Physics: Machines

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Physics: Machines
Mr. Fox’s 8th Grade Science
(I’ve got nothing witty for here.)
What is a machine?
 A device that is used to make work
easier.
 It
can multiply force
 Change direction of force
 Multiply distance
Input output
 Input Force – the force you exert on the
handle. What you put into it.
 Output Force - the force you get out of
the machine. What the machine puts
out.
Mechanical Advantage
 A machine’s mechanical advantage is
the number of times a force exerted on
a machine is multiplied by the machine.
Mech Adv: Multiplying Force
 Always greater than 1 for machines that
multiply force!
 Example: You are lifting an object with
a lever. Your input force is 35N and the
output force is 70N. What is the
Mechanical Advantage of the lever?
Mech Adv: Multiplying Distance
 Always less than 1. Because you are
getting out less force, but more
distance.
 Example: You are putting in 20N of
force, but the machine is only giving
back 5N of force. What is the
Mechanical Advantage?
Mech Adv: Changing direction
 Always equals one. You are getting
back what you put in, just in a different
direction.
 You hoist the main sail on the black
pearl using 150N of force. The pulley is
putting out 150N of force, what’s the
mechanical advantage?
Efficiency of Machines
 You compare the output work to the
input work.
Ideal Machines
 If there were no friction we would have
ideal mechanical advantage.
 Because there is friction, efficiency
gives us the actual mechanical
advantage.
 The closer to 100% efficiency, the better
the machine is. Nothing is lost to
friction.
Homework
 Practice Worksheet
 Also those that were absent can make
up the running lab today and TODAY
ONLY!
Simple Machines
 There are 6 kinds of simple machines.
 Each has their own use.
Inclined Plane
 A flat slanted surface.
 It’s meant to multiply your distance.
 Ideal Mech Adv = length of incline ÷
height of incline.
 Ex: ramps
Wedge
 A device that is thick at one end and
tapers to a thin edge at the other end.
 Two inclined planes put together.
 Meant to multiply force
 Ex: Splitting maul, teeth, zipper.
Screws
 An inclined plane that is wrapped
around a cylinder.
 Depends on how far apart the threads
are.
 Close
threads multiply distance.
 Far apart threads multiply force.
Levers
 Lever – rigid bar that is free to pivot, or
rotate, about a fixed point.
 Fulcrum – the fixed point that a lever
pivots
 There are 3 classes of lever
Classes of Levers
First Class Lever
 Fulcrum is in between the input and
output forces.
 Fulcrum is closer to the output force,
you multiply force
 Fulcrum is closer to the input force, you
multiply distance.
First Class Lever
Second Class Lever
 Output force is between the fulcrum and
then input force.
 Always multiply force!
Third Class Lever
 Input force is between the fulcrum and
output force.
 Always multiply the distance!
Mech Adv.
 Mech Adv lever = distance from fulcrum
to input force ÷ distance from fulcrum to
output force
Wheel and Axel
 A simple machine made of two different
sized circular objects.
 If you turn the wheel, you multiply force
 If you turn the axel, you multiply
distance.
 Mech adv = radius of wheel / radius of
axel.
Pulley
 Is a grooved wheel with a rope (chain,
steel cable) wrapped around it.
 Fixed Pulley – change direction
I.M.A=1
 Movable Pulley – Multiply Force
I.M.A=2
Pulley System
 A series of pulleys put together to
multiply force and change direction.
 The I.M.A of a pulley system is equal to
the number of sections of the rope that
support the object.
Compound Machines
 Machines that use 2 or more simple
machines.
– are wheel and axels that are all
connected in some way, either directly next
to each other, or by a chain.
 Gears
Homework
 Page 128 1 - 4
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