The Force What is it? Is It With Us? 9:33 AM 1 9:33 AM 2 1 2 - - PDF document

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9:33 AM 1

The Force

What is it? Is It With Us?

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Newton’s Laws of Motion

• An object remains at constant

velocity unless acted on by a net force

• The net force is equal to the mass

times the acceleration (change in the velocity with time)

• For every action, there is an equal

and opposite reaction

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Newton’s Law of Gravity

• Gravity always exerts a force
• The force causes an acceleration
• An object falling in Earth’s gravity field has constant

acceleration

• Unless air resistance is accounted for, when the

acceleration varies

• The weight of an object is the force of gravity on

the mass

• Weight is not the same as mass!

https://youtu.be/jwPc0kK9VHU

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Velocity and Acceleration

• Velocity is the speed in a

given direction

• Acceleration is the rate of

change of velocity with time

• They are different and can

even be in opposite direction!

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Cartoon Physics

• A character only falls under gravity when they

realize that they are about to fall.

• https://www.youtube.com/watch?v=wRSHzenjiNA

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What Actually Happens

• Parabolic (curved trajectories)
• Acceleration due to gravity constant
• Horizontal velocity constant
• Does not depend on the mass
• https://phet.colorado.edu/sims/html/projectile-

motion/latest/projectile-motion_en.html

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The Movie “Speed”: Bus Jump

https://www.youtube.com/watch?v=dKJa-KQNjQU

• Is it good physics?
• What minimum launch angle is needed to get

across a 16 metre gap at 116 km/h?

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Δ𝑦 = 16 𝑛 𝑤𝑗 = 116 𝑙𝑛. ℎ−1(70 𝑛𝑞ℎ) 𝜄 Angle of 4.5o needed

8

• Notice that the missing bridge section is horizontal,

so the bus would launch horizontally

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Bus has mysteriously tilted upward on launch!

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Superman

• Can fly
• Very strong
• X-ray vision

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Superman Flying

• Force of gravity pulls Superman downwards
• Must be a force acting upwards to make him hover.
• Must be generated by Superman

Gravity Anti-Gravity

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• Suppose Superman is flying through the air
• Needs a propulsive force as well as the antigravity

force

Gravity Anti-Gravity Air Resistance Propulsion

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Superman Catching Lois

• Superman catching her – deceleration causes

enormous force.

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• Newton’s Second Law restated:

𝐺𝑝𝑠𝑑𝑓 = 𝑛𝑏𝑡𝑡 × 𝑏𝑑𝑑𝑓𝑚𝑓𝑠𝑏𝑢𝑗𝑝𝑜

• Giving us

𝐺𝑝𝑠𝑑𝑓 = 𝑛𝑏𝑡𝑡 × 𝑑ℎ𝑏𝑜𝑕𝑓 𝑗𝑜 𝑤𝑓𝑚𝑝𝑑𝑗𝑢𝑧 𝑗𝑜𝑢𝑓𝑠𝑏𝑑𝑢𝑗𝑝𝑜 𝑢𝑗𝑛𝑓 𝐺𝑝𝑠𝑑𝑓 = 50 𝑙𝑕 × 20 𝑛/𝑡 0.05 𝑡 = 20000 𝑂 Equivalent to having 40 times your own weight land on you!

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Catching the Helicopter

Superman has antigravity AND antitorque

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Visible Laser Beams

Ocean’s Twelve - the burglar alarm https://youtu.be/mr834Cs9ncs Goldfinger – the Evil Genius with a death ray https://www.youtube.com/wa tch?v=Mx9z99YJ_7s

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LASER

• Light Amplification by Stimulated Emission of Radiation
• A photon (wave packet of light)

encounters an atom in an excited quantum state (electron in high energy level)

• If the energy of the photon matches

the difference in energy between two levels then

• Stimulated emission takes place

𝐹1 𝐹2

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LASER

• Light Amplification by Stimulated Emission of Radiation
• Stimulated emission:
• 2 identical photons
• Moving in the same

direction as the original one

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Helium Neon Laser

• Uses excited Helium atoms to collide with the Neon.
• The Neon does the emission

Red light 3𝑡 → 2𝑞

The excited level

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• The photons move in a straight line.
• You can only see the beam, if you look directly

along it (bad idea)

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• If there are particles that can scatter the photons,

then you can see the beam from the side

• Dust
• Aerosol droplets
• Water vapour
• Dry ice particles

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The Reality

• Industrial Laser for cleaning metal

https://www.youtube.com/watch?v=Q8q3DZB_l6M

• Beam visible because of rust particles being burned
• ff the surface

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Exploding Cars Falling Off Cliffs

• Dr No scene
• https://www.youtube.com/watch?v=RO88NI16g84&feat

ure=youtu.be

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• The fuel in a car is relatively safe
• An explosive mix requires a mix of the vapour and

air in a fairly narrow ratio

• Explosive mix 1.5 – 7 % fuel to air ratio
• Sparks needed
• Realistic car falling off a cliff

“Freedom” 1982 – Scott Hicks

• https://youtu.be/xmsnSRoWdUo

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Explosions

• In Movies, watch for:
• People outrunning explosions
• Simultaneous flash and sound
• Selective directions of the pressure wave resulting

from the explosion

https://www.youtube.com/watch?v=O9197iNEGSo

• A sudden release of energy

resulting in flames (usually) and a pressure wave moving outwards from the source taking fragments with it.

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The Speed of Sound versus the Speed of Light

• Not what happens in explosions in movies –usally

simultaneous.

• What actually happens:

https://youtu.be/D9CxYQ7OHB4

• See the flash first
• Then hear the sound

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• Speed of light 𝑑 = 300,000,000 𝑛𝑓𝑢𝑠𝑓𝑡 𝑞𝑓𝑠 𝑡𝑓𝑑𝑝𝑜𝑒
• Speed of sound in air 𝑤 = 330 𝑛𝑓𝑢𝑠𝑓𝑡 𝑞𝑓𝑠 𝑡𝑓𝑑𝑝𝑜𝑒

Light arrives very quickly The sound travels more slowly and arrives after the light

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Iron Man

• Explosion sound and flash
• No shockwave from the explosion
• Sparking bullets (they don’t)
• https://youtu.be/3o2ACEr9NmQ?t=85

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Running From Explosions

• A rapid release of chemical

energy into hot gases

• Forms a pressure wave which

moves rapidly

• Several thousand m/s

https://youtu.be/0R3N5ByTnIM?t=55 Usain Bolt 100 m in 9.58 s = 10.4 m/s

• You can’t outrun it…

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Pirate Treasure

Blackbeard the Pirate (1952) https://youtu.be/DwLnjw1ey7Q?t=2879

Moving very heavy objects

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Density

• “How much stuff there is in a given volume”
• The definition of density is

𝑒𝑓𝑜𝑡𝑗𝑢𝑧 = 𝑛𝑏𝑡𝑡 𝑤𝑝𝑚𝑣𝑛𝑓

• We measure this in grams per cubic centimetre or

kg per cubic metre

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• When the unit of the gram and the centimetre

were defined, the values were fixed so that the density of water would be 1 gram per cubic centimetre

Material Density (grams per cc) Balsa wood 0.12 Float on water Cork 0.25 Gasoline/petrol 0.72 Ice 0.92 Human body 0.995 Water (fresh) 1.0 Pearls 2.6 – 3.0 Sink in water Aluminium 2.7 Iron 8.0 Silver 10.4 Lead 11.3 Gold 19.7

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• Pearl density 2.6 g/cm3
• Estimate the volume of

the chest in the movie clip 𝑛𝑏𝑡𝑡 = 𝑒𝑓𝑜𝑡𝑗𝑢𝑧 × 𝑤𝑝𝑚𝑣𝑛𝑓 𝑊𝑝𝑚𝑣𝑛𝑓 = 60𝑑𝑛 × 40𝑑𝑛 × 40 𝑑𝑛 = 96,000 𝑑𝑛3 𝑛𝑏𝑡𝑡 = 250 𝑙𝑕 , 𝑏 𝑥𝑓𝑗𝑕ℎ𝑢 𝑝𝑔 550 𝑞𝑝𝑣𝑜𝑒𝑡 𝑛𝑏𝑡𝑡 = 2.6 × 96,000 𝑑𝑛3 = 249,600 𝑕𝑠𝑏𝑛𝑡

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Star Wars

• Is the Force really with us?

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The Kessel Run

• https://www.youtube.com/watch?v=fjYuw6zWk_Y

&feature=youtu.be

• Talking about a fast ship…
• Mentions it can do the Kessel Run in 12 parsecs

(implying fast)

• Unfortunately, the parsec is a unit of distance, not

time.

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• The parsec is the distance to

a star, if the parallax angle is 1 second of arc

• Equivalent to 3.26 light-years
• r
• 31 × 1012𝑙𝑛
• 31,000,000,000,000 𝑙𝑛

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Doing the Kessel Run

• https://www.youtube.com/watch?v=6LBCr4tTnPo
• Sound in space
• Visible laser beams
• Slow light pulses
• Banking Spacecraft

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Sound Waves

• Energy transfer between air molecules
• Can’t have sound where there are no molecules!

https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

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What Are Those Guns Firing?

• https://youtu.be/wtoHjGWc2s8?t=96
• Lasers can’t be seen from the sides
• Travel much too fast to be seen
• Charged Particles?
• Still travel very fast
• Tracer bullets?

Royal Navy training film 27624 https://youtu.be/ip3-4vhVnIE?t=634

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Atoms and Molecules

• An atom is the basic building block of matter
• The atom comprises of a small

nucleus with negatively charged electrons moving around it

• The nucleus has protons

(positively charged) and neutrons (no charge) in it

• Atoms may bond together to form molecules

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• A solid is lots of atoms strongly

bonded together.

• A liquid is lots of molecules fairly

close together, but only weakly

• bonded. They can move around,

and the fluid can flow

• A gas is a collection of atoms or

molecules which are very weakly bonded, usually at long distances from each other, and highly mobile

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Plasma: The 4th State of Matter

• The atoms are ionized, so there is a charged

“gas”

• https://youtu.be/EDD360PgDRg
• How do you keep the plasma in one place?
• Magnetic fields: but they would interfere

with nearby plasma charges!

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Spacecraft Banking Whilst Turning

• Aircraft need to do this to turn in the atmosphere
• In space there is no atmosphere, so the spacecraft

does not need to bank, just fire an engine in the appropriate direction

What they should do: Babylon 5 Star Fury https://youtu.be/nanfQ0mviaU

X-wing: https://youtu.be/ntG28o6DhcY

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• The lift force from the wing is partly directed to the

centre of the turn

• This pushes the aircraft round in a circle

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• Only works if there is

air flowing over the wings

• Useless in space

where there is no air

Giant Creatures

• Them!
• Giant ants (1954)
• https://youtu.be/KPw4U_IhWV4
• The chitin exoskeleton of an insect is not strong

enough when you scale up in size

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Giant Humans

• https://www.youtube.com/watch?v=h_ZDlJ61erY
• Blood Pressure in the legs
• Heat Regulation

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Hydrostatics

• Pressure increases as you go

further down in a fluid

• Due to the weight of the fluid

above you exerting an extra pressure 𝑄 = 𝑄𝑡𝑣𝑠𝑔𝑏𝑑𝑓 + 𝜍𝑕𝑒

Depth below the surface Density of fluid Acceleration due to gravity

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• Apply this to the human body
• Assume no fluid flow (small

correction needed for flow)

Measure blood pressure at the shoulder Similar height to the heart. Blood pressure at the legs must be higher, since the legs are lower down

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The Incredible Shrinking Man

• What happens to the mass?
• Smaller atoms?
• Smaller atomic bond lengths?

https://www.youtube.com/watch?v=VnDfrveCADo

• Shrinking the atom would

require different charges and masses of the protons, neutrons and electrons

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• Changing chemical bond

lengths to shrink the molecules?

• Again, change the mass and

charge of the atoms.

• Has nasty problems because

the shape of molecules is extremely important

• DNA spiral changes

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• If the weight stays the same, it all acts on a much

smaller area – the floor would deform!

𝑄𝑠𝑓𝑡𝑡𝑣𝑠𝑓 = 𝐺𝑝𝑠𝑑𝑓 𝐵𝑠𝑓𝑏

If the feet are ½ in both dimensions, then the area is ¼

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Creating New Elements

• What you have to do – have a nucleus with more

protons in it – the number of protons determines the chemical element

• Looks easy in the movies:
• https://youtu.be/y0brSA1cyzw

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Periodic Table

• http://www.rsc.org/periodic-table/history

https://www.gsi.de/en/researchaccelerators/research_an_overview/new_elements.htm

Creating Darmstadtium Element 110

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Conservation of Energy

• Energy cannot be created or destroyed.
• Maybe converted into different forms
• Actual bouncing ball
• Loses height on each bounce – energy loss
• Losses to sound, ball getting warmer
• https://www.youtube.com/watch?v=aY3TrpiUOqE

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The Absent Minded Professor: Flubber

• A ball that bounces higher when dropped!
• https://www.youtube.com/watch?v=QrwqIEIQuoY

&feature=youtu.be

• The Flubber would need to gain energy from

elsewhere – maybe remove heat from the floor

• Or the Flubber could cool down to compensate. But

then it would freeze eventually.

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Breaking Glass With Sound - Resonance

• Natural Frequency of Vibration
• External vibration at exactly the same frequency
• Causes high amplitude vibration
• Can break things

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• Some things oscillate with only
• ne natural frequency
• Mass on a spring
• Some things oscillate with a

harmonic series of frequencies

• Waves on a string

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• A wineglass has a set of natural

frequencies, when the glass vibrates

• Check this by tapping the glass
• Now use a sound wave of exactly

that frequency to make the

• scillation very big.

https://youtu.be/JiM6AtNLXX4

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Limitless Image Enhancement

• Blade Runner (1979)
• https://www.youtube.com/watch?v=hHwjceFcF2Q
• An example of quantization – the picture consists of

many discrete elements.

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3892 by 3265 pixels

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14 by 9 pixels

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And Finally: Spiderman

• https://youtu.be/nGAyUqgtUas
• Bad Physics: Spiderman must fall at the same rate

as Mary-Jane unless he is in powered flight

• Good Physics: Spiderman catches up to Mary Jane

so the impact is minimized

• Good Physics: Conservation of energy with the

elastic spider web to slow them down

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Enjoy the Movies: even if the physics is not so good

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