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Chapter 19 Simple Machines
Force –
¾ a push or pull ‘exerted’ on a body
¾ may cause a change in the body. (These changes are called
force effects.)
Motion –
¾ The effect of force on a body. No motion is possible without
some sort of force.
¾ Once a body is in motion, it will continue to move at the same
speed and in the same direction until other forces act upon it.
¾ Consider space probes. Once a space probe escapes earth’s
gravitational field, it will continue to travel in the same direction
unless something happens:
o Firing of steering thrusters.
o Runs into another probe
o Flies too near another planet and gets pulled into its
gravitational field and crashes.
Friction –
A force acting in the opposite direction of a body’s velocity, due to a
‘rub’ between the surface of the body and some other material like:
¾ The road
¾ A table top
¾ Air etc.
¾ The more uneven the interacting (or ‘rubbing’) surfaces, the
greater the amount of friction. Problem: EVERYTHING (at least
on Earth) is uneven! This means everything is affected by
¾ Everything has to overcome the effects of friction in order to
start moving.
¾ Everything will slow down and come to a stop because of
¾ Consider hockey pucks. Why do hockey pucks travel faster and
farther on the ice than they do on concrete? Because, even
though the puck is uneven, the concrete is more uneven than
the ice.
Gravity –
¾ The attractive effect that all bodies have on all other bodies.
Whoa! Does that mean we all have our own gravity? Yes, indeedy! Check out the computer
¾ However, for these effects to be noticeable, at least one of the
bodies must be massive.
Forces on a body:
Question: You are peddling uphill like crazy. You are going so fast
that you stop pedaling. After a while you notice that the bike slows
and eventually stops. Why? (2 reasons)
Newtons (N) –
¾ A unit of force approximately equal to unit of force in the
meter-kilogram-second system, or international system (SI), of
units; equals that force which, if applied to an object having a
mass of 1 kg, would give that object an acceleration of 1 m
per second per second in a vacuum. Sounds complicated?
¾ Okay how about this:
1 N = 1kg ⋅ m
¾ If you know how much something weighs you can convert that
to 1.0 kg = 9.8 N (because of the force of gravity…but who
cares anyway, right?)
1.0 kg = 9.8 N
19.2 Work
In order for work to happen (scientifically) two things must be present
¾ A force acting on a body
¾ The body moving a distance.
Work = force X distance
10 N of force
Suppose a force of 10 N was ‘exerted’ on a block of wood moving it 5
Q: How much work was done?
W = 10 N X 5 m
W = 50 N-m
N-m is too cumbersome so we say 50 J (joules).
1J = 1 Nm
Instant practice:
In class p417 # 3, #4.
Activity 19C
19.3 Machines
machine – a device that helps people do work more easily.
Functions of machines
All machines do at least one of the following:
¾ transfer force from one place to another.
(e.g. pedals Æ chain Æ rear wheel)
¾ transform energy from one form to another.
(e.g. mechanical Æ generator Æ electrical)
¾ change direction of the force.
(e.g. flagpole pulley)
¾ multiply speed or distance.
(e.g. one pedal revolution Æ one wheel revolution)
¾ multiply force.
(e.g. car jack)…How does this work?
Mechanical Advantage –
¾ The amount the machine can multiply Force.
Mechanical advantage = ___Load force___
Effort force
MA = ____LF____
Example: Suppose in the above jack I could lift a 1000 N car by
applying 20 N of force. What would the mechanical advantage
Load force = 1,000 N
Effort force = 20 N
MA = ___LF__ = ___1,000 N___ = 50
20 N
Review 19.3 ALL
19.4 The Six Simple Machines
- all complex machines are made up of six simpler machines:
¾ Inclined plane - the gentler the slope, the greater the MA.
¾ Wedge - the longer and narrower the wedge, the greater the
¾ Screw – an extremely long inclined plane. The finer the
pitch, the greater he MA
¾ Lever – the greater the ratio between effort arm and load
arm, the greater the MA.
¾ Wheel and axle – the greater the ratio between the wheel
diameter and the axle diameter, the greater the MA.
¾ Pulley – the greater the number of supporting strands, the
greater the MA.
In class and at home: Activity 19E
Review 19.4
What are these? How would you get a mechanical advantage fro
each one?
19.5 Efficiency of Machines
Machines are NEVER 100% efficient because they always have to
overcome friction. Some machines have a higher degree of friction
so they are less efficient.
There are two ways to calculate efficiency:
Efficiency = ___work output___ X 100%
work input
Efficiency = ___load force X load distance___
effort force X effort distance
X 100%
Review 19.5
Chapter Review.