Download Physics MCAS Study Guide Motion and Forces Distance

Motion and Forces
Physics MCAS Study Guide
Distance- how far an object went, in any direction (scalar), Ex: 230 miles
Displacement- how far an object went from where it started. Direction matters! (vector), Ex: 20m North
Speed is rate of change of distance (distance/time)
It is how fast an object went. Direction doesn’t matter (scalar), Ex: 12m/s
Velocity is the rate of change of position (displacment ÷ time) (m/s, km/h)
It is how fast, and what direction an object is moving (vector), Ex: 12m/s South
Acceleration is the rate of change of Velocity (change in velocity ÷ time) (m/s2)
Graphs of motion:
3 Ways Accelerate:
-slow down
-go faster
-change direction
Position vs time
The slope of a d-t graph is velocity.
Velocity vs time
The slope of a v-t graph is acceleration.
Vectors are quantities that require a direction to be fully understood. Ex: 35N left
Scalars are quantities described only by magnitude (how big). Do not need direction. Ex: 50J
Force- is a push or a pull measured in Newtons.
(Force is a vector quantity)
A free-body diagram is a vector diagram of all the forces acting on an
object.
If all the forces on an object add to zero, the forces are balanced and the
object will not change its motion (will stay at rest, or stay moving at a
constant speed in a straight line).
Forces in opposite direction subtract from one another.
Forces in same direction are added together
Newton’s 1st Law of Motion (Law of Inertia)
An object at rest will stay at rest, and an object in motion will stay with the same motion, unless an
unbalanced force acts upon it.
Inertia- Objects resistance to changes in motion. The more mass, the more inertia something has
Newton’s 2nd Law of Motion:
If all the forces on an object add to a non-zero value, the object will accelerate, according to F = ma.
The greater the force, the more acceleration. The more mass, the less the acceleration. More mass
requires more force to accelerate.
Friction is a force that opposes motion.
Static friction needs to be overcome to start an object moving.
sliding (kinetic) friction needs to be overcome to keep an object moving at constant velocity.
Static friction is always greater than kinetic friction, so it is always harder to start an object moving
than keep it in motion.
Newton’s 3rd Law of Motion:
For every force there is an equal and opposite force (reaction force).
Circular Motion:
An inward force is required to keep something in circular motion.
Centripetal Force- a force toward the center that keeps an object in circular
motion (an inward force)
Law of Gravitation:
Force of gravity is your weight. Weight = mg gravity on Earth = 10m/s2
The more mass two objects have, the greater the force of gravity between them.
The more distance between the two objects, the force of gravity between them is much less.
Energy and Momentum
Momentum is the product of mass times velocity. Unit- kg m/s
p = mv
The more mass, the more momentum
The faster, the more momentum
The Conservation of Momentum says that the total momentum of a system will stay constant. So the
momentum before a collision or explosion will be the same as the momentum after.
Inelastic Collision- objects stick after colliding
Elastic Collision- the objects bounce off one another after colliding
Work is a force over a distance. Measured in Joules (J). No distance, then there is no work
Power is the rate at which work is done, so it is work ÷ time. If the same work is done faster, it
requires more power. The unit of Power is Watts
Gravitational Potential Energy is the energy an object has due to its position. The higher up it is
the more potential energy it has. To calculate PE: multiply mass times g times height.
Kinetic Energy is the energy of motion. The faster an object is moving the higher its kinetic energy.
KE = ½ mv2.
Energy can change from PE to KE to PE, etc. For example, on a rollercoaster, you have maximum
potential energy at the top of the hills (highest height) and maximum kinetic energy at the bottom of
the hills (highest speed).
Energy is conserved! Energy start with = energy you end up with.
Heat
Heat is the flow of thermal energy, which always flows from hot to cold.
As a substance heats up (absorbs thermal energy) its molecules move faster and get farther apart.
Remember, molecules moving faster also means kinetic energy increases.
The amount of heat transferred is given by Q = mc∆T.
Q is the amount of heat that is transferred, m is mass, c is specific heat, and ∆T is change in
temperature.
Lower specific heat, easier to heat up.
More mass, harder to heat up.
Methods of heat transfer:
Conduction- transfer of heat by direct contact
Convection- “heat rises”. Transfer of heat by movement of fluids. Warm rise, cool sink.
Radiation-heat that is transferred by electromagnetic radiation (infrared rays). Only heat transfer
that can travel through a vacuum. Travels at speed of light.
Phase changes occur when substances are at their melting/freezing or boiling/condensing
temperatures and energy is absorbed or released. No temperature change occurs during a phase
change.
Evaporation is a cooling process (as water evaporates off you skin you are cooled)
Condensation is a warming process (you can be very badly burned when steam condenses on your
skin).
WAVES
Waves transfer energy.
Period (T)- time it takes for one complete wave. measured in seconds.
These two quantities are inverses of each other :
T = 1/f
f = 1/T
Amplitude- how high or low a wave is from rest
Frequency (f)- # of waves per second. Unit- Hz (1/seconds)
Wavelength- how far apart the waves are (measured crest to crest)
Relationship between frequency and wavelength is inverse
As frequency increases, the wavelength gets shorter!
Transverse wave- wave that vibrates up and down (perpendicular) as it moves across (picture a)
Longitudinal wave that vibrates left to right (parallel) as it moves across (picture b)
Sound waves are longitudinal.
Sound travels fastest in solids, then liquids….slowest in gases.
Doppler Effect
Apparent change in pitch of sound as object moves
Girl hears a low pitch because the waves she hears have a lower frequency
Boy hears a high pitch because the waves he hears have a higher frequency
Mechanical waves- waves that need a medium, like sound or water waves.
Mechanical waves cannot travel through a vacuum.
The speed of a mechanical wave is determined by the properties of the medium.
Electromagnetic wavesDo not need a medium to travel.
Travel at the speed of light. They travel at 3 x 108 m/s.
Includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.
ROYGBIVOrder of visible light spectrum
by wavelength, frequency, and
energy
Long Wavelength
Low Frequency
Low Energy
Shorter Wavelength
High Frequency
High Energy
As frequency increases, the energy increases! Frequency and Wavelength change opposite each other.
Law of Reflection
Angle of Incidence = Angle of Reflection
Refraction-happens when a wave bends due to the difference in wave speed
in different materials. Wave bends toward the normal if wave travels into a
more dense medium.
Electricity and Magnetism
Two charges, + and If something has a – charge, it has more electrons than protons, so it has gained electrons.
If something has a + charge, it has more protons than electrons, so it has lost electrons. NOTE:
substances cannot gain or lose protons.
Like charges repel
opposite charges attract.
The force that causes this is increased by more charge. The force is decreased by more distance.
Ohms Law:
V = IR
Voltage- energy to push the current (Volts)
Current is the flow of charge - Units of current – amps (A)
resistance – stops current from flowing (Ohms)
Conductors- allow flow of electricity, Ex’s: metals like copper
Insulators- do not allow electricity to flow, Ex’s: wood, plastic, rubber, air, etc.
Power is current x voltage
P = IV
Circuit symbols:
wire
battery
Light bulb
resistor
switch
series circuit- all components one after another on a single wire. (circuit pictured below on left)
if one lightbulb goes out, all others will go out- circuit is broken.
In a parallel circuit- components on parallel wires (circuit pictured below on right)
if one lightbulb goes out, the others can remain lit.
Electromagnets:
Changing magnetic field produces an electric current (this is how generators work)
Changing electric current produces an magnetic fields (this is how electromagnets work)
3 Ways to make an electromagnet stronger:
Add an iron core (nail)
More turns in a coil
More current (stronger batteries or more batteries)