SCI 206 - The Physics of How Things Work

Stefan Idziak

Estimated reading time: 22 minutes

Table of contents



A body in motion tends to remain in motion; a body in rest tends to remain at rest. How to describe motion?

  • Position: Where you are
  • Velocity: 2 parts:
    • 1.) How fast you are going (speed)
    • 2.) Which way you are going (direction)
  • Acceleration: How fast your velocity is changing: Speeding up, Slowing down, Changing direction

Newton’s law

  • First Law: An object that is not subject to any outside forces moves at constant velocity, covering equal distances in equal times along a straight line path.
  • Second Law: The force exerted on an object is equal to the product of the object’s mass times its acceleration. The acceleration is in the same direction of the force
    • F = ma
    • Easier to move a light object than a heavy one
  • Third Law: For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second objects exerts on the first object.


A man standing on the ground, not moving, no acceleration. By Newton’s second law, 0 force. Does not mean no gravity. Think of all forces, there’s normal force from floor pointing upwards. These two forces cancel out.

Consider falling ball. \(F=mg=ma \implies g=a\). Acceleration independent of mass. Feather falls as fast as a lead brick.


is ability to do work/damage. Two energy:

  • potential (stored): Higher has more potential energy.
  • kinetic (moving): Faster moving has more kinetic energy.


is how you transfer energy. work = force \(\times \) distance. If you are not pushing or it is not moving, then you are not working.


is tendency to continue moving in a certain direction. More momentum: Tend to win in collisions

Momentum = Mass \(\times \) Velocity


How to push a block. Gravity = normal force (block not falling). Push force - friction = ma (the block accelerate)

Friction slows the motion. Push force needs to be greater than friction for motion to occur. Friction only resists the motion.

Two kinds of friction

  1. Static friction: Not moving; Keeps the object in place (desk on the floor)
  2. Kinetic friction: while moving

Generally static friction is bigger than kinetic hard to get the motion started

Friction force does work \(\to \) HEAT

Circular Motion

Constant speed, but the direction of velocity changes. It is always accelerating. Thus requires an inward force, pointing towards the center of the circle. \(F=mv^2/R \). Bigger radius, smaller force. Bigger velocity, larger force.

Car going around a corner/circle. No strings, where does that inner force come from? friction between tire and road. Well, there’s force pushing me towards the center, but I feel I am being thrown out of the car while turning left. Is there a force pushing me out of the car? No. Imagine the seats are slippery. Then the person is moving straight but the car is going left by Newton’s first law.

Demo here shows the conservation of angular momentum. If it keeps spinning, it doesn’t fall. If it slows down, then gravity takes over.

Another demo shows momentum of inertia. We cannot change the mass, but we change the distribution: by moving mass further away/closer to the center. Putting hands and legs, it becomes faster.


Hold a bucket of water and rotate it around your head, the water doesn’t fall when the bucket is above your head. It goes fast enough, the force needed is greater than the gravity, then the water stays. If the gravitational force is smaller than that required for circular motion, the bucket will exert more force on the water to keep it moving in a circle.


Demo: wine glass with straw inside. Round circle speaker at the back, amplifier. We can see straw going back and forth, and finally the glasses breaks.

Travelling Waves

Pulse on a string.

there should be a image...

Can have a wave travelling down the string OR can have a standing wave.

Demo: fix a rope at one end and hold on the other end. The pulse hits that end and comes back upside down. If we pull tight, create more tension, it travels more faster. The pulse carries the energy.

  • Transverse Oscillations: String moves up and down forming a wave that travels along the string, similar to waves in the ocean
  • Longitudinal Oscillation:
    • Pressure (= Force divided by Area) is a force exerted over an area.
    • Strong force on a large area has same pressure as weak force on a small area
    • Sound is an example of this type of wave. Regions of low pressure and high pressure. Can see compression and expansion in different places.

Speed of a wave = Frequency \(\times \) Wavelength

Why does string move up and down? Restoring force from tension in the string

Doppler Effect

Change in Train Whistle.

  • Train not moving, Wavelength \(L \), Frequency \(f \)
  • Train moving right, towards you. Wavelength smaller, freq higher.
  • Train moving left, away from you. Wavelength larger, freq lower.

Speed of sound constant. Waves get compressed or expanded


  • String pulls the mass to the right
  • The mass keeps moving, past the bottom and off to the right
  • The string then pulls the mass to the left
  • The mass goes back and forth
  • Restoring force: always tries to bring the mass to equilibrium
  • Simple Harmonic Oscillator
  • Period of pendulum ( Time to go back and forth) only depends on length of the string
  • Long string, long period, low frequency

Harmonic Oscillator

Mass on a spring

  • Start with spring stretched
  • Spring pulls mass to the left
  • Spring gets squeezed and pushes the mass to the right
  • Spring applies a restoring force to the mass (tries to place the mass so the string is unstretched and relaxed
  • Frequency depends on mass and the stiffness of the spring
  • Bigger mass, lower frequency. Stiffer spring, higher frequency


Example: child on the swing and push the child. That’s pendulum. Because of the friction, eventually stop. If you take phys 112, then you will know the period changes if the amplitude becomes large enough.

You can’t push in any given time. Always push just as the swing moving away from you. Pushing does the work, which increase the mechanical energy. Here the kinetic energy gets increased, moving faster. At the top, lots of gravitational potential energy. If we push when it comes towards me, we decrease the kinetic energy.

Oscillators have a natural frequency at which they prefer to oscillate. This is the resonant frequency of the oscillator. If energy is applied to the oscillator at the same frequency, the amplitude of the oscillation will increase.

Standing Waves

string instruments. If string has two fixed ends, the waves will travel and hit the end and comes back flipped upside down. We got some waves travelling to the right, and some to the left. There are some interference. We see the sum of waves. String can’t move on two ends. For some specific freq, we see standing waves.

there should be a image...

Demo: 5 hz is the fundamental freq, first harmonic. 10 hz we see the second. 15 hz is the third, 3 antinodes.

In guitar, we got fret. Changing the fret, we can adjust the length of the string. Speed constant, wavelength different so frequency different (harmonics)

Wine Glass

The sound makes the glass vibrating. Plastic cup will not likely break because energy could loss in the cup. Little energy loss in good wine glass for every oscillation of the cup.

  • Rub finger along the rim of glass get friction
  • Stick / slip causes rim of wine glass to move
  • Size and shape of the glass determine the natural frequency
  • Rubbing finger adds energy and makes sound louder
  • Similar to bow on a violin

there should be a image...

Bowl of wine glass moves in and out like a water balloon. This moves air back and forth, producing a pressure wave SOUND

Sounds from a Tube

In wind instruments, sound is little different. In the pipe, we have standing waves of the actual air.

  • Pressure differences in tube provide a restoring force
  • Simple harmonic oscillator
  • Fundamental wavelength is twice length of tube
  • Blow air across bottom of tube
  • Adds energy to resonator, coupling complicated
  • Air moves in and out at the ends

Demo: freq depends the length of the column of air. The temperature and density of gas affect the speed of sound.

Sound from a beer bottle

Behaves like a mass on a spring. Air in a bottle is the mass. Air in the large part of the bottle provides the restoring forces as it expands and contracts.

Adding liquid to the bottle decreases the size of the air compartment, changes restoring force and therefore the frequency

Loud Speaker

there should be a image...

The pressure wave is created by the speaker cone moving back and forth.



Inside the air, we have oxygen molecules, nitrogen. Can be single atoms or molecules. All molecules are moving fast, and bouncing off each other. Different from liquid. Solid, molecules are regularly aligned.


Where does pressure comes from?

there should be a image...

We have a bunch of gas particles bouncing around the box. Some of them are bouncing off the walls in the box. Molecules have mass. When they hit the wall, there’s a collision, some force applied to the wall. So there’s a force pushing outwards on the box. Rather than talking about the force pushing the box, we talk about the pressure, which is \(force/area \). That’s a property of a gas, which is independent of the size of the box/area.

How we can change the pressure? We can change the pressure by changing the density. Or temperature. Higher the temperature, faster those particles are moving, more kinetic energy, faster motion, more force, greater pressure.

Density is how many particles I have in certain amount of volume. More particles inside the box, more collisions with the walls, more force, higher pressure.

Ideal Gas Law

Combine what we already know \[ PV = nRT \] where P is pressure, V is volume, n is number of moles, R is constant (fudge factor), T is temperature. Pressure is proportional to density of particles times temperature.


Why do we have atmosphere pressure. There’s gradual transmission between atmosphere and space. Gravity is giving us gravity air pressure.

there should be a image...

Let’s take a little box of air. Each stacks on each other. The gas has some weight.

What does pressure really mean?

  • Atmospheric pressure at sea level: 100,000 newtons per square meter, 100, 000 pascals (Pa) (~10,000 kg per square meter or 14.7 pounds per square inch)
  • Pressure at Denver: 83,000 Pa (12.2 psi)
  • Pressure at Chomolungma (Mount Everest): 33,000 Pa (4.9 psi)
  • At sea level, equivalent to a bus on top of you or 1 ton per square foot
  • Pressure difference between top and bottom of wing 2, 000 Pa (1/3 psi)

So we have ~10000kg air on us. Why we are not flattened into a pancake? It turns out air pressure presses us on all sides. Equally from all sides. We are made up of water. Water is compressible.

Hydraulic Pump

there should be a image...


there should be a image...

Archimedes’ Principle: An object partially or wholly immersed in a fluid is acted upon by an upward buoyant force equal to the weight of the fluid it displaces.

Consider part of the fluid: Total Force = force up – force down – force of gravity = 0

If the object has a different mass, the force of gravity will be different

  • if mass is greater than the fluid. Total Force is negative, object falls
  • if mass is less than the fluid. Total Force is positive, object goes up
  • if mass is the same Total Force = 0, object stays where is

Hot Air Balloon

Floating through the sky. Fire burner at the bottom, which heats the air. The bottom of the balloon is completely open. Air can come in and out. Why does hot air balloon go up? The density of the hot air balloon must be less than the density of the air. How’s that possible? The key is the fire. When we heat up the air inside the balloon, the temperature gets higher, the kinetic energy of the air particles gets larger. Then the pressure goes up, but the bottom is open. Air will then come out from the bottom. The density inside will decrease significantly.

pressure inside = pressure outside because of the hole on the bottom.

The density of the balloon then decreases. Pressure is proportional to density times temperature. Hot air has higher temperature and same pressure so density is lower. So weight of balloon is less than the air. Then balloon rises.

Regular Balloon

First, prof blows up a balloon. Pressure inside > pressure outside, because it needs to stretch balloon. We are doing work to inflate the balloon. Tension on balloon produces inward force, pressure of balloon provides outward force to balance.

Helium Balloon

Force due to gravity = weight of balloon + weight of helium

If weight of helium is less than weight of air (yes) and weight of helium + weight of balloon is less than weight of air (depends on balloon) then balloon goes up


A bubble is made up of soap and water. Surfactant molecules (SOAP)

there should be a image...

Why do bubbles pop? Water inside stablizes the bilayer. As water vaporizes, it starts to thin the membrane. Until the membrane is not strong enough, then it corrupts.

Air Cleaners and Xerox

Electric Charge & Force

Two types: Electron is negative, Proton is positive, (no smaller electric charges).

Benjamin Franklin (1700s): Silk rubbed glass is positive, Fur rubbed rubber is negative,

Charge is measured in Coulombs (C). 1 Coulomb = 6.28x1018 charges

  • Neutral objects have both negative and positive charges (an equal number of both)
  • Something positively charged has more positive charges than negative
  • Something negatively charged has more negative than positive

Coulomb’s Law \[ F= {kq_1 q_2\over R^2} \]


  • Power supply (battery) pushes current around the circuit
  • Battery keeps one side positive and other negative
  • Work is done in lighting the bulb
  • This work comes from the charges in the circuit which in turn comes from the battery
  • Voltage measures the potential energy of a unit of charge at a particular location

Higher voltage, positive charge more potential energy, releases more energy into the light bulb, light bulb glows better.


Dust is small and light, moving around in the air. Can be all sorts of different things: rock, dirt, organic matter in tiny pieces.

Soot: carbon, organic material that has been burned imperfectly, oily, greasy, tarlike

Ash: powdery non‐combustible residue of a fire

Basically neutral particles floating in the air

Then we have dust filters: Can filter dust by passing air through small pores: air goes through, large dust particles get trapped.

Two ways to stop the particles.

  • Inertial impaction. Imaging filter being lots of fibers intersecting each other, with pores in between. Larger pieces of dust have velocity, traveling in straight lines. At some point, it hit the filter and stick to it.
  • Diffusive motion/Brownian motion. Small particles zigging-zagging.

Not good with intermediate size particles. Small dust goes through clogging slows air flow, then air doesn’t move out.

Electronic Air Cleaner

Recall dust particles are neutrally charged. We could somehow charge the dust. For example, we give the dust negative charge. Then we pass the dust through positively charged cylinders. Then it sticks to the container. Then bang the wall to get the dust off.

How do we charge dust? Take a power supply, which can generate large voltage. Connect one end to ground, the other end to a pin (wire). Wire starts to emit charges, which spill out to the air. The charges repel each other and eventually end up on the tip where they are very close together. When the charge has built up to a large enough value, the electrons jump off the pin on their own.

Then corona discharge: A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage (from wiki). Potential energy is released when the electrons fly off the in: Light is emitted

Lightning Rod

is a stick sitting at the top of the building/tower. Attach that rod to the ground with a wire. Ground is a big source of charge: can release/store a lot of charge, and get charged. In a lighting storm, we get clouds, which are charged. Eventually, those charges want to go to different places: other clouds. We don’t want the lighting hit us/the house. So we make our ground less attractive to hit by lightning. Try to discharge any charge on the ground around us, which is where we use lightning rods. Any charge on the ground will go through wire and get corona discharge coming off the lightning rod, which will discharge the charge around the building. Then because it is neutral, less attractive for the big spark from the cloud to hit the rod.

Electrostatic Precipitator

  • neutral dust
  • Power supply between walls and corona wires
  • Corona discharge from the negatively charged wires releases electrons into the air that then charge the dust particles, making them negatively charged
  • This negatively charged dust is then attracted to the positive walls, trapping the dust
  • The electrostatic force is much stronger than gravity and any air currents

Other Dust Cleaners

Ion generator: Charges dust but does not trap it. Instead it sticks to the neutral walls of the room or the furniture. Make the room more dirty. Charged dust particle polarizes the neutral surface so that they attract

N95 Masks

Two kinds of standard face masks. One is like filter. Air goes through the fiber of the filter, common cloth. What goes through quite easily are intermediate sized of moisture.

N95: 95% gets blocked by it. Out layers block large pieces. Thick layer of filter: bunch of fibers. Then the key to N95 is the additional layer behind here, made up of particular type of polymer. Looks similar to the first one, but it is stiffer than the first thick layer. There are charges in these fibers. Then those intermediate sized particles can be electrostatically attached to the filter.

Xerox Machines

Ink is made up of toner particles (plastic particles that are coloured). When they melt, they stick to the paper. It works with electrostatics.

Photoconductor: in dark, insulator. In light, conductor. Lots of light, good conductor. Less light, less of a conductor (more resistance to charge flow)

  • Place charge on both sides of the photoconductor
  • illuminate one spot, that spot becomes a conductor, charge can move and gets neutralized
  • There is no isolated charge where the light shines

Laser Printer

there should be a image...

By KDS4444 - Own work, CC BY-SA 4.0, Link

  • Use laser to scan an image onto a photoconductive drum
  • This creates a charge image on the drum that can be covered with toner
  • This toner is then transferred to the paper, creating the image


Electric Circuits

How do charges move? Electrons orbit the nucleus Some electrons break free of the nucleus and can move freely through the metal. Conduction electrons.

In a conductor, the atoms inside release electrons, then electron is free to walk around the entire structure. Those free electrons can be moved around. We can use electric field to move electrons. Any time we have a current flow, we have the negative charges, electrons moving around the conductor.

In an insulator, charges don’t move. There are no free conductor electrons. All electrons are bound to the atoms or nucleus.

In an electric circuit, current moves from positive to negative. Actually electrons travel from - to +

Resistance and power

Resistance is a measure of the opposition to current flow in an electrical circuit.

Ohms Law: \(V=IR \). Voltage = current \(\times \) resistance

Constant voltage:

  • small resistance gives big current
  • big resistance gives small current

Power is energy released per time Big power, lots of energy released per second Small power, little energy released per second

\(P=VI \). Power = voltage \(\times \) current.

Thus \(P=I^2R \)

Magnet & Current & Electricity

Direct/Alternating Current: Direct current is what we have seen so far, current travels in one direction. For simple model, voltage difference is constant. Alternating current, direction of the current keeps changing. For example, \(V=k\cdot \sin(t) \). Different frequencies in different places.

Magnets: A little bar, one side north, one side south. North and north repel, south and south repel, north and south attract. You can make magnets out of magnetic material. You can also make magnetic field with a wire, like electromagnet, current in a coil. Magnet field around the earth.

Magnet and Electricity. Electric current can make magnetic field. Wind the wire around the soft magnetic material, then it can take things up. Hard magnetic materials: once magnetize it, it will stay magnetized. Soft magnetic materials: it doesn’t behave like a magnetic, paperclip is an example. Iron bar has magnetic domains that are normally randomly oriented when there is not external field. Domains oriented with the magnetic field from the coil grow (the others shrink) This amplifies the total magnetic strength of the electromagnet. When we remove the external magnetic field, soft magnetic material becomes randomized again.

Change magnetic field induces an electric field. Move permanent magnet in and out of coil to change magnetic field inside coil. Changing magnetic filed inside coil induces voltage across the coil. This voltage (and current) can power an electrical load (light bulb)

Lenz’s Law

(楞次定律) Current induced by a changing magnetic field always produces a magnetic field that opposes the change.

push magnet into coil \(\implies \) coil tries to push back. I need to do work, which is transferred into electrical energy. This can be used to make a generator.

there should be a image... Lenz’s Law: harder to turn when there is a large load

Another application: transformer: convert between different voltage.

there should be a image...

The ratio of the two voltages is the same as the ratio of the number of turns on the coils.

\[ V _ {in} / V _ {out} = \frac{\text{number of turns in primary}}{\text{number of turns in secondary}} \]

House Power (skipped)

Fuses and circuit breaker

Fuse wire heats (I2R) eventually melts and breaks the circuit \(\to \) protects the light bulb

Circuit Breaker, reusable

  • Bimetallic strip: two strips of metal that thermally expands at different rates
  • Bimetallic strip heats (I2R) bends and pulls the contacts apart protects the light bulb

They are designed to protect the house. The wires do not overheat, the houses do not burn down.

Ground Fault Circuit Interrupter (GFCI)

is designed to protect you from being hurt. wiki

If the currents are not perfectly no magnetic field is induced in the coil balanced (5mA difference), the coil will sense the difference and activate the solenoid, opening the switch and turning off the power, There are also mechanical pieces that make certain power can not turn on again unless the reset button has been pushed.

Live and neutral wires pass through a coil. If there is no leakage current (no short), the two currents cancel out and no magnetic field is induced in the coil.

Radio & TV

E & M review

Like charges repel, unlike charges attract.

Electric field lines move from +ve to -ve charges. A charged particle experiences a force along the field lines.

(new) the changes of electric field produces a magnetic field.


is a piece of wire, long pole.

Back to top

Copyright © 2017-2024 Sibelius Peng.