“Work and Power”
CHAPTER 14
LESSON 1
I. WORK
A. Books in hand Demo
B. Def – the product of force and distance
1. Not…a job, chores, school.
2. Formula: Work = Force x Distance
3. Force is in Newtons
4. Distance is in meters
5. Work is in Joules
6. Ex: Your backpack is full and weighs 30
Newtons. You lift it from the floor to the shelf, a
distance of 1.5 meters. How much work is
done on the backpack?
W=FxD
30 N x 1.5 m = ?
45 Joules
7. A backpack has a weight of 35 N and a
student who is 1.8 meters tall has it on her
back. How much work is the student doing on
the backpack?
W=FxD
35 N x 0 meters = 0 Joules
8. A carpenter lifts a 45-kg beam 1.2 meters.
Approximately how much work is done on the
beam? (be careful!!! Look at your units)
Hint … 1 kg = 9.8 N
45 kg x 9.8 N/kg = 441 Newtons
441 N x 1.2 meters =
=529.2 Joules
II. POWER
A. Def – Rate of doing work.
1. 2 variables
a. Work and Time
B. Formula
1. Power = Work (J) / Time (s)
a. Unit is the Watt (W)
b. A 60-watt light bulb requires 60 J per
second.
C. Ex: A machine puts forth 400 J of work in
0.25 seconds. What is the amount of power of
the machine?
P=W/T
P = 400 J
0.25 Sec
= 1600 Watts
D. A similar machine exerts 500 J of work with
1400 Watts of power. What is the time it takes
this machine to do its job?
T=W/P
T = 500 J
1400 Watts
= 0.36 sec
CHAPTER 14
LESSON 2
“Work and Machines”
I. WHY WE USE MACHINES
A.
What are Machines?
1. Machine – a device that changes a force.
 makes work easier
 change the size of a force needed
 change the direction of a force
B.
Advantages of a Machine
1. Increasing force…
tradeoff is you put in greater distance
-”easier”
Ex. Pg. 418 – changing a tire with a jack
2. Increasing distance…
tradeoff is you put in less distance
-”harder / more difficult”
Ex. Pg. 419 – rowing a boat with oars
3. Changing Direction…
movement in one direction causes movement in the
opposite direction
Ex. Pg. 419 rowing a boat with oars
II. WORK INPUT AND WORK OUTPUT
A.
Work Input to a machine
1. Formula…
Fi x Di
a. Input Force – the force you exert to the machine.
-Fi
-Ex. Pushing on a crowbar
b. Input Distance –the distance the input force acts
through.
-Di
-Ex. The length of a crowbar
B.
Work Output of a machine
1. Formula…
Fo x Do
1. Output force – the force exerted by the
machine.
Fo
2. Output distance – the distance the output
force is exerted through.
Do
**Work output is always less than work input**
Why???
 Friction (pg. 420 p. 1 end)
In-class Assignment…
1. Pg. 420 #’s 1-6
2. Read 14.3
CHAPTER 14
LESSON 3
“Mechanical Advantage and Efficiency”
I. Mechanical Advantage
A. Def – the # of times a machine multiplies
the input force. (MA)
B. Pg. 421 – nut cracker
C. MA of Force = Output force
Input force
D. Ideal – MA w/o friction
E. MA of Distance =
Input Distance
Output Distance
F. Ex. A carpenter uses a claw hammer to pull a
nail from a board. The nail has a resistance of 2500
N. The carpenter applies an input force of 125 N.
What is the mechanical advantage of the hammer?
MA = Output force
Input force
so…
2500 N
125 N
= 20 MA
G. Ex. A worker applies an input force of 20
N to pry open a window that has a output force
of 500 N. What is the mechanical advantage
of the crowbar?
500 / 20
= 25
H. Find the input force needed to lift a
2000 N rock, using a jack with a mechanical
advantage of 10.
Fi = Fo / MA
=2000 / 10
=200 N
II. EFFICIENCY
B. Def – How much work put into the machine is
changed to useful work put out by the machine.
1. Formula
Efficiency = Woutput
Winput X 100
2. Why doesn’t Wout = Win?
-friction
-dirt
-age
-bad parts
-improper use
3.
Ex: A sofa weighing 1000 N must be placed in a truck bed
1.2 m off of the ground. A worker uses a force of 500 N to
push the sofa up the inclined plane that has a slope of 3.5 m.
What is the efficiency of the inclined plane?
Fo =
Fi =
Do =
Di =
1000 N
500 N
1.2 m
3.5 m
1000 N x 1.2 m
500 N x 3.5 m
= 68.6 % efficient

A record setting buck weighing 1205 N is
pulled up a ramp to the back of a pick-up with a
force of 600 N by a hunter. If the back end of
the pick up is 1.2 meters high and the length of
the ramp is 3.2 meters, what is the efficiency of
the ramp?
75.3%
The Simple Machine
CHAPTER 14
LESSON 4
I. LEVER SYSTEM
A. Def – A lever is a bar that is free to pivot or turn about a
fixed point.
B. Parts of a lever
1. Fulcrum – fixed point of the lever
2. Effort arm – part where force is applied to the lever.
3. Resistance arm – part where the resistance is found on
the lever.
C. Ideal Mechanical Advantage
1. IMA = Length of effort arm
Length of resistance arm
or
Le
Lr
2. Ex: A worker uses an iron bar to
raise a manhole cover weighing 65 N.
The effort arm of the lever is 60 cm
long. The resistance arm is 10 cm
long. What is the ideal mechanical
advantage of the bar?
60 cm
Le =
10 cm
Lr =
= 6 (IMA)
D. Types of Levers
1. 1st Class
-E – F – R
-Common Ex:
teeter totter
scissors
pliers
claw of hammer
2. 2nd Class
-F–R–E
- Common Ex:
stapler
bottle opener
wheelbarrow
nut cracker
3. 3rd Class (increase speed)
-F–E–R
- Common Ex:
fishing rod
tweezers
mousetrap
baseball bat
softball bat
II. PULLEYS
A. Def – a grooved wheel with a rope or chain
running along the groove.
1. similar to 1st class lever
2. uses rope instead of bar
3. has effort arm and resistance arm.
B. Types of Pulleys
1. Fixed
-Fixed – pulley does not move
-MA = 1
2. Movable
-Movable – pulley is movable
-MA = 2 (can vary)
3. Block and Tackle
-uses fixed and
movable
-Large Mechanical
advantage
-MA = # of ropes in
the pulley system
-MA = 2
WHAT IS THE MA?
=4
III. WHEEL AND AXLE
A. Def – a simple machine consisting of 2 different sized
wheels that rotate together.
1. Parts
-small wheel = axle
2. Effort force is applied to large wheel
3. Large wheel turns the axle
4. IMA = rw / ra
5. Ex: Bike, doorknobs, ice cream makers, cars…
6. Ex: An ice cream maker has a wheel radius
of 20cm. The axle has a radius of 15 cm.
What is the ideal mechanical advantage of the
wheel and axle?
20 / 15
=1.3 IMA
IV. INCLINED PLANE, WEDGE, SCREW
A. Inclined Plane
1. Def – a sloped surface used to raise
objects
2. IMA = length of slope / height of slope
3. Ex: any ramp
4. Remember** Force is smaller but
distance is greater.
B. Screw
1. Def – Inclined plane wrapped in a spiral
cylindrical post.
2. Ex: screws, lids, drill bits
C. Wedge
1. Def – Inclined plane with 1 or 2
sloping sides
2. Used to cut or split
3. Ex: knives, ax blades, chisels…