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PHYSICS: CHAPTER 4.2 NOTE PACKET
Chapter 4.2: How Machines Do Work
I.
Vocabulary: Define the following terms from Chapter 4.2.
a. Machine-A device that allows you to do work in a way that is easier.
b. Input force-the force you exert on a machine.
c.
Output force-the force a machine exerts on an object.
d. Input work-the input force times the input distance.
e. Output work-the output force times the output distance.
f.
Mechanical advantage-the ratio of output force to input force.
g. Efficiency-the efficiency compares the output work to the input work.
Chapter 4.2: How Machines Do Work
II.
Notes:
a. What Is a Machine?
i. A device that allows you to DO WORK IN A WAY THAT IS EASIER.
ii. Input and Output Forces
1. Input Force
a. The force YOU EXERT on the machine.
2. Output Forces
a. The force the machine EXERTS ON AN OBJECT.
iii. Input and Output Work
1. Input Work
a. Input FORCE x Input DISTANCE
2. Output Work
a. Output FORCE x Output DISTANCE
i. The amount of OUTPUT work can NEVER BE
GREATER than the amount of INPUT work.
iv. A machine makes work easier by CHANGING at least one of THREE factors.
1. Change the AMOUNT OF FORCE you exert.
2. Change the DISTANCE over which you exert your force.
a. Exerting less force over a greater distance.
3. Change the DIRECTION in which you exert your force.
a. Exerting the same force over the same distance, but in an
opposite direction.
QUESTION: How does the cable system on a weight machine make raising the weights
easier? __The cable system enables you to raise the weights more conveniently by
changing the direction to make work easier._____________________________.
b. Mechanical Advantage
i. The NUMBER OF TIMES A MACHINE INCREASES THE FORCE exerted on
it.
1. Mechanical Advantage = OUTPUT Force/INPUT Force
a. Increasing Force
i. Output Force > Input Force
b. Increasing Distance
i. Output Force < Input Force
c. Changing Direction
i. Output Force = Input Force (no change in force)
c. Efficiency of Machines
i. Input Work is ALWAYS GREATER than Output Work
1. Force must be applied to OVERCOME THE FRICTION of the machine
itself
ii. Expressed as a percentage
1. To calculate the efficiency of a machine, DIVIDE the OUTPUT WORK
by the INPUT WORK and multiply the result by 100 percent.
a. Efficiency = Output Work / Input Work x 100
iii. Real and Ideal Machines
1. REAL
a. Efficiency is ALWAYS LESS THAN 100%
i. loss due to friction
2. IDEAL
a. Efficiency is ALWAYS 100%
i. does not exist.
QUESTION: Why is output work always less than input work in real situations?
_Because friction exists in every machine and reduces the machines efficiency.
Chapter 4.2: Work and Machines
III.
1.
Assessment Questions
What is a machine?
A machine is a device that allows work to be done in a way that is easier or more effective.
2.
In what three ways can machines make work easier?
Machines make work easier by changing the amount of force you exert, the distance over
which you exert the force, or the direction in which you exert the force.
3.
How does a screwdriver make work easier?
A screwdriver multiples force because you exert an input force on the handle over a greater
distance than the output force that is exerted on the tip of the screwdriver.
4.
What is the mechanical advantage of a machine?
A machine’s mechanical advantage is its output force divided by its input force.
5.
What is the mechanical advantage of a machine that changes only the direction of the applied
force?
If only direction changes, the mechanical advantage is 1.
6.
If a machine has an input force on 40N and an output force of 80N, what is its mechanical
advantage?
80 / 40 = MA 2
7.
What must you know in order to calculate a machine’s efficiency?
You must know its output work and its input work to calculate a machines efficiency.
8.
What is an ideal machine?
An ideal machine is a machine with 100% efficiency.
9.
How is a real machine like an ideal machine, and how is it different?
Both ideal and real machines perform work. However, because of friction, real machines
operate at less than 100% efficiency.
10.
The input work you do on a can opener is 12J. The output work the can opener does is 6J.
What is the efficiency of the can opener?
Eff = output/input x 100
6J/12J x 100 = 50%
11.
Suppose the efficiency of a manual pencil sharpener is 58%. If the output work needed to
sharpen a pencil is 4.8J, how much input work must you do to sharpen the pencil?
About 8.3J