Download 3-13 IOT

March 31, 2011
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 car by gravity.
IOT
3-13
POLY ENGINEERING
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
3-13
POLY ENGINEERING
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
3-13
POLY ENGINEERING
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
3-13
POLY ENGINEERING
WORK
WORK = FORCE x DISTANCE
W=Fxd
Consider the 1.3-lb ball below, sitting at rest. How much work is
gravity doing on the ball?
IOT
3-13
POLY ENGINEERING
WORK
WORK = FORCE x DISTANCE
W=Fxd
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
d = 1,450 ft.
W=?
W=Fxd
= (1.3 lb) x (1,450 ft.)
W = 1,885 ft-lb
IOT
3-13
POLY ENGINEERING
WORK
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.
Fw = ?
Wt = 3,000 lb
30o
IOT
3-13
POLY ENGINEERING
WORK
Fw = ?
60o
Wt = 3,000 lb
30o
IOT
3-13
POLY ENGINEERING
WORK
x
60o
cos 60o = x / (3000 lb)
x = (3000 lb)(cos 600)
3000 lb.
= (3000 lb)(1/2)
x = 1,500 lb.
IOT
3-13
POLY ENGINEERING
WORK
F = 1,500 lb
F = 1,500 lb.
d = 200 ft
Wt = 3,000 lb
W=?
30o
W = Fxd
= (1500 lb) x (200 ft)
W = 300,000 ft-lb
IOT
3-13
POLY ENGINEERING
EFFICIENCY
OUTPUT
EFFICIENCY =
x 100%
INPUT
EFFICIENCY
Back to our drill problem
F = 1,500 lb.
Wt = 3,000 lb
FORCE APPLIED = 3,000 lb
INPUT
EFFECTIVE FORCE = 1,500 lb
OUTPUT
IOT
3-13
POLY ENGINEERING
EFFICIENCY
Back to our drill problem
FORCE APPLIED = 3,000 lb
INPUT
EFFECTIVE FORCE = 1,500 lb
OUTPUT
EFFICIENCY =
EFF =
OUTPUT
x 100%
INPUT
1,500 lb
x 100%
3,000 lb
EFF = 50%
IOT
3-13
POLY ENGINEERING
POWER
1.
2.
3.
4.
5.
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
6. Who has greater power?
IOT
3-13
POLY ENGINEERING
POWER
Power is the rate of doing Work
W
P=
t
The less time it takes….
The more power
Units:
Watts, Horsepower, Ft-lbs/s
IOT
3-13
POLY ENGINEERING
HOMEWORK
WORKSHEET
Problem Solving Steps:
1. Write given, sketch and label a diagram
2. Write equation/formula
3. Substitute values and solve
4. Check answer
5. Box answer
IOT
3-13
POLY ENGINEERING