Download Work, Energy, and Machines

Work, Energy, and Machines
What is work?
W=F*D
Force = 200 N
Distance = 15 m
work
W=F*D
W = (200 N)(15 m)
Scientific work = force used to move an object;
familiar work = something you do, doesn’t require
motion
W = 3,000 N * m
N * m  joule
3,000 J
lifting your backpack
that’s full of books
because you’re so
excited to do your
homework
Backpack moves in the direction
of the force
Work occurs when the blade of the wind turbine
turn. The force comes from the wind and occurs
when the blades accelerate. The distance is the
amount that the blades move
Power = energy / time
P = E/t
P = 156 J / 6s
P = 26 W
The energy it uses and the
time it uses that energy in
CFL uses less power = less money in
electric bill.
work
distance
Same direction
energy
watts
work
energy
power
No, you haven’t done any
work  obj has not moved.
Force and distance
No, box has not moved a
distance
Power can be calculated
from energy and time
Moved a distance of 1.5 m
W= f*d
W = 150 N * 1.5 m
W = 225 J
P = e/t
P =2400 J/ 2 s
P = 1200 W
Moving a desk; work
transfers from my arm’s
energy into the desk,
transfer energy to the legs
to move desk.
On the Move
 Energy: ability to cause change
 Kinetic Energy  the energy of MOTION
 Every moving object has kinetic energy (i.e.
fast movement = more kinetic energy; slow =
less)
Kinetic Energy Calculated
 The kinetic energy of an object equals ONE HALF the
object’s mass (m) times the square of its velocity (v)
 KE = ½ mv2
 When mass is expressed in kilograms (kg) and speed is
expressed as meters per second (m/s) KINETIC ENERGY is
expressed in JOULES (J)
The foal has a mass of 100 kg and is moving at 8 m/s along
the beach. What is the kinetic energy (KE) of the foal?
 What do you know?  m = 100 kg; v = 8 m/s
 What are we looking for?  Kinetic energy
 What is the formula?  KE = ½ mv2
 KE = ½ (100 kg) * (8 m/s) 2
 KE = 3,200 J
100 m2/s2
5,000 J
100 m2/s2
40,000 J
225 m2/s2
90,000 J
Potential Energy
 An object that is NOT moving CAN still have energy
 Potential energy  the energy an object has in regards to its
position, condition, or chemical composition
 The ability to do WORK
 The use of force to move an object at a distance
Types of Potential Energy
 Elastic potential energy: an object that has the potential to be stretched or compressed
 Rubber bands | springs
 Gravitational potential energy: an object’s position
ABOVE THE GROUND
 An object held above ground has POTENTIAL to fall
 Higher object = higher potential energy
 Mechanical potential energy: potential energy
that depends on an object’s position
 Chemical potential energy: result of chemical
bonds
 Potential energy that can be released during chemical reactions
Chemical PE
Gravitational PE
Elastic PE
Calculating Gravitational PE
 The gravitational potential energy of an object is equal to its mass (m) times its height
above the ground (h) times the acceleration due to Earth’s gravity (g)
 GPE = mgh
 An objects mass is expressed in KILOGRAMS
 An objects height is expressed in meters
 The acceleration due to gravity is expressed as 9.8 m/s2
 Potential energy expressed in JOULES
The cat has a mass of 4 kg and is 1.5 m above the ground.
What is the gravitational potential energy of the cat?
 What do you know?  m = 4 kg; h = 1.5 m;
acceleration due to gravity = 9.8 m/s2
 What are we looking for?  Gravitational
Potential Energy
 What is the formula?  GPE = mgh
 PE = (4 kg) (1.5 m) (9.8 m/s2)
 PE = 58.8 J
3.92 J
5.88 J
58.8 J
Mechanical Energy
 Mechanical energy: energy possessed by
an object due to its motion and position
 Sum of kinetic energy and potential energy
 ME = KE + PE
 ME = (½ mv2) + mgh
0
18
18
18
0
18
Energy an obj has due
to motion
Energy an obj has due to its
position/chem comp.
Yes; if an obj is not moving it
has zero kinetic energy. The
sum of KE and PE would be
the same as the GPE
Sum of an obj’s PE and
KE
Greater the mass of an
obj. at a given speed,
the greater the PE
Object’s mass and the
object’s height
KE due to mass and
speed & GPE due to its
mass and height above
the ground
Helicopter travels up at a
steady rate for 5 sec and
then downwards at same
rate for 5 sec.
40 J
Machines
 A machine is any device that helps people do
WORK by changing the way work is done
 Machines that make up other machines 
simple machines
 Levers
 Wheel and axles
 Pulleys
 Inclined planes
 Wedges
 Screws
Simple Machines
 Work is done when a force is applied to an
object and MAKES IT MOVE
 Work done to a machine = work input
 Fore you apply to a machine through a distance =
input force
 Work done by a machine on an object = work output
 Force a machine exerts on an object = output force
What do simple machines do?
Work = f * d
Apply less force  longer distance =
work the same
Some machines increase the amount
of force needed (short distance)
Some machines decrease the
magnitude or size of the force
needed to move an object (longer
distance)
Mechanical Advantage
 A machine’s mechanical advantage is
the number of times the machine
multiples the input force
 Input force vs. output force
 Mechanical Advantage (MA)  divide
output force by input force
 MA = input/output
Mechanical Advantage (cont.)
 Mechanical advantage > 1 
easier task
 Output force greater than the input force
 Mechanical advantage = 1 
change direction of input force
 Mechanical advantage < 1 
greater force over shorter
distance
Output / input
Output / input
5N/5N
2 N / 6N
.334
1
Output / input
Output / input
5N/5N
2 N / 6N
1
.334
Mechanical Efficiency
 Mechanical efficiency: comparison of a machine’s
work output with the work input
 Mechanical efficiency (ME) = work output divided by
work input
 EXPRESSED AS A PERCENTAGE
 ME = work output/work input * 100%
 Ideally  work a machine does is the same as the
work you put into it
 Reality  work input is greater than the work output
because some work is done to OVERCOME
FRICTION
ME = work output/work input * 100%
ME = 475 J / 500 J * 100%
ME = .95 * 100%
ME = 95%
Gaining Leverage
 A lever is a simple machine that has a bar that pivots at a fixed point
 Fulcrum: fixed point
 Levers are used to apply a force to move an object
 Force of object = LOAD
 Ideal Mechanical Advantage  distance from input force to a fulcrum (dinput)
divided by the distance from the output force to a fulcrum (doutput).
 The mechanical advantage of a simple machine that does not take friction into
account
 Mechanical advantage is 100% efficient
Three classes of levers
There are three classes of levers that differ based on the
positions of the fulcrum, the load, and the input force
First-class
Second-class
Third-class
First-Class Lever
 First-class lever: the fulcrum is
between the input force and the
load
 Scissors, pliers, crowbar, hammer
Second-Class Lever
 Second-Class Lever: the load is between
the fulcrum and the input force
 Wheelbarrow, nut cracker, bottle opener,
paper cutter
Third-Class Lever
 Third-Class Lever: the input force is between
the fulcrum and the load
 Tweezers, tongs, fishing rod
Third-Class Lever
First-Class Lever
Second-Class Lever
Wheel and Axle
 Wheel and axle: machine that is made of a wheel
connected to a smaller cylindrical object  THE
AXLE
 The ideal mechanical advantage of a wheel and
axle equals the radius corresponding to the input
force (radiusinput) divided by the radius
corresponding to the output force (radiusoutput)
 Ideal Mechanical Advantage = radiusinput /radiusoutput
Wheel vs. Axle
 The radius of the wheel is ALWAYS
larger than the radius of the axle
 Mechanical advantage > 1 
input force applied to the wheel
 Mechanical advantage < 1 
input force applied to the axle
1 meter
20 meters
Mechanical Advantage = 1m/ 20m
Mechanical advantage = .05
Mechanical advantage  input
force applied to the axle
Pulley
 Pulley: simple machine that has a grooved wheel that holds a rope or cable
 A load is attached to one end of the rope, and an input force is applied to the other end
 Three types  fixed, movable, and block and tackle pulley
Fixed Pulley
 A fixed pulley is attached to something
that does not move
 Allows you to pull down on the rope to lift
the load up
Movable pulley
 The wheel of a moveable pulley is attached to the object
being moved
 One end of the rope is fixed
 The other end of the rope can pull the wheel and load to
move along the rope
Block and Tackle Pulley
 A block and tackle pulley is a pulley system made by combining
a fixed pulley and a movable pulley
Inclined Planes
 Inclined Plane = simple machine that is a straight,
slanted surface
 A smaller input is needed to move an object using an
inclined plane
 Force applied a longer distance
 SAME AMOUNT OF WORK DONE  easier task than
lifting
 Ideal Mechanical Advantage: dividing the length of
the incline by the height that the load is lifted
 Ideal Mechanical Advantage = length / height
Wedges
 A wedge is a pair of inclined planes that MOVE
 One thick end; one thin end
 Used to cut and split objects
 Output force of the wedge > input force  force
applied over a shorter distance
 Longer & thinner the wedge is = greater its ideal
mechanical advantage
Screw
 A screw is an inclined plane that is wrapped in a spiral
cylinder
 When a screw is turned; a small force is applied through
the distance along the inclined plane of the screw
 The screw applied a large force through the short
distance it is pushed
 The longer the spiral on the screw & closer the threads
are  greater the mechanical advantage
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3rd-class levers are useful when the output
force is to be applied over a greater
distance  baseball bats & rakes
Mechanical efficiency = (work
output/work input) *100% 
42/50*100 = 84%
Machines can change the way
work is done by changing the
size and distance of the force
used. They can also change the
direction of a force.
A first-class lever; the fulcrum is
between the input and the
load
MA = input force/output force =
245 N/245N = 1
Ideal mechanical advantage
= radius of input / radius of
output  radius of axle / radius
of wheel
Ideal mechanical advantage =
length/ heigh  120 m / 6 m = 6
MA = distance input
force/distance output foce =
1.5 m/ 2m = .75