Transcript Objective 5 - Physics

Objective 5
Motion, Forces & Energy
Physics the Study of Energy
Why things MOVE
• The faster things move the more energy they use.
• Moving objects have momentum (mass x velocity)..
• A force is required to change motion of an object.
• Work is done when motion is changed. Forces change
the energy of an object when they change its motion.
• Power is the rate at which work is done, thus power is
the rate at which energy is consumed.
Motion can be described as
• a change in an
object’s position
A Force is a push or a pull
• Forces can create
changes in motion
Balanced Force
• A force that produces no change in an
object’s motion because it is balanced by
an equal, opposite force.
Unbalanced Forces
Are forces that results in an
object’s motion being changed.
Vectors
• A vector is an arrow ---------->, it has both
magnitude, (length) and direction.
• <---------- is the same magnitude (length) as the earlier
vector, it's not the same because it's pointing in the opposition
direction.
• ---> is not the same as the first one because the two
magnitudes are different
Scalar
• A scalar quantities do not include direction.
They include the magnitude (length) only, and
are represented by a single number.
• The speed a car is driving is a scalar quantity
• The speed is 64 mph
Friction
• A force that acts in a direction opposite
to the motion of two surfaces in contact
with each other.
• Friction causes an object to slow down and stop.
• Since the amount of energy stays constant, the energy
becomes heat.
Momentum
Momentum is a property of a moving object that
depends on the object mass and its velocity.
The bigger and/or faster it is the harder it is to
stop
The formula for Momentum is as follows:
Momentum = mass x velocity
The units; kilograms x meters per second (Kg x m/s)
Momentum
A bullet has a large momentum because of its high
velocity.
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Two trains have the same mass but different velocities.
The train with the greater momentum will be more difficult to stop.
***The train on the bottom would be harder to stop**
Law of Conservation of Momentum
The momentum remains the same unless outside forces
(like friction) act on the object
The momentum lost by one object is gained by another
when they collide.
A ball moving at 30 m/s has a momentum of
15 kg·m/s. The mass of the ball is —
A 45 kg
B 15 kg
C 2.0 kg
Momentum = Mass x Velocity
D 0.5 kg
So 15 kg.m/s = M x 30 m/s
solving for M it is: D
WORK
Work is done when a force acts on an
object in the same direction that the object
moves.
Work
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•
•
Work: a force for a distance
W=Fxd
(The object must move)
if work =0 then no work is done
Work and Energy are measured in Joules
1 Joule=1 Newton • meter
How much work is performed when a 50 kg crate is pushed 15 m with
a force of 20 N?
F 300 J
Use the formula Work = Force x distance
G 750 J
H 1,000 J
Force of 20 N x 15 meters = 300 Joules
J 15,000 J
Answer:
If a force of 100 newtons was exerted
on an object and no work was done,
the object must have —
A accelerated
Work = Force x Distance
rapidly
Work = 0 Force = 100 N
B remained
so
motionless
0 J = 100 N x d
C decreased its
distance must be 0
velocity
D gained
It did not move!
momentum
Mechanical Advantage
• Man first started using machines to make work easier
and faster. How much easier and faster a machine
makes your work is the mechanical advantage of that
machine.
• Mechanical Advantage (M.A.) – the number of
times a machine multiplies the effort force.
Efficiency of a machine
The comparison of work output to work
input. It is expressed as a percent.
No machine is 100% efficient.
Because of friction, work output is always
less than work input
The diagram shows an electric
motor lifting a 6 N block a
distance of 3 m. The total
amount of electrical energy
used by the motor is 30 J. How
much energy does the motor
convert to heat?
F 9J
G 12 J
H 18 J
J 21 J
Answer on next slide
Work Output = Resistance Force x Resistance
Distance
Work Input = 30J done by the motor
Workout = 18J = 6N x 3m
The difference is lost as heat due to friction, which
is 30J – 18J = 12J
Answer G
Machines use power
• Power: the rate at which energy is used (work is
done)
• P=Work/time
• Power is measured in H.P. or watts
• 1 watt = 1 Joule
1 sec
Machines
One horsepower (hp) equals about 750 watts (745.56
W), the weight a strong horse could move a distance of
1 meter in 1 second.
6 Types of simple machines
Some Simple Machines:
• Inclined planes
• Screws
• Pulleys
• Wheel and axle
• Wedge
• Levers
Newton’s Three Laws of Motion
Newton’s 1st Law of Motion
Object in motion stays in motion
Objects at rest stay at rest
Until they are acted upon by
unbalanced forces.
Also known as inertia
The mass of an object
determines its inertia
Newton’s 2nd Law of Motion
The greater the mass or acceleration of an object,
the greater the force required to change its motion.
• Force = Mass X Acceleration
• F=ma
Newton’s 3rd Law of Motion
• For every action force there is an
equal and opposite reaction force.
The frog leaps from its resting position at the lake’s bank
onto a lily pad. If the frog has a mass of 0.5 kg and the
acceleration of the leap is 3 m/s2, what is the force the
frog exerts on the lake’s bank when leaping?
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A 0.2 N
B 0.8 N
C 1.5 N
D 6.0 N
F=ma, m is mass in kg, a is acceleration in m/s2.
.5 kg x 3 m/s2= 1.5 N answer- C
how fast an object is moving
Speed
• Speed – the distance traveled by a
moving object per unit of time.
• speed units: m / sec , km / hr , miles /
hr
Speed = Distance
Time
S=D
T
Velocity
Velocity is speed with a direction
• Velocity = Distance/Time
• S= D/T
• The units for velodity are meters per second (m/s)
• Example; Bill drove west at 45mph.
Velocity/Speed Graphs
V = distance
time
• Velocity (v) is the slope (rise over run) of a
position (d) vs. time (t) graph
The slope of the graph is related to speed.
The steeper the slope the faster the speed.
Constant speed
speed that does not change
The graph for constant speed is a straight line.
At any point on the line if you divide distance by time you get the same
answer.
Average Speed is the
total distance divided by total time
Speed that changes is not constant speed.
Average speed is not the speed at any one instant.
40 The diagram represents the total travel of a
teacher on a Saturday. Which part of the trip is
made at the greatest average speed?
FQ
How do we work this one?
GR
Calculate v = d/t for each segment.
HS
J T
Acceleration is a change in an objects
velocity (speed or direction)
• Speeding up, slowing down or turning
• When an object’s speed changes over time it is
accelerating (or decelerating)
• A = vfinal – vinitial
time
• Units for acceleration m/s/s or m/s2
Circular Motion
velocity is continuously changing because the direction of
motion is continuously changing.
An object in circular motion is accelerating even though its
speed may be constant because the direction of velocity is
continuously changing
Centripetal acceleration
Centrifuge
acceleration that is directed toward the center of a circular path.
Example: swinging a ball on a string in a circle. The
acceleration is in the direction of the center of the circle.
Velocities that are in opposite directions combine
by subtraction.
Suppose a boat is going upstream at 10 km / hr and the
river is flowing downstream at 5 km / hr.
Subtract the speed of the river from the speed of the boat.
Then velocity = 10 km / hr – 5 km / hr = 5 km / hr
Velocities that have the same direction combine by
addition.
Suppose a boat is going down a river at 10 km / hr, and the
river is flowing in the same direction at 5 km / hr.
Add the speed of the river to the speed of the boat.
Then velocity = 10 km / hr + 5 km / hr = 15 km / hr
the ability to do work
The Law of Conservation of energy
energy can be neither created nor destroyed.
Energy can only be converted from one form to another.
Einstein’s equation
C = speed of light
shows that mass and energy are two forms
of the same thing and can be converted into
each other.
If a small amount of matter is destroyed, a
large amount of energy is created as
happens in a nuclear bomb explosion.
If a large amount of energy is destroyed, a
small amount of matter is created
Five main forms of energy -mechanical, heat, chemical, electromagnetic, nuclear
Mechanical energy -- energy associated with motion
Heat energy -- the internal motion of particles of matter
Chemical energy -- the energy that bonds atoms or ions together.
Electromagnetic energy – moving electrical charges
Examples: electricity, light, X-rays, radiowaves, laser light
Nuclear energy – energy in the nucleus of the atom. Released
during nuclear fission (splitting) or nuclear fusion (combining).
Example: sun’s energy
Heat energy to electricity
Energy
Is defined as the ability to do Work
Energy has Two Types:
-Kinetic (Energy of Motion)
- Potential (Stored Energy)
Exogonic - more energy is given off than is used to start
Endogonic reaction absorbs energy and causes cooling
Energy Conversions
Changes from one form of energy to another.
Examples of conversions between kinetic and potential energy:
Example: a swinging pendulum, ball thown in the air
A falling object speeds up as it falls
to the ground;
PE decreases as KE increases,
KE decreases as PE increases
the KE at impact with the ground is equal to the
PE it had before it fell
Potential Energy – stored energy
PE = mgh
41 What is the potential energy of
the rock?
A 59,900 joules
B 64,600 joules
C 93,100 joules
D 121,600 joules
m = 95 kg g = 9.8 m/s2 h = 100 m
95 kg x 9.8 m/s2 x 100 =
93,100 joules C
Kinetic Energy- moving energy
A moving car has the ability to do work on the
light pole if it hits it.
KE = ½ m v 2
Gravity
a natural phenomenon by which physical
bodies attract
with a force proportional to their mass.
Universal Law of
Gravitation
Gravity varies depending on
two factors
1) the mass of the object
: doing the pulling, and
2) the distance from the center
of that object
On Earth gravity = 9.8 m/s/s
For every second that
an object falls its
speed increases by
9.8 m/s
all objects dropped from the same
height will land at the same time
Weight= Mass (m) X gravity (g)
Weight is due to gravity
• Weight Unit of mass = kg
• Unit of acceleration = m/s/s
• Unit of weight = Newton
• 1 Newton= about ¼ pound
Electricity
Flow of electrons that provide power
to your home
Electrical Energy - Electricity
• Electrical Energy - Moving electrons in a path is electricity
Electric Current (I)
• The flow of electric charges
• This charge is carried by moving electrons
in a conductor such as wire
• measured in Amperes
electricity flows
• electricity flows from the positive (+)
terminal of a battery to the negative (-)
terminal of the battery
Volt - Electrical Potential Difference
• The positive terminal of the battery has an
electric potential that is equal to the voltage
rating of the battery.
Resistance (R)
• Resistance is a measure of how much an
object opposes the passage of electrons.
• The unit of electrical resistance is the ohm
Volt
V=IR
V- volt, I- current, R-resistance
• What is the current in a copper wire that has
a resistance of 2 ohms and is connected to a
9 volt electrical source?
A. 0.22 amp
V- volt, I- current, R-resistance
B. 4.5 amps
V = I R so,
C. 11.0 amps
9V = I x 2 ohms or 4.5 amps
D. 18.0 amps
Series circuits
• are the most simple.
• One path for the current to travel.
• Contains an energy source, a path, and a load (like
a lamp)
Parallel circuits
• provide more than one path for the current to travel.
• Most circuits are parallel, since if one lamp goes out, the
others can stay lit.
Notice the symbols used
6. Which switches, if opened, will
cause the light bulb to stop glowing?
F. Q
G. R
H. S
J. T
Q It is the
only switch
in series to
both the
battery and
light.
Static electricity
• the build-up of electrical charge on the
surface of objects. The static charges remain
on an object until it is quickly neutralized
by a discharge
Heat is energy
When you add heat to a substance, you are adding energy.
• When heat (energy) goes into a substance one of two things
can happen:
• 1. The substance can experience a rise in temperature- an
increase in the kinetic energy of the molecules..
• 2. The substance can change state.
Thermal Energy - Heat
A body contains internal KE due to the motion of its atoms
(they are constantly wiggling and jiggling)
Thermal Energy is the total internal KE of a body
Temperature
a measure of the average kinetic
(moving) energy of molecules.
Adding heat to something increases
–.
its temperature
Heat is not temperature
Temperature scales
Fahrenheit - the freezing of water to ice is 32 degrees, boiling
point of water is 212 degrees
Celsius (°C )– metric unit -the boiling point of water (100 °C)
freezing point of water (0 °C)
Kelvin ( K ) – another metric unit of temperature based on
absolute zero, the lowest possible temperature
Calorie – a unit of heat.
• Calorie – a unit of heat.
• The amount of heat needed to raise the temperature of
one gram of liquid water one degree Celsius.
• 20 calories will raise the temp. of 1 g of water by 20 °C.
• 20 calories will raise the temp. of 10 g of water by 2 °C.
• The amount of heat needed depends on the mass.
Heat transfer - Movement of heat from a warmer
object to a colder one, by conduction, convection, or
radiation.
Warmer molecules move faster than cooler ones.
When warmer molecules collide with cooler ones,
energy is transferred.
Heat Transfer of Thermal Energy
1. Conduction-direct contact
(Usually Solids)
2. Convection- heating by circulating
fluids
3. Radiation – Transfer of
Electromagnetic (E.M.) Energy
The primary way liquids and
gases transmit heat is by the
process of —
J Fluid heat movement is
F reflection
convection.
G conduction Fluid motion occurs in liquids and
gases.
H radiation
J convection
Phase
change
Phase change– the physical change of
matter from one state (Solid, liquid, or gas)
to another.
Melting – solid to liquid
Freezing – liquid to solid
Vaporization – liquid to gas
Condensation – gas to liquid
Sublimation - solid to gas
Deposition - gas to solid
Heat convection occurs in gases and liquids. Heat
convection does not occur in solids because solids are
unable to —
A absorb heat by vibrating
B transfer heat by fluid motion
C emit radiation by reflecting light
D exchange heat by direct contact
B - Heat moves by conduction in solids since the particles
are close together and vibrate. .
Heat Formula
Heat gained or lost = Mass x change ( ) in
temperature(T) x specific heat (SH)
H = m x T x SH
• Example:
H = 4g x 5°C x 0.22cal / g x °C = 4.4cal
WAVES
Waves: How energy moves
• Waves - traveling disturbances that carry energy
from one place to another.
• They can move through matter but do not carry
matter with them.
• Waves get their energy from a source of
repeated motion called a vibration.
• Examples: ocean waves, waves on a rope, sound
waves, microwaves
Some wave types require a medium through which they are
transmitted.
Water is a medium for ocean waves.
Air is a medium for sound waves.
All phases of matter (solid, liquid, or gas) can act as a medium.
Waves that require a medium are called mechanical waves
electromagnetic waves.
• Other waves do not require a medium. They can be
transmitted through a vacuum. (space)
• Instead of matter these waves disturb electric and magnetic
fields.
• Examples include radio waves, visible light, infrared,
ultraviolet, microwaves, and X-rays.
Waves - 2 Types
Transverse Waves
• In Transverse Waves particles vibrate at right
angles to the direction the wave travels.
• E. M. Waves, rope coil, ocean waves
Longitudinal or
Compression Waves
Vibrating particles move back and forth along the
direction of the wave velocity
Parts consist of compressions and rarefactions
Ex. Sound Waves
At 0°C sound travels through air at a speed of
330 m/s. If a sound wave is produced with a
wavelength of 0.10 m, what is the wave’s
frequency?
F 0.0033 Hz
G 33 Hz
H 330 Hz
J 3300 Hz
Velocity = f λ OR
330 m/s = f x 0.10 m
And the answer is? J 3300 Hz
Sound acts like other waves
• Echoes are reflected sound waves
• Sonar uses echoes to judge distance to
obstructions
• Human hearing is 20-20,000 Hz, below 10 Hz is
infrasonic, and above 20,000 Hz is ultrasonic.
the Doppler effect
• Heard an ambulance go? Remember how
the siren's pitch changed as the vehicle
raced towards, then away from you? First
the pitch became higher, then lower
resonance
when two things vibrate with same frequency,
Interference of Waves
• when two waves collide
Constructive / destructive interference
Constructive
• Waves add together to make
a bigger wave
Destructive
• Waves cancel each other out
Standing Wave
• A stationary wave – is a wave that remains
in a constant position
Standing waves- Nodes and Antinodes
• Areas on a standing wave that appear to
stand still
One tuning fork is struck and placed next to an
identical fork. The two forks do not touch. The
second tuning fork starts to vibrate because of —
F interference
G the Doppler effect
H resonance
J standing waves
Resonance is the vibration of another object struck by a wave of the correct
frequency. Since the forks are identical, the second one receives the
correct frequency to begin vibrating
Electromagnetic Radiation
Electromagnetic Spectrum
Is a name that scientists give a bunch of types of radiation when
they want to talk about them as a group.
Radiation is energy that travels and spreads out as it goes
-visible light that comes from a lamp in your house
-radio waves that come from a radio station
- Infrared and Ultraviolet light, X-rays and Gamma Rays
.
Speed of light
• All radiant energy travels at
3.0 x 108 m/sec in space
Reflection
• Reflection involves a change in direction of
waves when they bounce off a barrier.
Refraction
Refraction- waves change
direction when going thru media
of different densities
Diffraction
• Diffraction is the slight bending of light as
it passes around the edge of an object.
Visual Spectrum
• The part of the electromagnetic spectrum
we can see as color
Color is refection of light
• White is the reflection
of all colors
• Color a reflection of
one wave length
• Black is the
absorption of all color
Lens
Reduces size and flips
them over – your eyes
Magnifiesmagnifying glass