Transcript Work

Ch 14
Work, Power,
and Machines
Ch 14 WORK AND POWER
• Work – transfer of
energy
• through motion
• a. Force must be
exerted through a
distance
• b. Amount of
Work depends
on:
–1. Amount of
applied Force
–2. Distance
over which
the force is
applied
Work
F
d
Work
F
d
• c. W=Fd
(Force multiplied by distance)
• d. Unit is the Joule (1J = 1Nm)
• e. In order for work to be done:
–1. Something has to move
–2. Motion must be in the
direction of the force
• How much work is done when a car is
pushed with 600 N of force for a distance
of 50m?
• How far will a ball roll if 17J of work was
done to push it with 2 N of force?
Power – rate of
doing work
a. To increase
Power:
–1. increase
work done
– 2. do work in
less time
b. P=W/t
• (Work divided by time)
Work
P
c. SI unit is Watt
(1W=1J/s)
d. common unit is horsepower
(1hp=746W)
t
• How much power is needed to do 40
Joules of work in 5 seconds?
• How long will it take to do generate 70
Watts of power if I do 4900 Joules of
work?
• How much power will I need if I want to do
the same amount of work in half the time?
– Twice as much
Reference table check
Work
P
Work
t
F
Fd
P
t
d
• You exert a vertical force of 72 N to lift a
box a height of 1 meter in a time of 2
seconds. How much power is used to lift
the box?
Star questions
• True or False
–Decreasing the amount of time it takes
to do work, increases the power
–True
–Motion must occur in order for work to
be done
–True
Machines
• Make
work
easier
• Device
that
changes a
force
How do machines make work easier?
• Increasing force
–Rising a car using a jack
• Increasing distance
–Using oars to row a boat
• Changing direction
–Pulling back on the
oars to row a boat
More details later-this is an overview :o)
Work Input (on) vs Work Output (by)
• Work done on a machine as the
input force acts through the
input distance
• Work done by a machine as the
output force acts through the
output distance
You cannot get more work out of
a machine than you put into it!
Mechanical advantage
• # of times the machine increases
an input force
• Actual (AMA) = output force( FR)
input force (FE)
• FR = resistance (output) force
• FE = effort (input) force
• You test a machine and find it exerts a
force of 5N for each 1N of force you exert
operating the machine. What is the Actual
Mechanical Advantage of the machine?
• 5N/1N = 5
•
If the output force is 100 N and the input
force is 15 N, what is the AMA of the
simple machine?
•
Output/input=100N/15N=
• Ideal (IMA) = MA in absence of
friction
• IMA = input distance( dE)
Output distance (dR)
• dE = effort (input) distance
• dR = resistance (output) distance
• bc friction is always present,
AMA is always less (<) than IMA
• If x = 3 cm and y = 15 cm, what is the ideal
mechanical advantage of the pliers?
x
• 15cm / 3cm = 5
y
•
A student working in a grocery store after
school pushes several carts together
along a ramp. The ramp is 3 m long and
rises 0.5 m. What is the mechanical
advantage?
•
Input/output=3m/0.5m=6
• What is the
MA?
• 12 / 3=4
3m
12 m
• What is the MA of the pulley?
1
2
Efficiency
no machine can be
100% efficient, why not?
work output
Efficiency = ------------------------- X 100%
work input
Thanks Cambridge Physics Outlet for amazing graphics!
• You have just designed a machine that uses
1000J of work from a motor for 800J of useful
work the machine supplies. What is the
efficiency of your machine?
800J / 1000J X 100 = 80%
STAR Questions
• What three things make machines work
easier
• Increasing force
• Increasing distance
• Changing direction
• What two things does the amount of work
depend on?
– Force & distance
• To increase Power I must do work in
________ time
– less
Types of simple machines
1. Lever – rigid bar that is free to
move around a fixed point
• Fulcrum=the fixed
point the bar rotates
around
• Input arm (effort) =
distance between the
input force and fulcrum
• Output arm (load) =
distance between the
output force and the
fulcrum
• Position of
fulcrum
identifies
class of
lever
First class levers
• Fulcrum located between the input
force (effort) and output force (load)
Second class levers
• Output force
(effort) is located
between the
input force (load)
and fulcrum
The input distance your
• MA is >1
hands move to lift the
• Increases force wheelbarrow is larger
than the output distance
the wheelbarrow moves
to lift its load
Third class levers
• Input force is
located
between the
fulcrum and
the output
force
The output distance over
which the lever exerts its
• MA <1
force is always larger than
•
Increases
the input distance you move
distance
the lever through
2. Wheel and Axle
• Variation of a lever
• Consists of two disks (cylinders)
each one with a different radius
3. Pulley
• Variation of a
lever
• Consists of a rope
that is different in
size, direction, or
both from that of
the input force
3 types of pulleys
1. Fixed pulley = changes only the
direction of the input force
2. Movable pulley = changes both
direction and the size of the
input force
3. Pulley System= made up of both
fixed & movable pulleys
4. Inclined Plane
• Slanted surface along which a
force moves an object to a
different elevation
5. wedge
• Variation of inclined plane
• A V-shaped object whose
sides are two inclined
planes sloped toward
each other
* A thin wedge has a
greater IMA than a thick
wedge of the same length
6. screw
• Variation of inclined plane
• Wrapped around a cylinder
*Screws w/ threads that are closer
together have a greater IMA
Identify which class lever goes
with each picture
STAR Questions
Complex (Compound) Machines
• Combination of 2or more simple
machines to operate together
FYI: Rube Goldberg machine
• device, or apparatus is a deliberately over-engineered
machine that performs a very simple task in a very
complex fashion, usually including a chain reaction.
• The expression is named after American cartoonist &
inventor Rube Goldberg.
STAR Questions
Lever
Wheel and axle
Pulley
Inclined Plane
Screw
Wedge