Download II. Forces

Domain 5 Teacher Notes
I. Energy
A. Law of conservation of energy.
1. Energy can neither be created nor destroyed.
2. It can be changed into other forms.
B. Two kinds of energy
Click here for video clip
1. Potential energy is stored energy or energy of
position. Examples: Magnetic, gravitational,
chemical, elastic, and nuclear
PE = mgh where m= mass
g = acceleration due to gravity
h = height
2. Kinetic energy is energy of motion or energy in action.
Examples: a moving baseball or a roller coast going
downhill
KE = ½ mv2
C.Forms of energy
1. mechanical energy
a. Any object which possesses mechanical energy whether it be in the form of potential energy or
kinetic energy - is able to do work. That is, its
mechanical energy enables that object to apply a
force to another object in order to cause it to be
displaced
b. simple machines help us lift, pull, increase
elevation of heavy things, change the direction of
the force, increase the force, split things, fasten
things, and cut things
i. inclined plane - Ramp, stairs
ii. wedge – two inclined planes back to back –
screwdriver, knife, axe
iii. screw – inclined plane wrapped around a
Revised Friday, June 1, 2007
cylinder
iv. pulley – rope revolves around a fixed point;
more pulleys make work easier
v. lever – has a fulcrum – see-saw
vi. wheel and axle – bicycle, car, doorknob,
screwdriver in use
Click here for Simple Machines video clip
a. mechanical advantage-the number of times a
machine multiplies an effort force.
b. Formulas from formula sheet:
Actual Mechanical Advantage:

F 
 AMA  R 
FE 

where FR is Force due to resistance and FE is Force due to effort
IMA= Effort Length
Resistance Length
Sample Problem: In a pulley set-up, an effort force of
50 Newtons lifts a resistance force of 100 Newtons.
What is the MA of the pulley system?
Think about which formula to use
IMA= Effort Length

FR 
 AMA 

Resistance Length
or
F

E

Revised Sunday, July 8, 2007
AMA= Fr
Fe
M.A. = 100N
50N
M.A = 2
1. Heat, Temperature and Internal Energy
a. The temperature of an object is directly proportional
to the average kinetic energy of its particles. As
temperature goes up, particles move faster.
b. The Internal Energy of a substance is total of the
potential and kinetic energy of all its particles.
c. Energy moving from one location to another is
known as heat. Objects do not contain heat.
Instead they contain internal energy.
2. Chemical energy relates to potential energy stored in
the bonds between atoms in a compound
Click for Video Clip
D. Wave energy
1. Basic Wave Vocabulary:
a) Amplitude – the height of a wave as
measured from its equilibrium position.
b) Frequency – the number of wave cycles per
unit time
c) Hertz – common unit for frequency, means
the same as cycles per second
d) Wavelength – the distance between two
identical points on adjacent waves. It is
often measured “crest to crest.”
e) Period – the time required to complete one
cycle
Revised Sunday, July 8, 2007
2. Types of waves. A wave is a disturbance of a
medium which transports energy through the
medium without permanently transporting matter. In
a wave, particles of the medium are temporarily
displaced and then return to their original position.
A. Waves in which the particles of the medium
move in a direction that is perpendicular to the
direction of the wave are known as transverse
waves. Transverse waves require a relatively
rigid medium in order to transmit their
energy. Examples of transverse waves include
Waves on a string
B. A longitudinal wave is a wave in which particles
of the medium move in a direction parallel to the
direction which the wave moves. Examples of
longitudinal waves include sound waves.
Revised Sunday, July 8, 2007
C. A surface wave is a wave in which particles of the
medium undergo a circular motion. Surface waves
are both longitudinal and transverse. Seismic
(earthquake) and water waves are examples of
surface waves.
Click here for Nature of Waves clip
4. Light
a. Electromagnetic Spectrum
i. colors of visible light - ROYGBIV
red, orange, yellow, green, blue, indigo, violet
Revised Sunday, July 8, 2007
ii. all electromagnetic waves travel at the speed of
light 3.0 x 108 m/s in a vacuum.
Click here for Electromagnetic Spectrum clip (4 min)
b. Reflection occurs when a wave encounters a new
medium and bounces off of it.
i. The Law of Reflection states that the angle of
incidence (the angle between the incoming
wave and the normal) must equal the angle of
reflection (the angle between the reflected
wave and the normal). The normal is a line
perpendicular to the surface.
c. Refraction occurs when a wave travels at an angle
from one medium to another in which its speed is
different. The difference in speed causes the wave
to bend. Refraction is why objects in a pool are
difficult to locate when viewed from above and why
a pencil looks broken when part of it is placed in a
glass of water.
Revised Sunday, July 8, 2007
d. Diffraction involves the bending of waves around
obstacles. Diffraction can occur with any kind of
wave. Diffraction explains why sound can be
heard around corners.
Revised Sunday, July 8, 2007
Diffraction of waves through a slit
e. Interference
i. Constructive interference occurs when two
waves disturb the medium in the same
way. The disturbance is larger than the
disturbance of either wave separately.
ii. Destructive interference is canceling interference
that occurs when two waves disturb the
medium in opposite ways. The disturbance is
smaller than the disturbance of either wave
separately.
Click here for Wave Interference clip
Stop clip after Doppler Effect
Revised Sunday, July 8, 2007
5. Sound
a. Sound waves are mechanical waves, meaning
they must have a medium to travel through.
(There is no sound in space.) A sound wave is a
compressional or longitudinal wave. This means
the particles of the medium move in the same
direction as the wave.
Click here for a Sound Energy video clip
b. Sound travels fastest in solids, second fastest in
liquids, and slowest in gases. As the temperature
increases, the speed of sound also increases.
Different sounds appear different because they
have different pitch or frequency.
Click here for video clip
II. Forces
A. gravitational
1. The force of gravity between any two objects
increases as the mass of either object increases. The
force of gravity decreases as the distance between
the objects increases.
2. The force of gravity experienced by something is also
known as its weight. Weight can be calculated
multiplying mass by the acceleration of gravity (g).
W=mg
Weight depends on both mass and the acceleration
of gravity. Mass depends only on the amount of
matter in an object. Mass does not change when the
location of an object changes.
Click here for video clip
Revised Sunday, July 8, 2007
The force and weight are both measured in Newtons.
Mass is measured in kilograms. Acceleration is
measured in m/s/s (m/s2).
3. Free Fall
a. A free-falling object is an object which is falling
under the sole influence of gravity.
b. Free-falling objects do not encounter air
resistance.
c. All free-falling objects (on Earth) accelerate
downwards at a rate of approximately 10 m/s/s
(to be more exact, 9.8 m/s/s). This quantity
known as the acceleration of gravity has a
special symbol to denote it - the symbol g.
The distance traveled by a falling object is
calculated using the formula, d = ½ gt2
d. Sample problem: A rock is dropped from the
top of a cliff and strikes the ground 6.5
seconds later. How high is the cliff in meters?
g=9.8 m/sec2
t = 6.5 sec
d = ½ gt2
= ½ (9.8 m/sec2) (6.5 sec)2
= ½ (9.8 m/sec2) (42.25 sec2)
= ½ (414.05 m)
= 207 m
B. Electromagnetic Forces – Like charges repel each
other, opposite charges attract.
1. As the distance between the charges increase, the
magnitude of the force decreases. The same holds
true for magnets.
2. Magnets will always have a North Pole and a South
Pole. Just like with electrical charges, opposite
poles attract. While it is possible to separate
Revised Sunday, July 8, 2007
positive and negative charges, it is impossible
to separate north and south magnetic poles.
3. Electric and Magnetic Field Lines- The lines always
go from positive charges to negative charges and
from north poles to south poles. The closer the
lines, the stronger the field.
C.Electromagnet
1. List a use for electromagnets.
Lift and move cars in a junk car
2. How can you make a stronger
electromagnet?
a. More coils of wire around the nail
b. More batteries
Click here for video clip
D. Work
1. two conditions must be met for work to occur
a. the object must move through a distance
b. a force must act upon the object in the direction
the object moves
Click here for video clip
Revised Sunday, July 8, 2007
2. SI unit for work is the joule, J. (Newton-meter)
3. Formula: work = force X distance W=F X d
4. When an object is lifted to a new location or pushed
up a ramp, the work equals the potential energy
gained.
5. Sample Problem: What work is done if Hernando
uses 88 N of force to pull a table 12 meters?
F = 88N
d = 12 m
Use the formula
from the
SCIENCE FACTS AND FORMULAS sheet
W=F X d
W=88N X 12 m
W= 1056 N-m or
1056 J
E. Power
1. the rate of work
2. SI unit is the watt, W (joule/second)
3. formula: power = work/time (work divided by time)
4. Sample Problem: When doing a chin-up, a physics
student lifts her 40-kg body [which has a force (weight) of
400 N] a distance of 0.25 meters in 2 seconds. What is the
power delivered by the student's biceps?
You must first calculate the work done to lift her body
W=FXd
= (400 N) (0.25 m)
W = 100 J
To calculate power
Power= work
time
Power=100 J
2 sec
Power = 50 Watts
III. Motion
A. Newton’s Laws of Motion
Revised Sunday, July 8, 2007
1. The Law of Inertia or Newton’s First Law: An
object at rest tends to stay at rest and an object
in motion tends to remain in motion in a
straight-line path unless acted on by an
unbalanced force.
a. Inertia is another word for mass. The more
mass an object has, the greater its tendency
to maintain its current state.
Click here for video clip
b. Applications:
i. People are often thrown from automobiles
in wrecks because the car comes to a
sudden stop, but the person has a
tendency to stay in motion.
ii. The ride is much smoother on a cruise ship
than a fishing boat, because the cruise ship
is more massive and is not affected as
much by the waves.
2. Newton’s Second Law: The acceleration of an
object is directly proportional to the applied force
and inversely proportional to its mass.
F=ma
F= force
m = mass
a = acceleration
Click here for video clip
Sample Problem: What is the force exerted by a
2 kg mass that accelerates at 3 m/sec/sec?
Revised Sunday, July 8, 2007
mass=2 kg
F=ma
acceleration=3 m/sec/sec F =2 kg x 3 m/sec/sec
F=6 kilogrammeter/sec/sec
Check the SCIENCE FACTS AND FORMULAS sheet
1 newton = 1 kilogrammeter/second/second
So the correct answer is 6 newtons.
3. Newton’s Third Law: For every action there is an
equal and opposite reaction.
a. If object A exerts a force on object B, then
object B exerts an equal force on object A in
the opposite direction.
Click here for video clip
b. Consequences: Forces always exist in pairs. It
is impossible for you to push on something
without it pushing back. Newton’s Third law
can be used to explain the motion of rockets
and balloons. As the gases exit the balloon or
rocket they push it in the opposite direction.
B. Motion depends on the observer’s frame of
reference
C. Speed
1. a measure of how fast something is moving
2. the distance traveled in a given amount of time
3. formula speed = distance
time
4. Sample Problem: A bicyclist rides for 1.5 hours
from Snellville to downtown Atlanta. He travels 21
Revised Sunday, July 8, 2007
miles. What is his average speed?
d=21 miles
t=1.5 hours
speed=distance
time
speed= 21 miles
1.5 hours
Speed = 14 mi/hr
D. Velocity
1. speed in a particular direction
2. formula: velocity = distance and direction
Time
The formula from the SCIENCE FACTS AND FORMULAS
sheet is Velocity (V) = V0 + at, where V0 = Initial Velocity, a =
Acceleration, and t = Time
3. Sample Problem: What is the average velocity of
a commercial jet that travels west from New York
to Los Angeles (4800 km) in 6.00 hours?
Velocity = distance
time
= 4800 km
6.00 hours
= 800 km/hr west
E. Acceleration
1. the rate at which velocity changes
2. formula:
final velocity-initial velocity
Revised Sunday, July 8, 2007
acceleration =
or
time
acceleration = Vf-Vi
t
from SCIENCE FACTS AND FORMULAS sheet

Acceleration = Change in Velocity/Time Elapsed  a 

V  V0 

t 
3. Acceleration occurs if either of these two
conditions exist.
a. The speed of an object is changing. It can be
increasing or decreasing.
c. The direction of the movement is changing.
Click here for Video Clip (4 min)
4. Sample Problem: If a car accelerates from 5 m/s to
15 m/s in 2 seconds, what is the car's average
acceleration?
V = 15 m/s
Vo = 5 m/s
t = 2 sec
V  V0 

a 

t 

a= 15 m/s - 5 m/s
2 sec
a= 5 m/sec/sec
Revised Sunday, July 8, 2007