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Work in Mechanical System by ewghwehws


									    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
   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
 Work in Mechanical System
6. How do we calculate work?
  - work = applied force x distance
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 = (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 = (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%
   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
 - Work = Applied Torque x Angle of
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

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