IOT POLY ENGINEERING 3-14 DRILL March 18, 2009 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 30 o 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)?

IOT POLY ENGINEERING 3-14 WORK 30 0 F = 12 lbs W t = 18 lbs d = 40’ STEP #1: SKETCH and GIVEN t = 1 min = 60 s W t = 18 lbs d = 40 ft. F = 12 lbs W = ? EFF = ? P = ? F x = (12 lbs)cos 30 o

IOT POLY ENGINEERING 3-14 WORK 30 0 F = 12 lbs W t = 18 lbs d = 40’ STEP #1: SKETCH and GIVEN t = 1 min = 60 s W t = 18 lbs d = 40 ft. F x = 10.4 lbs W = ? EFF = ? P = ?

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

IOT POLY ENGINEERING 3-14 WORK 1.Velocity, acceleration, force, etc. mean nearly the same thing in everyday life as they do in physics. 2.Work means something distinctly different. 3.Consider the following: 1)Hold a book at arm’s length for three minutes. 2)Your arm gets tired. 3)Did you do work? 4)No, you did no work whatsoever. 4.You exerted a force to support the book, but you did not move it. 5.A force does no work if the object doesn’t move

IOT POLY ENGINEERING 3-14 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.

IOT POLY ENGINEERING 3-14 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.

IOT POLY ENGINEERING 3-14 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?

IOT POLY ENGINEERING 3-14 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 lbsW = F x d d = 1,450 ft. = (1.3 lb) x (1,450 ft.) W = ?W = 1,885 ft-lb

IOT POLY ENGINEERING 3-14 A 3,000-lb car is sitting on a hill in neutral. The angle the hill makes with the horizontal is 30 o. 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. W t = 3,000 lb 30 o F w = ? d = 200’ WORK Back to our drill problem

IOT POLY ENGINEERING 3-14 W t = 3,000 lb 30 o F w = ? d = 200’ WORK 60 o

IOT POLY ENGINEERING 3-14 WORK 60 o 3000 lb. x cos 60 o = x / (3000 lb) x = (3000 lb)(cos 60 0 ) = (3000 lb)(1/2) x = 1,500 lb.

IOT POLY ENGINEERING 3-14 W t = 3,000 lb 30 o F = 1,500 lb. d = 200’ WORK F = 1,500 lb d = 200 ft W = ? W = F x d = (1500 lb) x (200 ft) W = 300,000 ft-lb

EFFICIENCY

EFFICIENCY = x 100% OUTPUT INPUT

IOT POLY ENGINEERING 3-14 W t = 3,000 lb F = 1,500 lb. EFFICIENCY FORCE APPLIED = 3,000 lb EFFECTIVE FORCE = 1,500 lb Back to our drill problem INPUT OUTPUT

IOT POLY ENGINEERING 3-14 EFFICIENCY FORCE APPLIED = 3,000 lb EFFECTIVE FORCE = 1,500 lb Back to our drill problem INPUT OUTPUT EFFICIENCY = x 100% OUTPUT INPUT EFF = x 100% 1,500 lb 3,000 lb EFF =50%

IOT POLY ENGINEERING 3-14 POWER 1.Three Buddhist monks walk up stairs to a temple. 2.Each weighs 150 lbs and climbs height of 100’. 3.One climbs faster than the other two. 4.Who does more work? 5.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 6.Who has greater power?

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 IOT POLY ENGINEERING 3-14

IOT POLY ENGINEERING 3-14 HOMEWORK WORKSHEET Problem Solving Steps: 1. Write given and sketch a diagram 2. Write equation/formula 3. Substitute values and solve 4. Check answer 5. Box answer