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IOT POLY ENGINEERING

March 18, 2009. IOT POLY ENGINEERING. 3-14. DRILL. Solve the following problem in your notebook.

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IOT POLY ENGINEERING

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  1. March 18, 2009 IOTPOLYENGINEERING 3-14 DRILL Solve the following problem in your notebook. It takes a girl 1 minute to pull her 18 lb wagon a distance of 40 ft. The force required to pull the wagon is 12 lbs, which the girl applies to the wagon handle at 30o with the horizontal. How much work does she do? How efficient is her force being applied in the direction of displacement? What power is used (units for power are ft-lb / s)?

  2. WORK IOTPOLYENGINEERING 3-14 STEP #1: SKETCH and GIVEN t = 1 min = 60 s Wt = 18 lbs d = 40 ft. F = 12 lbs W = ? EFF = ? P = ? F = 12 lbs Fx = (12 lbs)cos 30o 300 d = 40’ Wt = 18 lbs

  3. WORK IOTPOLYENGINEERING 3-14 STEP #1: SKETCH and GIVEN t = 1 min = 60 s Wt = 18 lbs d = 40 ft. Fx = 10.4 lbs W = ? EFF = ? P = ? F = 12 lbs 300 d = 40’ Wt = 18 lbs

  4. WORK IOTPOLYENGINEERING 3-14 STEP #4: CHECK / BOX ANSWER STEP #2: WRITE EQUATIONFORMULA STEP #3: SUBSTITUTE / SOLVE W = F x d = Fx x d = (10.4 lb)(40’) W = 416 ft-lb EFF = (Output/Input)x100% = (F / Fx) x 100% = [(10.4 lb)/(12lb)] x 100% EFF = 86.7% P = W / t = (416 ft-lb) / (60 s) P = 6.93 ft-lb/s

  5. WORK IOTPOLYENGINEERING 3-14 • Velocity, acceleration, force, etc. mean nearly the same thing in everyday life as they do in physics. • Work means something distinctly different. • Consider the following: • Hold a book at arm’s length for three minutes. • Your arm gets tired. • Did you do work? • No, you did no work whatsoever. • You exerted a force to support the book, but you did not move it. • A force does no work if the object doesn’t move

  6. WORK IOTPOLYENGINEERING 3-14 • The man below is holding 1 ton above his head. • Is he doing work? • No, the object is not moving. • Describe the work he did do: • Lifting the 1 ton from the ground to above his head.

  7. WORK IOTPOLYENGINEERING 3-14 • WORK = FORCE x DISTANCE • The work W done on an object by an agent exerting a constant force on the object is the product of the component of the force in the direction of the displacement and the magnitude of the displacement.

  8. WORK IOTPOLYENGINEERING 3-14 • WORK = FORCE x DISTANCE • W = F x d • Consider the 1.3-lb ball below, sitting at rest. How much work is gravity doing on the ball?

  9. WORK IOTPOLYENGINEERING 3-14 • WORK = FORCE x DISTANCE • W = F x d • Now consider the 1.3-lb ball below, falling 1,450 ft from the top of Sears Tower. How much work will have gravity done on the ball by the time it hits the ground? • F = 1.3 lbs W = F x d • d = 1,450 ft. = (1.3 lb) x (1,450 ft.) • W = ? W = 1,885 ft-lb

  10. WORK IOTPOLYENGINEERING 3-14 • Back to our drill problem A 3,000-lb car is sitting on a hill in neutral. The angle the hill makes with the horizontal is 30o. The distance from flat ground to the car is 200 ft. Begin with a free-body diagram. Then, calculate the weight component facing down the hill. Finally, calculate the work done on the car by gravity. d = 200’ Fw = ? Wt = 3,000 lb 30o

  11. WORK IOTPOLYENGINEERING 3-14 d = 200’ Fw = ? 60o Wt = 3,000 lb 30o

  12. WORK IOTPOLYENGINEERING 3-14 cos 60o = x / (3000 lb) x = (3000 lb)(cos 600) = (3000 lb)(1/2) x = 1,500 lb. 60o x 3000 lb.

  13. WORK IOTPOLYENGINEERING 3-14 d = 200’ F = 1,500 lb d = 200 ft W = ? F = 1,500 lb. Wt = 3,000 lb 30o W = F x d = (1500 lb) x (200 ft) W = 300,000 ft-lb

  14. EFFICIENCY

  15. OUTPUT INPUT EFFICIENCY = x 100%

  16. IOTPOLYENGINEERING 3-14 EFFICIENCY • Back to our drill problem F = 1,500 lb. Wt = 3,000 lb FORCE APPLIED = 3,000 lb EFFECTIVE FORCE = 1,500 lb INPUT OUTPUT

  17. EFFICIENCY OUTPUT INPUT 1,500 lb 3,000 lb EFFICIENCY = x 100% EFF = x 100% IOTPOLYENGINEERING 3-14 • Back to our drill problem FORCE APPLIED = 3,000 lb EFFECTIVE FORCE = 1,500 lb INPUT OUTPUT EFF = 50%

  18. POWER IOTPOLYENGINEERING 3-14 • Three Buddhist monks walk up stairs to a temple. • Each weighs 150 lbs and climbs height of 100’. • One climbs faster than the other two. • Who does more work? • They all do the same work: • W = F x d (force for all three is 150 lb) • = (150 lb)(100’) • W = 15,000 ft-lb • Who has greater power?

  19. POWER IOTPOLYENGINEERING 3-14 • Power is the rate of doing Work • P = • The less time it takes…. • The more power • Units: • Watts, Horsepower, Ft-lbs/s • W t

  20. HOMEWORK IOTPOLYENGINEERING 3-14 WORKSHEET • Problem Solving Steps: • Write given and sketch a diagram • Write equation/formula • Substitute values and solve • Check answer • Box answer

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