Transcript Simple Machines - 8th Grade Physical Science

Warm Up: 1/14/13

Give an example of a machine
you use on a daily basis. Then
say how you thinks it helps to
make work easier.
MACHINES
Chapter 8, Sections 2
What is a machine?

a device that makes work easier by changing the
size or direction of a force.
 Imagine
trying to change a tire without a jack or tire
iron
 Everyday machines:
 Wheel
chair
 Chopsticks
 Scissors
Work and Machines

Work Input: the work that you do on a machine


Work Output: the work done by the machine


You apply an input force through a distance on a machine.
The machine applies an output force through a distance
Example: We use a can opener to help open a jar.
What is the input? What is the output?
Do machines for work for us??

No, Work output is ALWAYS less than work input.
 Machines


do not make work less, just EASIER.
How?
Some machines allow force to be applied over a
greater distance, so less force is needed to do the
work. But you have to go a longer distance so you
still have to do more work than the machine!
 EX:
when loading something into a truck. If you push it
up a ramp it will be easier. You need less force but
must go a farther distance.
Why is work output always less than
work input?


All machines have moving parts that create friction
as they move.
SO: some work input always goes into overcoming
friction in the machine therefore reducing the work
output.
How do machines make work easier?

When using a machine 3 things could happen…
 1.
The machine could increase your force by decreasing
your distance. (ex. Ramp)
 2. The machine could decrease your force but increase
your distance. (ex. Baseball Bat or Feather Duster)
 3. The machine could change the direction of your input
force. (ex. Flagpole)
Mechanical Advantage

The number of times the machine multiplies the
force. It compares the output and input force.
 MA
 MA
= output force / input force
> 1 means the machine makes lifting or moving a
heavy object easier.
 MA < 1 means the machine can still help, it just reduces
the output force.
Mechanical Efficiency

A comparison of a machines work output to its work
input.
 ***the
less work a machine must do to overcome
friction, the more efficient it is.
 ME = (work output/work input) x 100
 Your
 Ideal
answer will be a precentage
efficiency would be 100%
 However
due to friction this cannot be done.
 What can be done to REDUCE friction?

Use lubricants such as oil or grease
Work Input = Fi x Di
Work Ouput = Fo x Do
Effort Force
= 800 N
2000 N
1 meter
Calculating Advantage and Efficiency

Efficiency = (Work Output/Work Input) x 100
E
= (2000/2400) x 100
 E = 0.83 x 100
 E = 83.3 %

Mechanical Advantage = Force Output/Force Input
 MA
= 2000N/800 N
 MA = 2.5
Warm Up: 1/16/13

David picks up his 45 N dog and raises him 2.5
meters and then carries him 10 m down the stairs.
How much work did David do on Rocky?
Machines Help us in 3 Ways
The machine may:

Multiply the input force by increasing the input
distance
1.
1.
Multiply the input distance by increasing the input
force
2.
1.
3.
Meaning you use less force but more distance – the machine
uses more force and less distance
Meaning you use more force and less distance – the machine
uses less force and more distance
Change the direction of the input force
Types of Simple Machines
Chapter 8, Section 3
Lever
☺Pulley
☺Wheel and Axle
☺
Inclined Plane
☺Wedge
☺
The Pulley


A machine that consists of a grooved wheel that
holds a rope or cable.
A load is attached to one end of the rope, an input
force applied to the other end.
Fixed Pulley


This pulley is attached to something that does not
move.
It changes the direction of the input force.
 Pulling
down on the rope pulls the load up.
Movable Pulley



This pulley is attached to the object being moved.
It does NOT change the direction of the force
It increases force by increasing the distance the
work is done over.
Block and Tackle Pulley


When a fixed pulley and a moveable pulley are
used together.
The mechanical advantage depends on the number
of rope segments.
Wheel and Axle




Consists of two circular objects of different sizes
MA = radius of wheel / radius of axle
MA >1 means the radius of the wheel is larger than
the axle.
Multiplies input force!
Inclined Plane



straight, slanted surface that helps us to do work.
Multiplies input force by spreading the work over a
greater distance!
MA = length of ramp / height of ramp
 The
greater the ratio the greater the mechanical
advantage
The Wedge



A type of inclined plane.
Consists of a pair of inclined planes that move.
Applies an output force that is greater than the
input force, but the input force is over a greater
distance.
Screw


An inclined plane wrapped around a cylinder.
Applies a small input force over a long input
distance.
 This
increases output force but decreases distance.
The Lever


A simple machine that consists of a bar that pivots
at a FIXED point, called a fulcrum.
There are 3 types…
st
1



Class Lever
The fulcrum is between the input force (you) and the
load (the work to be done)
This lever changes the direction of the input force
It may also increase force OR distance depending
on the location of the fulcrum.
Effort
Load
Fulcrum
nd
2




Class Lever
The load is between the fulcrum and the input force.
This lever does NOT change the direction of the
input force.
Allows you to apply less force over a longer
distance.
Multiplies input force!
Effort
Load
Fulcrum
rd
3




Class Lever
The input force is between the fulcrum and the load.
Does NOT change the direction of the input force or
increase the size of the input force.
Allows for a small input distance but a large output
distance.
Reduces input force in exchange for a greater
output distance.
Load
Effort
Fulcrum
Compound Machine

A machine made up of two or more simple
machines.
Warm Up: 1/22/13

You apply a 90 N force on a lever which moves the
lever 1.5 m. If the bookshelf weighs 175 N and it
moves .75 m, what is the efficiency of the lever?