Transcript Chapter 13 PP - Moline High School

Ch. 13
Work and Energy
Work, Power and Machines
• Work - Amount of energy transferred when a
force causes an object to move in the
direction of the applied force.
– Work = Force x distance
–W= Fxd
-work is measured in Joules (J)
* 1J = 1N x 1m or 1J = 1 Nm
• Ex. A student lifts a 12N book 1.5m.
How much work did he do on the book?
• Ex. A student pushes against a desk
with 50N of force. The desk doesn’t
move. How much work did he do on the
desk?
• Ex. A mechanic uses a lift to raise a
1,200 kg car .50 m off the ground. How
much work does the lift do on car?
• Power - The rate at which work is done
– Power = Work / time
– P= W / t
Power is measured in watts (W)
Kilowatts(kW)=1000W (what electric companies use)
Can also measure in horsepower
* 1hp = 750W
Power Example
• 1. While rowing across the lake during a
race, John does 3,960 J of work on the
oars in 60.0 s. What is his power output
in watts?
P = W/t
P = 3,960J/ 60.0s
P = 66.0 W
Machines
• Machine - Any device that helps make
work easier
-Makes work easier by changing
the size or direction of the
applied force.
• 2 forces involved in using a machine
1. effort force (Fe) ( also called input
force)
- the force applied to a
machine (your force)
2. resistance force (Fr) (also called output force)
- the force applied by the machine
-usually equal to the weight
of the object being moved
We use machines to overcome gravity and
friction
Mechanical Advantage
• Mechanical Advantage (MA) The number of times a machine multiplies
the effort force.
-Input force (effort force)
-Force you put in
-Output force (resistance force)
-Force machine does
-Input distance
-Distance you do
-Output distance
-Distance machine does
MA equation
• Ratio that measures how much a
machine multiplies force or distance.
MA = output force or input distance
input force
output distance
MA that is greater than 1 means the machine multiplies the
input force
MA that is less than 1 means the machine increases the
distance or speed
• Ex. A student lifts a 12 N book 1.5 m in
1.5 s and carries the book 5 m across
the room in 7 s.
• How much work does the student do on
the book?
• What is the power output of the student?
Simple Machines
• 6 simple machines
2 Families of simple machines
Lever family and Inclined plane family
Lever Family
1. Lever Made up of a rigid beam (arm) and
a fulcrum(fixed point).
3 classes:
st
1
class lever
Fulcrum is in the middle.
Input on one end, and the
Output on the other end.
Ex.
1st class lever
2nd class lever
Output is in the middle.
Input on one end, and the
Fulcrum on the other end.
Ex.
2nd class lever
3rd class lever
Input is in the middle.
Output on one end, and the
Fulcrum on the other end.
Ex.
3rd class lever
2. Pulley -
Changes the direction of
the force.
The middle is like the fulcrum
The wheel is like the short
end of the lever arm
Pulley cont.
Single pulley Doesn’t magnify force,
it just changes direction.
MA = 1
Moving pulley Magnify force,
but doesn’t change
direction.
-can increase MA
3. Wheel and Axel -a lever/pulley (wheel)
connected to
a rigid arm (axel)
-Changes a small
input force into a
large output force.
-Ex.
Wheel and axel examples
Inclined Plane Family
4. Inclined plane -A slanted board used to lift an object
-It increases the distance but
decreases the input force.
-MA is equal to the length of the
plane divided by the height
-Ex.
Inclined plane
5. Wedge- modified inclined plane
-Two inclined planes, back to back
-changes a downward force into two
forces directed out to the sides
Ex.
Ex. wedge
6. Screw
- Inclined plane wrapped around a cylinder
-increases distance so you have
a small input force
The closeness of the ridges determine
The MA
- far apart = small force, large distance
- close together = large force, short distance
Ex.
Examples of screws
Compound Machine
• Combination of 2 or more simple
machines
Energy
Energy- the ability to do work.
Measured in Joules (J)
Potential Energy
• Potential Energy (PE)
Stored energy
Potential Energy
• 2 types of Potential Energy (PE)
1. Elastic PE
- energy stored in compressed
or stressed material
2. Gravitational PE
- energy stored because of the
location of the object
- amount of gravitational PE (GPE)
depends on mass and distance of the
objects
- Gravitational PE =
mass x gravity x height
Kinetic Energy
• Kinetic Energy (KE)
- Energy of motion
- Kinetic Energy = 1/2 mv2
Potential and Kinetic Energy
Mechanical Energy
• Mechanical Energy -when you can see the change
from PE into KE.
- large scale
Non-mechanical Energy
• Non-mechanical Energy -when you can not see the
change from PE into KE.
- small scale (dealing with atoms)
Conservation of Energy
• Conservation of Energy Energy of transformation
- PE

KE PE  KE
-When energy is transformed
some of the energy is changed
into other forms of energy.


Conservation of Energy
Law of Conservation of
Energy
- Energy can not be created or
destroyed it just changes form
Forms of Energy
1. Chemical Energy
- Depends on the position of atoms
in the substance
-Non mechanical energy
- PE when it is the substance
- KE when bonds break
Ex.
2. Electric Energy
- Flow of electron particles
-Non mechanical energy
-KE because the electrons
are moving
Ex.
3. Nuclear Energy
-Energy stored by forces holding
nucleus together
- fusion- combining two nuclei
- fission - splitting a nucleus
- PE
4. Solar Energy
-Energy that plants use to create food
(chemical energy)
-Non mechanical energy
-PE
5. Light Energy
-Electromagnetic waves that travel
over great distances
- mechanical energy
-KE