G7- Unit M- Chap 4- Work and Machines – Section 2 What is a Machine?
Section 2: What is a machine?– 4 Sessions
Objectives
Activities / Lab work
Vocabulary
• Explain how a machine makes work easier.
• Describe and give examples of the force-distance trade-off that occurs when a machine is
used.
• Calculate mechanical advantage.
• Explain why machines are not 100% efficient.
Different types of kitchen tools
Machine, work input, work output, mechanical advantage, mechanical efficiency
Starter:
Main Lesson
Launch the Lesson
Brainstorm
The class will first be
engaged with an activity
involving unfamiliar tools
used to complete a task.
The teacher will provide
enough utensils to provide
each group of 3 to 4
students with an objectto
decide on its use and
come up with two other
Inquiry skills: Compare and contrast / Predict/ Observe critically/ Model and label
Instructional strategy of the concept
Session 1
Power point presentation introducing machines and their uses to
students introducing different types of machines
Session 2
Plenary:
Practice & Apply
Internet Activity
Summary of the lesson
Calculating Mechanical
Lesson Review sheet
Advantage
Lesson Review
Internet Activity – The form will be given from the textbook by the
Assessment
practice
Lesson quizzes
End of chapter tests
teacher to help the students create the brochure
Writing Prompt: Use the internet and library
resources to find out more about simple machines and
how they are put to use in common objects, such as
the ones listed in the image selection boxes. Then
create a brochure promoting your favorite simple
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uses for it. The group’s
will present their
hypothesis of their object’s
use and its other uses to
the class after 10 minutes
of brainstorming and
collaborating
machine and offer three examples of common tools
that use it.
Directions: Use this form to create a brochure. Type
or paste text into the text fields. Select images from
the drop-down lists. Be sure to complete the layout in
one
class period.
Session 3 & 4
Demonstration on Calculating Mechanical Advantage
and Mechanical Efficiency
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Calculating Mechanical Advantage of Levers
We use simple machines, like levers, to make tasks easier. While the output work (work done by the machine) of a simple machine can never
be greater than the input work (work put into the machine), a simple machine can multiply input forces OR multiply input distances (but never
both at the same time). You can use this skill sheet to practice calculating mechanical advantage (MA) for levers.
The general formula for the mechanical advantage (MA) of levers:
MAlever = Fo(output force)
Fi (input force)
Or you can use the ratio of the input arm length to the output arm length:
MAlever = Li (length of input arm)
Lo (length of output arm)
Most of the time, levers are used to multiply force to lift heavy objects.
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Example 1: A construction worker uses a board and log as a lever to lift a
heavy rock. If the input arm is 3 meters long and the output arm is 0.75 meters
long, what is the mechanical advantage of the lever?
MA = 3 meters
= 4
0.75 meter
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Example 2: Sometimes levers are used to multiply distance. For a broom, your
upper hand is the fulcrum and your lower hand provides the input force: Notice
the input arm is shorter than the output arm. The mechanical advantage of this
broom is:
MA = 0.3 meter = 0.25
1.2 meters
A mechanical advantage less than one does not mean a machine isn’t useful.
It just means that instead of multiplying force, the machine multiplies
distance. A broom doesn’t push the dust with as much force as you
use to push the broom, but a small movement of your arm pushes
the dust a large distance.
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Lever problems
1. A lever used to lift a heavy box has an effort arm of 4 meters and an resistance arm of 0.8 meters. What is the mechanical advantage of the
lever?
2. What is the mechanical advantage of a lever that has an effort arm of 3 meters and a resistance arm of 2 meters?
3. A lever with an effort arm of 2 meters has a mechanical advantage of 4. What is the resistance arm’s length?
4. A lever with an resistance arm of 0.8 meter has a mechanical advantage of 6. What is the length of the effort arm?
5. A rake is held so that its effort arm is 0.4 meters and its resistance arm is 1.0 meters. What is the mechanical advantage of the rake?
6. A broom with an effort arm length of 0.4 meters has a mechanical advantage of 0.5. What is the length of the resistance arm?
7. A child’s toy rake is held so that its resistance arm is 0.75 meters. If the mechanical advantage is 0.33, what is the effort arm length?
Answers:
1. 5 2. 1.5
3. 0.5
4. 4.8
5. 0.4
6. 0.8
7. 0.25
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Homework Practice Problems #2—Simple Machines, Efficiency & Mechanical Advantage
Calculations
1.
A 100N effort force is applied to a machine lifts a 400N object. What is the MA of the machine?
2.
What is the percent efficiency of a machine in which work input is 4800J and work output is 3600J?
3.
A machine moves a 600N object 6m. If the amount of work put into the machine is 4500J, what is the efficiency of the machine? First find the work output of the machine and then
find efficiency!
4.
A machine has a mechanical advantage of 3. If a 20N force is applied to the machine, how much force will the machine produce?
5.
A machine used to lift crates of oranges has an efficiency of 65%. If a crate of oranges weighing 1300N is lifted 50m, how much work must be put into the machine?
6.
A force of 10,000N is applied to a stationary wall. How much work is performed?
7.
A 950N skydiver jumps from an altitude of 3000m. What is the total work performed on the skydiver?
8.
A bulldozer performs 80,000N-m of work pushing dirt 16m. What is the force of the dirt?
9.
An ant does 1N-m of work dragging a 0.002N grain of sugar. How far does the ant drag the sugar?
10. A horse performs 15,000J of work pulling a wagon for 20s. What is the horse’s power?
11. A 750N pole vaulter lifts himself 5m high in 2.5s. What is his power?
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12. A pump drains a small pond by performing 120,000J of work. The power rating of the pump is 1000W. How long does it take the pond to drain?
13. A tow truck pulls a car out of a ditch in 6.5s. If 6000W of power is used, how much work does the truck do?
14. An elevator lifts five passengers 30m in 24s. The power is 15,000W. What is the total weight of the elevator and passengers?
15. Find the kinetic energy using 20 kg and velocity of 2 m/s?
Answers To Homework Practice Problems #2—Simple Machines, Efficiency & Mechanical Advantage
1.
A 100N effort force is applied to a machine lifts a 400N object. What is the MA of the machine?
2.
What is the percent efficiency of a machine in which work input is 4800J and work output is 3600J?
3.
A machine moves a 600N object 6m. If the amount of work put into the machine is 4500J, what is the efficiency of the machine? First find the work output of the machine and then
find efficiency!
400 / 100 = 4
3600 / 4800 = 75%
3600 / 4500 = 80%
4.
A machine has a mechanical advantage of 3. If a 20N force is applied to the machine, how much force will the machine produce?
5.
A machine used to lift crates of oranges has an efficiency of 65%. If a crate of oranges weighing 1300N is lifted 50m, how much work must be put into the machine?
6.
A force of 10,000N is applied to a stationary wall. How much work is performed?
3 x 20 = 60 N
1300 x 50 = 65000J / .65 = 100,000 J
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0 none
7.
A 950N skydiver jumps from an altitude of 3000m. What is the total work performed on the skydiver?
8.
A bulldozer performs 80,000N-m of work pushing dirt 16m. What is the force of the dirt?
9.
An ant does 1N-m of work dragging a 0.002N grain of sugar. How far does the ant drag the sugar?
950 x 3000 = 2,850,000 J
80,000 / 16 = 5000 N
1 / .002 = 500 m
10. A horse performs 15,000J of work pulling a wagon for 20s. What is the horse’s power?
15000 / 20 = 750 W
11. A 750N pole vaulter lifts himself 5m high in 2.5s. What is his power?
750 x 5 = 3750 / 2.5 = 1500 W
12. A pump drains a small pond by performing 120,000J of work. The power rating of the pump is 1000W. How long does it take the pond to drain?
120,000 / 1000 = 120 s
13. A tow truck pulls a car out of a ditch in 6.5s. If 6000W of power is used, how much work does the truck do?
6000 x 6.5 =39,000 J
14. An elevator lifts five passengers 30m in 24s. The power is 15,000W. What is the total weight of the elevator and passengers?
15000 x 24 = 360,000 J / 30 = 12000 N
15. Find the kinetic energy using 20 kg and velocity of 2 m/s?
½(20 kg x 4) = ½(80) = 80 / 2 = 40 J
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Section: What Is a Machine?
MACHINES: MAKING WORK EASIER
______ 1. What is a device that makes work easier by changing the size or direction of force?
a. a machine
c. an engine
b. a load
d. a computer
2. Name three examples of everyday machines.
______________________________________________________________
______ 3. What type of common machine is a screwdriver that is used to pry off the lid on a paint can?
a. a pulley
c. a lever
b. a wheel
d. a screw
4. The work you do on a machine is called ______________________
5. The work done by a machine on an object is called ______________________
6. Work output can never be greater than ______________________
7. Why do machines need less force to do the same amount of work?
______________________________________________________________
8. When a screwdriver is used to open a can, both the size and direction of the ______________________ change
9. A ramp will decrease the size of the input force needed to lift a box and ______________________ the distance over which the force is
exerted.
10. When a machine changes the size of the force, the ______________________ through which the force is exerted must also change.
MECHANICAL ADVANTAGE
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______11. What is the number of times a machine multiplies force called?
a. output force
b. input force
c. mechanical advantage
d. work output
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______12. Which of the following is the formula for finding mechanical advantage?
a. MA = input force  output force
b. MA = output force  input force
c. MA = input force  output force  100
d. MA = output force  input force  100
13. A machine that has a mechanical advantage of greater than 1 has an output force that is ______________________ than the input
force.
14. A machine that has a mechanical advantage of less than 1 reduces the output force but can increase the ______________________ an
object moves.
MECHANICAL EFFICIENCY
______15. What is the quantity that measures the ratio of work output to work input called?
a. mechanical work
c. mechanical force
b. mechanical efficiency
d. mechanical energy
______16. Which of the following is the equation for finding mechanical efficiency?
a. mechanical efficiency = work input  work output
b. mechanical efficiency = work output  work input
c. mechanical efficiency = work input  work output  100
d. mechanical efficiency = work output  work input  100
17. When a machine drills holes in metal, some of the work input is used to overcome ______________________ between the metal and
the drill.
18. What would a machine that had 100% mechanical efficiency be called?
______________________________________________________________
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19. Why is it impossible to build an ideal machine?
______________________________________________________________
20. What do some machines use to lower friction between moving parts?
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