IOT
POLY ENGINEERING3-14
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 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)?
IOT
POLY ENGINEERING3-14
WORK
300
F = 12 lbs
Wt = 18 lbs
d = 40’
STEP #1: SKETCH and GIVENt = 1 min = 60 s
Wt = 18 lbs
d = 40 ft.
F = 12 lbs
W = ?
EFF = ?
P = ?
Fx = (12 lbs)cos 30o
IOT
POLY ENGINEERING3-14
WORK
300
F = 12 lbs
Wt = 18 lbs
d = 40’
STEP #1: SKETCH and GIVENt = 1 min = 60 s
Wt = 18 lbs
d = 40 ft.
Fx = 10.4 lbs
W = ?
EFF = ?
P = ?
IOT
POLY ENGINEERING3-14
STEP #2: WRITE EQUATIONFORMULASTEP #4: CHECK / BOX ANSWERSTEP #3: SUBSTITUTE / SOLVE
WORK
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)/(12 lb)] x 100%
EFF = 86.7%
P = W / t
= (416 ft-lb) / (60 s)
P = 6.93 ft-lb/s
IOT
POLY ENGINEERING3-14
WORK1. 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 ENGINEERING3-14
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 ENGINEERING3-14
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 ENGINEERING3-14
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 ENGINEERING3-14
WORKWORK = 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 dd = 1,450 ft. = (1.3 lb) x (1,450 ft.)W = ? W = 1,885 ft-lb
IOT
POLY ENGINEERING3-14
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.
Wt = 3,000 lb
30o
Fw = ?d = 200’
WORKBack to our drill problem
IOT
POLY ENGINEERING3-14
WORK
60o
3000 lb.
x cos 60o = x / (3000 lb)
x = (3000 lb)(cos 600)
= (3000 lb)(1/2)
x = 1,500 lb.
IOT
POLY ENGINEERING3-14
Wt = 3,000 lb
30o
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
IOT
POLY ENGINEERING3-14
Wt = 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 ENGINEERING3-14
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 ENGINEERING3-14
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 ENGINEERING3-14