VIEWS: 3 PAGES: 17 POSTED ON: 9/3/2012
Work in Mechanical System Work in Mechanical System 1. What is work? - exerting a force over a distance 2. When is work done? - work is only done when a force causes something to move or change - the force and the movement MUST be in the same direction for work to occur Work in Mechanical System 3. A man picks up a stack of books. Does he do work on the books? - yes; the force and the movement were in the same direction 4. The man holds the books at his waist. Is he doing work on the books? - no; the books are not moving Work in Mechanical System 5. The same man carries the books across the room. Is work done on the books? - no; the force on the books is not in the same direction as the books are moving Work in Mechanical System 6. How do we calculate work? -W=FxD - work = applied force x distance moved 7. What are the units for work? - English – foot-pounds (ft-lbs) - SI – joule (J) – “jewel” - Newton-meters are also used in SI Work in Mechanical System 8. Example – A weight lifter applies a vertical force to raise a 200 lb barbell 5 feet high. How much work is done? -W=FxD W = (200 lb)(5 ft) W = 1000 ft-lb Work in Mechanical System 9. Example – The weight lifter holds the weight for 10 minutes. How much work is done? - none – the weight is not moving Work in Mechanical System 10. Example – An electric truck is used to pull a cart a distance of 100 meters. A force of 900 newtons is required to keep the cart moving at a constant speed. How much work is done on the cart? -W=FxD W = (900N)(100m) W = 90,000 J Work in Mechanical System 10. What is efficiency? - the relationship of output work to input work of a machine - efficiency is always less than 100% 11. What is input work? - work done on a machine by a person 12. What is output work? - work done by the machine Work in Mechanical System 13. How do we calculate efficiency? % Efficiency = output work x 100% input work Work in Mechanical System 14. Example – A block and tackle (series of pulleys) is used to lift an engine. The engine weighs 600 lbs and is 0.9 ft. The operator pulls with a force of 100 lbs over a distance of 6 ft. Find the A) input work, B) the output work, and C) the efficiency of the machine. A. Input Work = F x D Input Work = (100 lbs)(6 ft) Input Work = 600 ft-lbs B. Output Work = F x D Output Work = (600 lbs)(0.9 ft) Output Work = 540 ft-lbs C. Efficiency = Output x 100% Input Efficiency = 540 ft-lbs x 100% 600 ft-lbs Efficiency = 90 % Work in Mechanical System 15. What else can do work in the MS? - torque does work when it causes an object to rotate 16. What are some examples of torque doing work? - turning wheels on car, a record player, helicopter blades, power drill Work in Mechanical System 17. How do we calculate work done by torque? - Work = Applied Torque x Angle of Rotation W=Txq 18. What is q? - angle of rotation measured in radians; Greek letter theta Work in Mechanical System 19. What is a radian? - another way to measure angles other than degrees - one revolution (360o) = 2p radians - to find the angle of revolution in radians, take the number of revolutions and multiply by 2p 20. Example – An electric motor is used to drive a water pump. The motor provides a torque of 150 N-m. How much work is done while the motor through 40 revolutions? - You must 1) Find the q 2) solve for the work 1) q = (# of revolutions)(2p) q = (40 revolutions)(2p) q = 251.2 radians 2) Work = Torque x Angle W = (150 N-m)(251.2) W = 37680 J - note – the radians is dropped - note – Joules are used because the torque was given in metric units