Transcript III. Using Machines

Chapter 15 – Work, Power & Simple Machines
Essential Questions:
I. What is Work? (In Physics Terms!)
II. What is Power? (In Physics Terms!)
III. How do machines make work
easier and how efficient are they?
IV. What are the 5 types of simple
machines?
V. What are compound machines?
15-1 What is Work?
 Work
 Def. – Work is done when a force
acts on an object along the
parallel direction the object
moves
 In order for work to be done, a
force must be exerted over a
distance.
 Ex – you can push on a wall for
hours, you’ll be real tired, but
you haven’t done any work – in
the scientific sense, anyway…
15-1 Work
 Work
 The amount of work done in moving an
object is equal to the force applied to
the object along the direction the
object moves times the distance
through which the object moves
Work  Force x Distance
 Units
 Force is measured in Newtons, Distance
is measured in meters. So, the unit is
Newton X meters. A Newton•meter is
known as a Joule (J)
15-1 Work
 A 700 N person climbs a 50 m cliff. How
much work did she perform?
GIVEN:
WORK:
W=F*d
F = 700 N
d = 50 m
W=F*d
W = (700 N) (50 m)
W = 35,000 J
15-1 Work
 An object weighing 200 N is lifted 0.5 m.
How much work was required?
GIVEN:
WORK:
W=F*d
F = 200 N
d = 0.5 m
W=F*d
W = (200 N) (0.5 m)
W = 100 J
15-1 Work
 A dog does 50 N-m (Joules) of work
dragging a 0.05 N bone. How far did the
bone move?
GIVEN:
W=F*d
W = 50 J
F = 0.05 N
WORK:
W=F*d
d=W
F
d = (50 J)
(0.05 N)
d = 1,000 m
15-1 Work
 Mrs. O’Gorman’s superhuman strength
allows her to lift a pickup truck 2.0 m above
the ground. How much force was required if
25.0 Joules (J) of work was done?
GIVEN:
W=F*d
W = 25.0 J
d = 2.0 m
WORK:
W=F*d
F=W
d
F = 25.0 J
2.0 m
F = 12.5 N
15-2 Power
 Power




Def: The rate at which work is done, or
the amount of work per unit time.
Power tells you how fast work is being
done – so it is a rate – similar to the way
speed, velocity and acceleration are
rates. Power is work per unit time.
Any measurement per unit time is a
rate!!
Formula:
Powe r W ork
Tim e
15-2 Power
 Power
 rate at which work is done
 measured in watts (W)
W
P
t
P: power (W)
W: work (J)
t: time (s)
15-2 Power
Formula:
Since work’s formula is force X Distance,
the formula for Power may ALSO be
written as:
Powe r Force x Distance
Time
15-2 Power
Units
Work is measured in
Joules (J), So, the unit for
Power is a Joule per
second (J/s).
The short way to write a
J/s is a Watt (W).
15-2 Power
When do we use Watts in our
Daily Lives?
They are used to express
electrical power.
Electric appliances and
lightbulbs are rated in Watts.
Ex: A 100 Watt light bulb does
twice the work in one second as
a 50 Watt lightbulb.
15-2 Power
 A small motor does 4000 J of work in
20 sec. What the power of the motor
in Watts?
GIVEN:
W = 4000 J
T = 20 sec
P=?
WORK:
P=W÷t
P = 4000 J ÷ 20 s
P = 200 J ÷ s
So P = 200 W
15-2 Power
 An engine moves a remote control car
by performing 120,000 J of work. The
power rating of the car is 2400 W.
How long does it take to move the car?
GIVEN:
WORK:
P = 2400 W
t=W÷P
W = 120,000 J t = 120,000 J ÷ 2400 W
T=?
t = 50 sec
15-2 Power
 A figure skater lift his partner who
weighs 450 N, 1.5 m in 3.0 sec. How
much power is required?
GIVEN:
P=?
F = 450 N
d = 1.5 m
t = 3.0 sec
WORK:
Fxd
P t
P=Fxd
t
P = 450 N x 1.5 m
3.0 sec
P = 625 J (N•m)
P = 225 W
3.0 sec
15-2 Power
 A sumo wrestler lifts his competitor, who
weighs 300 N, 2.0 m using 300 Watts of
power. How long did it take him to
accomplish this show of strength?
W
GIVEN:
WORK:
F = 300 N
d = 2.0 m
P = 300 W
t=?
P=W÷t
P t
W=Fxd
W = (300 N)(2.0 m) = 600 J
t = 600 J ÷ 300 W
t = 2.0 s
15-3 Machines
Machine – def. – Any device that
changes the size of a force, or
its direction, is called a machine.
Machines can be anything from
a pair of tweezers to a bus.
15-3 Machines
There are always 2 types of
work involved when using a
machine
 Work Input - The work that
goes into it.
 Work Output - The work that
comes out of it.
The work output can NEVER be
greater than the work input!!!
15-3 Machines
So, if machines do not increase
the work we put into them, how
do they help us?
Machines make work easier
because they change either the
size or the direction of the force
put into the machine.
15-3 Machines
Let’s analyze this…
Machines can not increase the
amount of work, so work either
stays the same or decreases.
The formula for work is:
 Work = force x distance
15-3 Machines
Again, the formula for work is:
 Work = force x distance
So, mathematically speaking, to
end up with the same or less
work:
 If the machine increases the
force then the distance must
decrease.
 If the machine increases the
distance, then the force must
decrease.
15-3 Machines
Why is it that machines can’t have
more work output than input?
Where does all the work disappear
to?
A machine loses some of the
input work to the force of friction
that is created when the machine
is used.
Part of the input work is used to
overcome the force of friction.
There is no machine that people
have made that is 100% efficient
15-3 Machines
If machines make our work
easier, how much easier do they
make it?
The ratio of how much work
output there is to the amount of
work input is called a machine’s
efficiency.
Efficiency is usually expressed
as a percentage (%).
15-3 Machines
 Efficiency
 measure of how completely
work input is converted to work
output
Wout
Efficie ncy
 100%
Win
 It
is always less than 100% due
to the opposing force of friction.
Wout
E
 100%
Win
15-3 Machines
 A worker exerts a force of 500 N to push
a 1500 N sofa 4.0 m along a ramp that is
1.0 m high. What is the ramp’s
efficiency?
GIVEN:
WORK:
Fi = 500 N
Win = (500N)(4.0m) = 2000 J
di = 4.0 m
Wout = (1500N)(1.0m) = 1500 J
Fo = 1500 N
E = 1500 J × 100
do = 1.0 m
2000 J
1.0m
1500N
E = 75%
15-3 Machines
Mechanical Advantage is
another way of expressing how
efficient a machine is.
Mechanical advantage is the
ratio of resistance force to the
effort force.
resistance force
Mechanical Advantage 
effort force
Fres
MA 
Feffort
15-3 Machines
 A worker exerts a force of 500 N to push
a 1500 N sofa 4.0 m along a ramp that is
1.0 m high. What is the mechanical
advantage of the ramp?
GIVEN:
WORK:
Fe = 500 N MA = F resistance
Fr = 1500 N
F effort
MA = 1500N
500 N
MA = 3
1.0m
1500N
15-4 Simple & Compound Machines
 Simple Machines
 There are six types of
simple machines.
They are the:
 1 - Inclined plane
 2 - Wedge
 3 - Screw
 4 - Lever
 5 - Pulley
 6 - Wheel and axle
15-4 Simple & Compound Machines
 1 - Inclined Plane
 Def - A slanted
surface used to
raise an object.
 The force
needed to lift
the object
decreases
because the
distance
through which
the object
moves
increases.
15-4 Simple & Compound Machines
 2 - Wedge Inclined Plane
Type #1
 Def – an
inclined plane
that moves in
order to push
things apart.
 Tines of a
fork, axe,
knife.
15-4 Simple & Compound Machines
 3 - Screw - Inclined Plane Type #2  Def - An inclined plane wrapped
around a central bar or cylinder, to
form a spiral.
 Ex – screw –duh!!!
15-4 Simple & Compound Machines
 4 - Lever
 Def - A rigid bar
that is free to
pivot, or move
around a fixed
point called a
fulcrum.
 Ex – see saw
 There are three
main types
(classes) of
levers.
15-4 Simple & Compound Machines
 3 classes of levers:
 First-class levers have
the fulcrum placed
between the load and
the effort, as in the
seesaw, crowbar, and
balance scale.
 Ex - a see-saw or
scissors
15-4 Simple & Compound Machines
 3 classes of
levers:
 Second-class
levers have
the load
between the
effort and the
fulcrum.
 Ex - a
wheel
barrow
15-4 Simple & Compound Machines
 3 classes of levers:
 Third-class levers have the effort
placed between the load and the
fulcrum. The effort always travels
a shorter distance and must be
greater than the load.
 Ex - a hammer or tweezer
15-4 Simple & Compound Machines
 5 - Pulley
 Def - A rope, chain
or belt wrapped
around a grooved
wheel.
 It can change the
direction of force
or the amount of
force needed to
move an object.
15-4 Simple & Compound Machines
 To calculate how
much
mechanical
advantage a
pulley system
creates… Count
the number of
ropes that are
attached to the
MOVEABLE
pulley – that # is
your mechanical
advantage!!!
15-4 Simple & Compound Machines
 6 - Wheel &
Axle
 Def - Made of
2 circular
objects of
different sizes
attached
together to
rotate around
the same
axis.
15-4 Simple & Compound Machines
 Compound Machine
 Def - A combination of 2 or
more simple machines