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

22/10/2019 The Force What is it? Is It With Us? 9:33 AM 1 9:33 AM 2 1 2 Newton’s Laws of Motion Newton’s Law of Gravity • An object remains at constant • Gravity always exerts a force velocity unless acted on by a net • The force causes an acceleration force • An object falling in Earth’s gravity field has constant acceleration • The net force is equal to the mass • Unless air resistance is accounted for, when the times the acceleration (change in the acceleration varies velocity with time) • The weight of an object is the force of gravity on the mass • For every action, there is an equal • Weight is not the same as mass! and opposite reaction https://youtu.be/jwPc0kK9VHU 9:33 AM 3 9:33 AM 4 3 4 Velocity and Acceleration Cartoon Physics • Velocity is the speed in a • A character only falls under gravity when they given direction realize that they are about to fall. • https://www.youtube.com/watch?v=wRSHzenjiNA • Acceleration is the rate of change of velocity with time • They are different and can even be in opposite direction! 9:33 AM 5 9:33 AM 6 5 6 1

22/10/2019 What Actually Happens The Movie “Speed”: Bus Jump • Parabolic (curved trajectories) https://www.youtube.com/watch?v=dKJa-KQNjQU • Acceleration due to gravity constant • Is it good physics? • Horizontal velocity constant • What minimum launch angle is needed to get across a 16 metre gap at 116 km/h? • Does not depend on the mass • https://phet.colorado.edu/sims/html/projectile- motion/latest/projectile-motion_en.html 𝜄 Δ𝑦 = 16 𝑛 𝑤 𝑗 = 116 𝑙𝑛. ℎ −1 (70 𝑛𝑞ℎ) Angle of 4.5 o needed 9:33 AM 7 9:33 AM 8 7 8 • Notice that the missing bridge section is horizontal, so the bus would launch horizontally Bus has mysteriously tilted upward on launch! 9:33 AM 9 9:33 AM 10 9 10 Superman Superman Flying • Can fly • Force of gravity pulls Superman downwards • Must be a force acting upwards to make him hover. • Very strong • Must be generated by Superman • X-ray vision Anti-Gravity Gravity 9:33 AM 11 9:33 AM 12 11 12 2

22/10/2019 • Suppose Superman is flying through the air Superman Catching Lois • Needs a propulsive force as well as the antigravity force • Superman catching her – deceleration causes Anti-Gravity enormous force. Propulsion Air Resistance Gravity 9:33 AM 13 9:33 AM 14 13 14 • Newton’s Second Law restated: Catching the Helicopter 𝐺𝑝𝑠𝑑𝑓 = 𝑛𝑏𝑡𝑡 × 𝑏𝑑𝑑𝑓𝑚𝑓𝑠𝑏𝑢𝑗𝑝𝑜 • Giving us 𝐺𝑝𝑠𝑑𝑓 = 𝑛𝑏𝑡𝑡 × 𝑑ℎ𝑏𝑜𝑕𝑓 𝑗𝑜 𝑤𝑓𝑚𝑝𝑑𝑗𝑢𝑧 𝑗𝑜𝑢𝑓𝑠𝑏𝑑𝑢𝑗𝑝𝑜 𝑢𝑗𝑛𝑓 𝐺𝑝𝑠𝑑𝑓 = 50 𝑙𝑕 × 20 𝑛/𝑡 = 20000 𝑂 Superman has antigravity AND antitorque 0.05 𝑡 Equivalent to having 40 times your own weight land on you! 9:33 AM 15 9:33 AM 16 15 16 Visible Laser Beams LASER • Light Amplification by Stimulated Emission of Radiation Goldfinger – the Evil Genius with a death ray https://www.youtube.com/wa • A photon (wave packet of light) tch?v=Mx9z99YJ_7s encounters an atom in an excited quantum state (electron in high 𝐹 2 energy level) Ocean’s Twelve - the burglar alarm • If the energy of the photon matches the difference in energy between two https://youtu.be/mr834Cs9ncs levels then 𝐹 1 • Stimulated emission takes place 9:33 AM 17 9:33 AM 18 17 18 3

22/10/2019 LASER Helium Neon Laser • Light Amplification by Stimulated Emission of Radiation • Uses excited Helium atoms to collide with the Neon. • The Neon does the emission • Stimulated emission: The excited level • 2 identical photons Red light 3𝑡 → 2𝑞 • Moving in the same direction as the original one 9:33 AM 19 9:33 AM 20 19 20 • If there are particles that can scatter the photons, • The photons move in a straight line. then you can see the beam from the side • You can only see the beam, if you look directly • Dust along it (bad idea) • Aerosol droplets • Water vapour • Dry ice particles 9:33 AM 21 9:33 AM 22 21 22 The Reality Exploding Cars Falling Off Cliffs • Industrial Laser for cleaning metal https://www.youtube.com/watch?v=Q8q3DZB_l6M • Dr No scene • Beam visible because of rust particles being burned • https://www.youtube.com/watch?v=RO88NI16g84&feat off the surface ure=youtu.be 9:33 AM 23 9:33 AM 24 23 24 4

22/10/2019 • The fuel in a car is relatively safe Explosions • An explosive mix requires a mix of the vapour and air in a fairly narrow ratio • A sudden release of energy • Explosive mix 1.5 – 7 % fuel to air ratio resulting in flames (usually) and a • Sparks needed pressure wave moving outwards from the source taking fragments • Realistic car falling off a cliff with it. https://www.youtube.com/watch?v=O9197iNEGSo “Freedom” 1982 – Scott Hicks • In Movies, watch for: • https://youtu.be/xmsnSRoWdUo • People outrunning explosions • Simultaneous flash and sound • Selective directions of the pressure wave resulting from the explosion 9:33 AM 25 9:33 AM 26 25 26 The Speed of Sound versus the • Speed of light 𝑑 = 300,000,000 𝑛𝑓𝑢𝑠𝑓𝑡 𝑞𝑓𝑠 𝑡𝑓𝑑𝑝𝑜𝑒 Speed of Light • Speed of sound in air 𝑤 = 330 𝑛𝑓𝑢𝑠𝑓𝑡 𝑞𝑓𝑠 𝑡𝑓𝑑𝑝𝑜𝑒 • Not what happens in explosions in movies – usally simultaneous. Light arrives • What actually happens: very quickly https://youtu.be/D9CxYQ7OHB4 The sound • See the flash first travels more slowly and • Then hear the sound arrives after the light 9:33 AM 27 9:33 AM 28 27 28 Iron Man Running From Explosions https://youtu.be/0R3N5ByTnIM?t=55 • Explosion sound and flash • No shockwave from the explosion • A rapid release of chemical • Sparking bullets (they don’t) energy into hot gases • Forms a pressure wave which • https://youtu.be/3o2ACEr9NmQ?t=85 moves rapidly • Several thousand m/s Usain Bolt 100 m in • You can’t outrun it… 9.58 s = 10.4 m/s 9:33 AM 29 9:33 AM 30 29 30 5

22/10/2019 Pirate Treasure Density • “How much stuff there is in a given volume” Moving very • The definition of density is heavy objects 𝑒𝑓𝑜𝑡𝑗𝑢𝑧 = 𝑛𝑏𝑡𝑡 𝑤𝑝𝑚𝑣𝑛𝑓 Blackbeard the Pirate (1952) • We measure this in grams per cubic centimetre or https://youtu.be/DwLnjw1ey7Q?t=2879 kg per cubic metre 9:33 AM 31 9:33 AM 32 31 32 • Pearl density 2.6 g/cm 3 • 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 • Estimate the volume of centimetre the chest in the movie clip Material Density (grams per cc) Balsa wood 0.12 = 96,000 𝑑𝑛 3 𝑊𝑝𝑚𝑣𝑛𝑓 = 60𝑑𝑛 × 40𝑑𝑛 × 40 𝑑𝑛 Cork 0.25 Gasoline/petrol 0.72 Float on water Ice 0.92 𝑛𝑏𝑡𝑡 = 𝑒𝑓𝑜𝑡𝑗𝑢𝑧 × 𝑤𝑝𝑚𝑣𝑛𝑓 Human body 0.995 Water (fresh) 1.0 Pearls 2.6 – 3.0 Aluminium 2.7 𝑛𝑏𝑡𝑡 = 2.6 × 96,000 𝑑𝑛 3 = 249,600 𝑕𝑠𝑏𝑛𝑡 Iron 8.0 Sink in water Silver 10.4 Lead 11.3 Gold 19.7 𝑛𝑏𝑡𝑡 = 250 𝑙𝑕 , 𝑏 𝑥𝑓𝑗𝑕ℎ𝑢 𝑝𝑔 550 𝑞𝑝𝑣𝑜𝑒𝑡 9:33 AM 33 9:33 AM 34 33 34 Star Wars The Kessel Run • Is the Force really with us? • 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. 9:33 AM 35 9:33 AM 36 35 36 6

22/10/2019 Doing the Kessel Run • The parsec is the distance to a star, if the parallax angle is 1 second of arc • https://www.youtube.com/watch?v=6LBCr4tTnPo • Equivalent to 3.26 light-years • Sound in space or • Visible laser beams • 31 × 10 12 𝑙𝑛 • Slow light pulses • 31,000,000,000,000 𝑙𝑛 • Banking Spacecraft 9:33 AM 37 9:33 AM 38 37 38 Sound Waves 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? • Energy transfer between air molecules • Can’t have sound where there are no molecules! Royal Navy training film 27624 https://youtu.be/ip3-4vhVnIE?t=634 https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html 9:33 AM 39 9:33 AM 40 39 40 Atoms and Molecules • A solid is lots of atoms strongly bonded together. • An atom is the basic building block of matter • A liquid is lots of molecules fairly close together, but only weakly • The atom comprises of a small bonded. They can move around, and the fluid can flow nucleus with negatively charged electrons moving around it • The nucleus has protons • A gas is a collection of atoms or molecules which are very weakly (positively charged) and bonded, usually at long neutrons (no charge) in it distances from each other, and highly mobile • Atoms may bond together to form molecules 9:33 AM 41 9:33 AM 42 41 42 7