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## Chapter 8 Work and Machines

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### Chapter 8 Work and Machines Section 1 Work and Power Mechanical Efficiency of Compound Machines Generally, the more moving parts a machine has, the lower its ... – PowerPoint PPT presentation

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Title: Chapter 8 Work and Machines

1
Chapter 8 Work and Machines
• Section 1
• Work and Power

2
To Work or Not to Work
• If you had a choice what would you prefer to do,
work or not work?
• Give some examples of work.

3
The Scientific Meaning of Work
• Work occurs when a force causes an object to move
in the direction of the force.
• The object must move for work to occur!

4
Force and Motion in the Same Direction
• Youll know work is being done on an object if
two things occur
• 1. the object moves as the force is applied
• 2. the direction of the objects motion is the
same as the direction of the force applied.

5
Force and Motion in the Same Direction
6
How do we calculate work?
• Work force x distance
• Force expressed in Newtons
• Distance is expressed in meters
• So the expression for work is the Newton meter
• or simply the joule
• Work Newtons x meters Joules

7
Practice
• A man applies a force of 500N to push a truck
100m down the street. How much work does he do?
• In which situation do you do more work?
• a. You lift a 75 N bowling ball 2m off the
floor.
• b. You lift two 50 N bowling balls 1m off
the floor

8
Power-How Fast Work Is Done
• Power is the rate at which work is done
• To calculate power (P) divide the amount of work
done (W) by the time (t) it takes to do that
work or
• P W/ t
• Unit for work is joule
• Unit for time is second
• So the unit for power is joules / second or J/s
• Or more commonly called a watt (W)

9
Chapter 8 Work and Machines
• Section 2
• What is a Machine?

10
Machines - Making Work Easier
• A machine is something that makes work easier by
changing the size or direction of a force

11
Machines - Making Work Easier
• Work input the work you do on a machine
• Work output the work done by the machine
• Output force the force that the machine applies
that opposes the forces of you and the machine
• Machines do not increase the amount of work that
is done because work output can never be more
than work input

12
Machines - Making Work Easier
• Machines make work easier because they change the
size or direction of the input force
• If force decreases then distance must increase
and vice versa

13
(No Transcript)
14
• A machines mechanical advantage tells how many
times a machine multiplies forceit compares the
input force to the output force
• Mechanical Advantage (MA) output force
• input force

15
The larger the mechanical advantage, the easier
16
1. The output force is the same as the input force
if the machine changes direction of a force and
the MA is 1.
2. The input force is greater than the output force
allows lower force over a longer distance and has
a MA less than 1.
3. The output force is greater than the input force
gives a MA of greater than 1.

17
Mechanical Efficiency
While output cannot be greater than input the
input is always greater than the output because
some of the work done by the machine is to
overcome gravity. The less work a machine has to
do to overcome friction the more efficient it
is. Mechanical Efficiency work output x
100 work input
18
Mechanical Efficiency
Mechanical Efficiency work output x
100 work input Mechanical efficiency tells
what percentage of the work input gets converted
into work output. So reducing friction in a
machine makes it more efficient.
19
Chapter 8 Work and Machines
• Section 3
• Types Of Machines

20
Types of Machines Levers
• Lever a simple machine consisting of a bar that
pivots at a fixed point called a fulcrum
• There are three classes of levers
• First class levers
• Second class levers
• Third class levers

21
Types of Machines Levers
• First class levers always change the direction
of the input forces

22
Types of Machines Levers
• Second class levers do not change the direction
of the input forces output force is greater than
the input force

23
Types of Machines Levers
• Third class levers do not change the direction
of the input forces and do not increase the
input force

24
Types of Machines Inclined Planes
• An inclined plane is a simple machine that is a
straight, slanted surface
• A ramp is an example of an incline plane
• MA of an inclined plane

25
Types of Machines Wedges
• A wedge is a double inclined plane that moves
• A wedge applies an output force that is greater
than the input force
• MA of wedges
• The longer and
• thinner the
• wedge, the better.

26
Types of Machines Screws
• A screw is an inclined plane that is wrapped in a
spiral
• MA of screws same advantage as a long inclined
plane

27
Types of Machines Wheel and Axle
• A wheel and axle is a simple machine consisting
of two circular objects of different sizes

28
Types of Machines Wheel and Axle
• The MA of this machine can be determined by
the axle

29
Types of Machines Pulleys
• A pulley is a simple machine consisting of a
grooved wheel that holds a rope or a cable. A
load is attached to one end of the rope and an
input force is applied to the other end.
• There are two types of pulleys
• 1) fixed
• 2) movable

30
Types of Machines Pulleys
31
Types of Machines Pulleys
32
Compound Machines
• Compound machines are made of two or more simple
machines.
• The block and tackle we just talked about is a
compound machine

33
Compound Machines
34
Mechanical Efficiency of Compound Machines
Generally, the more moving parts a machine has,
the lower its mechanical efficiency What would
happen if the mechanical efficiency of a car was
too high?