Transcript Chapter 5 Work and Machines

Chapter 5 Work and Machines
Page 126-131
Review
• Motion
– Distance, time, speed (velocity), acceleration
– Speed = rate of change of distance
– Acceleration = rate of change of velocity
• Force
– Force, mass, acceleration
– A force is required to change the velocity of an object.
• F = ma
– Weight is the gravitational attraction on an object
• W = mg (g= 9.8 m/s2 the acceleration of gravity)
Review of Energy
• Energy is the ability to do work.
• Different forms of energy
• Different types of mechanical energy
– KE = ½ mv2, PE (GPE)= mgh
• Energy is always conserved
• Energy may be conserved but it is often changed
into a useless form (heat) by friction and air
resistance.
• But Einstein said, E = mc2
Work
• Work makes something move
• Work is the transfer of energy when a force makes
an object move.
• Two conditions
– A force must make something move
– The direction of motion must be in the direction of the
force
– Carrying books
• Work is a transfer of energy
• W= Fd (force in Newtons (kg m/s2, distance in m)
Practice Problems P 128:1,2,3
• You push a refrigerator with a force of 100N.
If you move the refrigerator a distance of 5m
while you are pushing, how much work is
done?
• Given:
• Asked:
?units
• Formula:
Page 128 Problem 1 A couch is pushed with a force of
75N and moves a distance of 5m across the floor. How
much work is done in moving the couch?
• Given:
• Asked:
• Formula: W=Fd
• Substitute:
• Answer:
Problem 2 A lawn mower is pushed with a force of 80N.
If 12,00J of work is done in mowing the lawn, what is
the total distance the lawn mower was pushed?
• Given:
• Asked:
• Formula: W=Fd
• Substitute:
• Answer:
Problem 3 The brakes on a car do 240,000J of work in
stopping a car. If the car travels a distance of 50m while the
brakes are being applied, what is the force the brakes exert on
the car?
• Given:
• Asked:
• Formula: W=Fd
• Substitute:
• Answer:
Problem 4 The force needed to lift an object is equal in
size to the gravitational force on the object. How much
work is done in lifting an object with a mass of 5.0 kg a
vertical distance of 2.0 m?
• Given:
• Asked:
• Formula: W=Fd
• Substitute:
• Answer:
Power
•
•
•
•
•
•
•
•
Power is the rate of doing work.
How much work is done in 1 sec
P = W/t
W (joules) joules = kg m2/s2
Unit for power is joules/second called a Watt
A Watt is kg m2/s3
Since work is energy transferred
P= E/t
Power Example
• You push a box with a force of 100 N across
the floor 5 meters it takes you 45 seconds.
Your friend pushes a similar box also with a
100 N force also 5 meters across the floor but
it only takes your friend 30 seconds.
– How much work do you each do?
– How much power do you each expend?
Problem 1 Page 130
In lifting a baby from a crib, 50 J of work are done.
How much power is needed if the baby is lifted in 2 s?
– Given:
– Asked:
– Formula: P= W/t
– Substitute:
– Answer
Units?
Problem 2 Page 130
If a runner’s power is 130 W as she runs, how much
work is done by the runner in 10 minutes?
– Given:
– Asked:
– Formula: P= W/t
– Substitute:
– Answer
Units?
Problem 3 Page 130
The power produced by an electric motor is 500 W.
How long will it take the motor to do 10,000 J of work?
– Given:
– Asked:
– Formula: P= W/t
– Substitute:
– Answer
Units?
Energy and Work
Power Practice Problems P130: 1,2,3
• You do 900 J of work pushing a sofa. If it took 5s
to move the sofa, how much power did you use?
– Given
– Asked?
– Formula
– Substitute
– Answer
Units?
Section Review P131: 5,6,7
•
•
•
•
•
•
•
•
•
Work is a transfer of energy
W= Fd
(force in Newtons (kg m/s2, distance in m)
N*m or Joules
Power is the rate of doing work.
How much work is done in 1 sec
P = W/t
W (joules) joules = kg m2/s2 , t= seconds
Joules/second = Watt
Machines (P132)
• A machine is a device that makes work easier.
• Simple machines (not motorized)
• How machines make work easier
– A machine can increase the force that is applied to an
object.- Car jack (p 132) Multiplies force
– A machine can make work easier by increasing the
distance which reduces the force needed. Ramp
(p133)
– A machine can change the direction of the forcesplitting wedge (p133)
Work Done by Machines
• Figure 7 Page 133
• Choice – lift the chair straight up the height of
the truck.
• W = Fg x height
• Or- slide the chair up the ramp
• Work will be the same
• Distance will be greater so force will be less
Work, Distance and Force
Machines don’t decrease the work (real machines increase
the work), but they can decrease the force required.
Ideal machine (p135)
Wout= 100 N * 3m =
300J
3m
Workin= Workout
Win = Fin *5 m ,
Wout= Win
Wout= 300J but d=5m
so F= 60N
5m
100 N
4m
Factors of Work
• Input force – the force applied to the machine
• Output force – the force applied by the
machine
• Input distance – the distance the input force
moves
• Output distance – the distance the output
force moves.
• work in = input force x input distance
• work out = output force x output distance
Pulling a Nail
• work out = work in
•
•
•
•
•
Figure 10 page 135
hammer claw moves 1 cm to pull a nail
Handle moves 5 cm
Output force of 1,500 N
Input force = ?
Ideal Machines (P135)
• Workin= Workout
Forcein X distancein= ForceoutX distanceout
• Fin X 5cm = 1500N X 1 cm
• Fin= ?
• distance units cancel out if the same unit
Ideal Mechanical Advantage
Workin= Workout
Fin X din= FoutX dout
F out d in
=
Fin
dout
IMA = F out = d in
Fin
dout
Real Machines
• Work out< Workin
• Some of the input work is always
converted into heat by friction.
• Efficiency tells us how much of the
input work is converted into output
work.
Workout x 100% < 100%
efficiency =
Workin
Assignment
•
•
•
•
•
Chapter 4 Practice Problems
Practice Problems P 128:1,2,3
Power Practice Problems P130: 1,2,3
Section Review P131: 5,6,7
Page 137 – Applying math 5-6-7
Simple Machines
Section 3
Page 138-146
Types of Simple Machines
• Simple machine does work with only one
movement.
• Lever
• Pulley
• Wheel and Axle
• Inclined Plane
• Compound Machines
Lever
• A bar that is free to pivot (turn) about a fixed
Point
• The fixed point is called the fulcrum
• Input arm – distance from the input force to
the fulcrum
• Output arm – The distance from the output
force to the fulcrum.
• If output arm is shorter than the input arm
then the output force is greater than the input
force.
Types of Levers (P138-139)
First-class Lever
Output Force
Second-class Lever
Output
Distance
Output
Distance
Input Distance
Output Force
Input Distance
Third-class Lever
Input Force
Output
Distance
Input Distance
Input Force
Output Force
Input Force
Ideal Mechanical Advantage
of a Lever
Force output
IMA =
Force input
Force output x Length output=Force input X Length input
L input
IMA =
L output
Pulley Types (P 141-142)
Fixed Pulley
Input Force 4N
Output Force 4N
4N
Movable Pulley
2N
Input Force 2N
Output Force 4N
4N
Types of Pulleys (cont)
Block and Tackle
1N
1N
1N
Input Force 1N
1N
Output Force 4N
4N
Ideal Mechanical Advantage
of a Pulley
Force output
IMA =
Force input
IMA =
Number of Strings Lifting the Load
Wheel and Axle (P143)
Input Force
rw
ra
Output Force
Radius of wheel
IMA =
Radius of axel
Inclined Plane
F= 100N, height = 3m
W= 100 N * 3m = 300J
W= 300J but d=5m so F= 60N
3m
100 N
Length of Slope
IMA =
Height of Slope
5m
Types of Inclined Plane
•
•
•
•
•
Ramps, inclines, road grades
Screw
Wedge
IMA = length of slope/height of slope
Page 144
Compound Machines
• Two or more simple machines
that operate together.
–Can opener
–Automobile
–Space shuttle
Section 3 Review
• Page 146 Applying Math 5-6-7
Assignment
•
•
•
•
•
•
•
•
•
•
•
Practice Problems P 128:1,2,3
Power Practice Problems P130: 1,2,3
Section Review P131: 5,6,7
Page 137 – Applying math 5-6-7
Page 146 Applying Math 5-6-7
Practice Problem Page- Due Monday
Note Taking Worksheet- due Monday
Lab on Tuesday
Chapter 5 Review Page 152-153
1-20, 24-28 Due Wednesday
Test on Chapter 5 Friday
Formulae
F = m*a , FG= m*g
unit kg m/s2 = Newton
W= F*d units N*m = Joule
P= W/t or P= E/t
unit J/s = Watt (kW)
IMA = F out = d in
Fin
dout
Workout x 100% < 100%
efficiency =
Workin
IMA of Simple Machines
All Simple Machines MA = Force output
Force input
>1
Lever
IMA = L input
L output
Length of Slope
IMA
=
Inclined Plane
Height of Slope
Wheel and Axle IMA = Radius of wheel
Radius of axel
Pulley
IMA = Number of Strings Lifting the Load
Workout x 100% < 100%
efficiency =
Workin
Chapter 5 Review
Lever
Input Arm
Output Arm
A
25
75
B
53
42
C
36
36
D
32
99
E
10
30
IMA
Problems Page 153
• Given:
• Asked:
• Formula:
• Substitute:
• Answer
Units
Friday November 7
• Get out your calculators and Reference Sheets
• Put away your books and notes.
•
•
•
•
•
Answer all the questions on the test sheets
Don’t overlook problems on the back of the test.
When you have finished, place test in the tray.
Place all homework in the tray
After the test sit quietly and begin reading chapter 6
on page 158.