that they must do 1800 J of work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp? A: Q: If the machine has an efficiency of 40%, and you do 1000 J of work on the machine, what will be the work output of the machine? A: Objectives Name, describe and give an example of each of the six types of simple machines Describe how to determine the ideal mechanical advantage of different types of simple machines Define and identify compound machines Recognize simple machines within compound machines Simple Machines Definition - one of the six basic types of machines 2 types or families 1. 2. Levers Definition - a rigid bar that is free to move around a fixed point ex. - all levers have a rigid arm that turns around a point called the __________ - force is transferred from one part of the arm to another - original input force can be multiplied or redirected into output force - levers are divided into __ classes, based on the locations of the input force, output force, and the fulcrum Lever Family Cont. 3 classes 1. First class - fulcrum is in the ___________ of an arm - the input force acts on one end - the other end applies an output force - MA can be <1, >1, =1 ex. 2. Second class - _________ is at one end of the arm - input force is applied to the other end - output force is in the __________ - MA will always be greater than 1 ex. Lever Family Cont. 3. Third class - input force is in the _________ - output force is one one end - fulcrum is on the other end - multiply distance rather than force MA always ____ 1 ex. Wheel and Axis Definition - simple machines that consist of two ________ or cylinders, each one with a different radius ex. - made of a level or a pulley (_________) connected to a shaft (______) - small input force, multiplied to become a large output force ex. Inclined Planes Definition - slanted surface along which a force moves an object to a different elevation ex. knife, ax, zipper, wedge, screw - ramp __________ the force applied to lift object upward - turns a small input force into a large output force by spreading the work out over a large ___________ - wedge: functions as _____ inclined planes back to back, turning a downward force into two forces directed out to the sides Pulleys Definition - a simple machine that consists of a rope that fits into a groove in a wheel. - very similar to a lever - point in the middle of the pulley is like a fulcrum - rest of the pulley acts like the rigid arm 3 Types of Pulleys - ________ pulleys - ________ pulleys - ________ system Types of Pulleys - fixed pulleys: changes the ___________ of the input force - moveable pulleys: changes both the ________ and the _________ of the input force - pulley system: made of both fixed and moveable pulleys Compound Machines Definition - a machine that is made of more than one simple machine ex.