Transcript Chapter 13: Work and Machines

Chapter 13: Work and Machines
Aim: How does height affect an
object’s gravitational potential
energy?
Gravitational Potential Energy
• Is any time an object is raised above the
ground
• The greater an object’s height and weight the
more gravitational potential energy
• You can calculate it by multiplying the
object’s weight by its height
• Unit is joules when the height is meters and
weight is newtons
• The higher an object is, the more speed it will
pick up on its way down
Aim: How does energy change?
• Kinetic energy depends on both an objects
speed and mass
• Greater the mass, greater the speed, greater
the kinetic energy
• Potential energy changes to kinetic energy
• When a roller coaster is coming over a hill it
has potential energy
• When the roller coaster begins moving down
the hill its energy is changing to kinetic
energy
• It reaches its highest speed at the bottom of
the hill because the potential energy has
completely changed to kinetic energy
• Food stores potential energy stored in
chemicals
• Our bodies change the food energy into other
forms, such as kinetic energy, electrical
energy and heat energy
Aim: How do batteries produce
electricity?
Batteries
• Change chemical energy into electricity
• A chemical reaction inside the cell(battery)
causes a flow of electrons
• This flow is electricity
1. Dry-cell batteries
• Called a primary battery b/c it can only be
used once
• Dead dry-cell batteries should be thrown
away after being used
• This will prevent toxins from leaking out
2. Secondary Battery
• Can be recharged
• In order to recharge the battery needs to be
hooked up to an outside source of electricty
• During recharging chemicals are turned back into
their original form, enabling the battery to be used
again
• Lead-acid storage batteries found in cars
• Nickel-cadmium battery found in laptops,
camcorders, portable tools and space shuttles
Aim: What is work?
Work
• Is applying a force to an object to move it
through a distance
• W= distance x force
• Units is joules (d-meters, f-newtons)
• Just holding a box is not work
• Picking up a box you are doing work
• When work is done energy is added to it
• When throwing a ball your work is equal to
the kinetic energy gained by the ball
• Work can be changed into heat
• Friction between the table and the moving books
turns mechanical energy, used to do work, into
heat energy
Aim: How do we calculate work?
Aim: What is the law of conservation
of energy?
Law of Conservation of Energy
• Total amount of energy in the universe
remains constant
• Energy can change its form
• Energy can never be created nor destroyed
Aim: What are simple machines?
Simple Machines
• A machine is a device that makes it easier for
us to do work
• Simple machines are the most basic kinds
• Have none or a few moving parts
• Changes the direction of a force we apply to
something
• Can also increase the strength of an applied
force
• Ex: pulling a nail out with a hammer
• Effort force is the force that you apply to a
simple machine
• Force the machine applies to an object in
response to the effort force is called the
output force
• The force against which the machine acts is
called the resistance force
• 6 kinds of simple machines:
• Lever, pulley, wheel and axle, inclined plane,
screw and the wedge
Levers
• All levers have a rigid bar that rests on a
fulcrum, which is the pivot point
• Effort arm-side that applies an effort force
• Resistance arm-side that produces an output
force
• 3 classes of levers:
1. First-class levers
• Fulcrum lies between the effort force and
the output force
• Change the direction of the effort force
• Ex: hammer pulling a nail out
• Ex: seesaw
2. Second Class Lever
• Output force is between the effort force and
the fulcrum
• Do not change the direction of the effort
force
• Output force is greater than the effort force
b/c the effort arm is longer
• Exs: wheelbarrow, nut cracker, bottle
opener, paper cutter
3. Third Class Lever
• Effort force is between the fulcrum and
the output force
• Do not change direction of effort force
• Produce an output force that is less than
the effort force
• Distance of the force is multiplied
• Exs: fishing pole, tweezers, human
forearm and a broom
Aim: How do levers multiply force?
• Moving the position of the fulcrum
changes the amount of the output force
• First-class levers can produce an output
force that is greater than the effort force
when the fulcrum is placed closer to the
output force than the effort force
• Effort arm must be longer than the
resistance arm b/c input work must equal
output work
Aim: What is mechanical advantage?
Mechanical Advantage
• Is the number that tells you how much a
simple machine should multiply your effort
• Lever’s mechanical advantage is found by
dividing the distance the effort arm moves by
the distance the resistance arm moves
• When the output force is greater than the
input force, the mechanical advantage is
greater than one
• Using a broom has a mechanical advantage
less than one
• The other way to find the mechanical
advantage is dividing the length of the
effort arm by the length of the resistance
arm
Aim: How do pulleys work?
Pulleys
• A rope is threaded through a wheel and axle
• Pulling on the rope can lift an object which is
tied to the other end of the rope
• Can be either fixed or movable
1. Fixed pulley
• The wheel is attached to a fixed support
• It does not multiply the effort force
• It changes the direction of the effort force
2. Movable Pulley
• Pulley is attached to a movable object and
moves with it
• Multiplies the effort force by 2
• Has a mechanical advantage of 2
• Does not change the direction of the effort
force
Pulley System
• Is made up of fixed and movable pulleys
• The pulleys act together
• Has the same mechanical advantage as a
single movable pulley
• It does change the direction of the effort
force
Aim: What is the mechanical
advantage of a pulley?
• By increasing the number of wheels and
ropes, we can increase the mechanical
advantage
• While mechanical advantage increase, the
effort force required decreases
• 2 ways to find a pulley’s mechanical
advantage
1. By dividing the distance the effort rope
moves by the distance the object moves
2. By counting the number of strands of
rope that feel a downward pull from the
load
Aim: How does a wheel and axle work?
Wheel & Axle
• Fulcrum lies between the effort arm and the
resistance arm
• The wheel applies the effort force
• Small axle produces the output force
• The mechanical advantage can be found by
dividing the length of the effort arm by the
length of the resistance arm
• Effort arm is the radius of the wheel
• Resistance arm is the radius of the axle
• Has a large mechanical advantage
Aim: What is an inclined plane?
Inclined Plane
• Is a straight, slanted surface, like a ramp
• Have no moving parts
• Makes it easier to do work because they
multiply the effort force
• By using an inclined plane to move a heavy
object to a different height, you need less
force
• When using an inclined plane the force has to
be exerted over a longer distance
• Lifting heavy objects can hurt your back
• To find the mechanical advantage divide
the output force by input force
• The output force is the weight of the object
• The work put into a machine must equal
the work produced by the machine,
therefore the effort force must act over a
greater distance than the output force
Aim: What is the mechanical
advantage of a ramp?
Ramp’s Mechanical Advantage
• The steeper the inclined plane, the more force is
needed to move an object up the incline
• Both girls are doing the same amount of work, even
though the effort force is different
• You can find the MA of a ramp by dividing the
output force by the input force or by dividing the
length of the inclined plane by its height
• The longer the length of the ramp, the less the effort
Aim: How does a screw work?
Screws
• They multiply effort force
• They have a high mechanical advantage
• Are created by wrapping an inclined plane
around a central bar, which we called threads
• The head of the screw is the part we turn
• The distance from thread to thread is called
the pitch
• To find the mechanical advantage we divide
the distance around the head by the pitch
• Screws with a larger pitch have a lower
mechanical advantage
• Screws with large heads and very close pitch
have a very high mechanical advantage
• Friction keeps screws in place
• Without friction, the screw would unscrew and
the object would fall
Aim: What is a wedge?
Wedges
• A wedge is a single inclined plane or 2
inclined planes joined back to back
• A wedge must be moved by an effort force
• Wedges that are thin have a high mechanical
advantage
• Ex: knife blades, ax heads, and chisels
• These work best when they are sharpened
• A wedge changes the effort force and
increases it’s strength
• The effort force is downward
• The output force is horizontal
Aim: What is a compound machine?
Compound machines
• A compound machine is a combination of 2 or
more simple machines
• Ex: scissors (2 first-class levers and wedges)
• Ex: screwdriver and screw (screwdriver is a
wheel and axle, screw is an inclined plane)
Aim: What is efficiency?
Efficiency
• No machine runs without friction
• When there is friction some of the input
work is changed into heat energy, therefore
not all of the input work becomes output
work
• To find efficiency write a ratio: the work
done by a machine over the work put into a
machine and multiply by 100 to get a
percentage
• The closer the efficiency is to 100% , the
less energy the machine wastes
• The more friction there is, the lower the
efficiency of a machine
Aim: How do machines help us?
• Simple machines make tasks easier
• Machines, whether compound or simple,
allow us to do things that we could not do
with our bare hands