1. 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.

2. 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

3. 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.

4. 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.

5. 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?

6. 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

7. 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

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

9. 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.

10. 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

11. EFFICIENCY

13. EFFICIENCY = x 100%
OUTPUT INPUT

14. 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

15. 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 ,000 lb
EFF =
50%

16. 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?

17. 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

18. 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