March 17, 2009 IOT POLY ENGINEERING 3-13 DRILL Copy the following problem in your notebook, then solve. 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 care by gravity.

IOT POLY ENGINEERING 3-13 WORK 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

WORK 3-13 The man below is holding 1 ton above his head. IOT POLY ENGINEERING 3-13 WORK 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.

WORK 3-13 WORK = FORCE x DISTANCE IOT POLY ENGINEERING 3-13 WORK 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.

WORK 3-13 WORK = FORCE x DISTANCE W = F x d IOT POLY ENGINEERING 3-13 WORK 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?

WORK 3-13 WORK = FORCE x DISTANCE W = F x d IOT POLY ENGINEERING 3-13 WORK 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

Back to our drill problem WORK IOT POLY ENGINEERING 3-13 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

IOT POLY ENGINEERING 3-13 WORK d = 200’ Fw = ? 60o Wt = 3,000 lb 30o

WORK 3-13 cos 60o = x / (3000 lb) x = (3000 lb)(cos 600) IOT POLY ENGINEERING 3-13 WORK cos 60o = x / (3000 lb) x = (3000 lb)(cos 600) = (3000 lb)(1/2) x = 1,500 lb. x 60o 3000 lb.

WORK 3-13 F = 1,500 lb d = 200 ft W = ? W = F x d IOT POLY ENGINEERING 3-13 WORK 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

EFFICIENCY

EFFICIENCY = x 100% OUTPUT INPUT

Back to our drill problem IOT POLY ENGINEERING 3-13 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

Back to our drill problem EFFICIENCY IOT POLY ENGINEERING 3-13 Back to our drill problem FORCE APPLIED = 3,000 lb EFFECTIVE FORCE = 1,500 lb INPUT OUTPUT EFFICIENCY = x 100% OUTPUT INPUT EFF = x 100% 1,500 lb 3,000 lb EFF = 50%

POWER 3-13 Three Buddhist monks walk up stairs to a temple. IOT POLY ENGINEERING 3-13 POWER 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?

P = W t Units: POWER Watts, Horsepower, Ft-lbs/s 3-13 IOT POLY ENGINEERING 3-13 POWER Power is the rate of doing Work P = The less time it takes…. The more power Units: Watts, Horsepower, Ft-lbs/s W t

Problem Solving Steps: IOT POLY ENGINEERING 3-13 HOMEWORK WORKSHEET Problem Solving Steps: Write given and sketch a diagram Write equation/formula Substitute values and solve Check answer Box answer