Download TT work and machines notes with answers 12

Chapter 8 Notes: Work and Machines
Work
 In science, work is done only if the object moves in the same direction
in which the force is exerted
Examples of work and no work:
Example
Work? Why?
You pull your books out yes
Force in same
of your book bag.
direction as
movement
Carrying your
backpack to class
no
Picture
Force is in a
different direction
of motion
gravity
movement
Rolling a bowling ball
yes
Force in same
direction as
movement

Carrying stuff = NO WORK!
o You don’t do any work when you carry an object because you
exert an upward force to hold the object so that it doesn’t fall
due to gravity pulling down on the object. The motion of you
walking is a forward direction. Since the force is up and down
and the direction of movement is forward you don’t do any work.

The amount of work you do depends on both the amount of force you
exert and the distance the object moves.

Formula (factors that determine work): Work (W) = Force (f) x
distance (d)

Units: joules (J)
Chapter 8 Notes: Work and Machines
How Machines Do Work:
 Machines make work easier; they do not make less work!
Word to Know
Input Force (also
called the effort
force)
Output Force
(also called the
resistance force)
*Mechanical
Advantage
Definition and *equation if there is one
The force YOU PUT IN to a machine
The force the MACHINE does
The number of times the machine multiplies the input
force.
Mechanical advantage (MA) = OUTPUT force
INPUT force
*Efficiency
How well the machine works
Efficiency is always a percent!!! (90%, 70%...)
The higher the percent, the better the machine!

Machines make work easier by
1. change how the force is applied
2. change the distance that the force is applied
3. change the direction that the force is applied
 The amount of work stays the same!!!!!!! You only change force
and distance!!!!!
If the machine allows you to…
Then the trade-off is…
Use less force to move an object
You have to apply that force over a
longer distance
Chapter 8 Notes: Work and Machines
Move an object a long distance while
you move a short distance
You have to use a bigger force
Changing Direction of Force
 The meaning: Your work is made easier! It is easier to pull down than
to push up on an object
 example: raising a flag on a flagpole, weight machine, sailboat
Efficiency:
 Machines have moving parts, therefore experience the force of friction on them

Therefore, the output is always LESS than the input!!
 The more friction in a machine, the LESS efficient the machine!

Real machines can never be 100% efficient because of friction!
Simple Machines:

A simple machine is an unpowered mechanical device

A machine that utilizes two or more simple machines is called a
compound machine.


There are 6 types of simple machines:
Simple
Machine
Inclined
Plane
Description
How it makes work easier
Examples
Flat, slanted surface
the longer the inclined
plane, the less force you
need to push or pull an
object
Ramp
Stairs
Hills
Chapter 8 Notes: Work and Machines
Longer the ramp = greater
mechanical advantage!
Wedge
device that is thick at one
end and tapers to a thin
edge at the other end
The longer and thinner a
wedge is, the greater its
mechanical advantage
2 inclined planes that are
put together and move
Less force over longer
distance
Knife
Ax
Zipper
Pencil
Door stop
Screw
Inclined plane wrapped
around a central cylinder,
forming a spiral.
The closer together the
threads of the screw are,
the greater the mechanical
advantage
Screws
Bolts
Jar lids
Light bulbs
Less force over longer
distance
Lever
Rigid bar that is free to
pivot, or rotate, about a
fixed point.
The fixed point that a lever
rotates around is called the
fulcrum
See below chart
See saws
Baseball bats
Scissors
Doors
wheelbarrows
Chapter 8 Notes: Work and Machines
Pulley
Consists of a rope or cable
that is wrapped around a
grooved wheel.
Fixed (does not move):
change direction
Movable: increase force
over longer distance
Block and tackle (has both
fixed and movable parts):
does both
The mechanical advantage
of a pulley is equal to the
number of rope that
supports the object.
Wheel
and Axle
Simple machine made of two
circular objects that are
fastened together and that
rotate about a common axis.
Bigger the wheel, the
greater the mechanical
advantage
Fixed: Flag
poles,
Sailboats,
Weight
machines
Movable:
ziplines,
Cameras used
at football
games that
go across the
field
Block and
tackle:
elevators,
construction
cranes
Screwdriver
Tires
Door knob
skateboards
Chapter 8 Notes: Work and Machines
Types of Levers:
Class/Examples Diagram
First class:
-scissors
-seesaws
-pliers
Second class:
-wheelbarrow
-doors
Third class:
-baseball bats
-hockey sticks
-fishing poles
-shovels
1-2-3
F-R-E
How it makes work
easier
changes the
direction of the
input force
Fulcrum is in the
middle
Increases your
force
Resistance (output)
is in the middle
Effort (input) is in
the middle