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 • Does not depend on the mass across a 16 metre gap at 116 km/h? • https://phet.colorado.edu/sims/html/projectile- motion/latest/projectile-motion_en.html 휃 Δ푥 = 16 푚 −1 푣푖 = 116 푘푚. ℎ (70 푚푝ℎ) Angle of 4.5o 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 • • Very strong Must be a force acting upwards to make him hover. • 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 • Needs a propulsive force as well as the antigravity Superman Catching Lois 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: 퐹표푟푐푒 = 푚푎푠푠 × 푎푐푐푒푙푒푟푎푡표푛 Catching the Helicopter • Giving us 푚푎푠푠 × 푐ℎ푎푛푔푒 푛 푣푒푙표푐푡푦 퐹표푟푐푒 = 푛푡푒푟푎푐푡표푛 푡푚푒 50 푘푔 × 20 푚/푠 퐹표푟푐푒 = = 20000 푁 0.05 푠 Superman has antigravity AND antitorque 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 Goldfinger – the Evil Genius • Light Amplification by Stimulated Emission of Radiation 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 퐸2 energy level) Ocean’s Twelve - the burglar alarm • If the energy of the photon matches https://youtu.be/mr834Cs9ncs the difference in energy between two levels then 퐸1 • 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 3푠 → 2푝 • 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 off the surface • https://www.youtube.com/watch?v=RO88NI16g84&feat 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 • An explosive mix requires a mix of the vapour and Explosions 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 • with it. Realistic car falling off a cliff 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 푐 = 300,000,000 푚푒푡푟푒푠 푝푒푟 푠푒푐표푛푑 Speed of Light • Speed of sound in air 푣 = 330 푚푒푡푟푒푠 푝푒푟 푠푒푐표푛푑 • 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 • Then hear the sound slowly and 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 • https://youtu.be/3o2ACEr9NmQ?t=85 • Forms a pressure wave which moves rapidly • Several thousand m/s • You can’t outrun it… Usain Bolt 100 m in 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 heavy objects • The definition of density is 푚푎푠푠 푑푒푛푠푡푦 = 푣표푙푢푚푒 Blackbeard the Pirate (1952) https://youtu.be/DwLnjw1ey7Q?t=2879 • We measure this in grams per cubic centimetre or kg per cubic metre 9:33 AM 31 9:33 AM 32 31 32 • When the unit of the gram and the centimetre • Pearl density 2.6 g/cm3 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 Material Density (grams per cc) clip Balsa wood 0.12 3 Cork 0.25 푉표푙푢푚푒 = 60푐푚 × 40푐푚 × 40 푐푚 = 96,000 푐푚 Gasoline/petrol 0.72 Float on water Ice 0.92 Human body 0.995 푚푎푠푠 = 푑푒푛푠푡푦 × 푣표푙푢푚푒 Water (fresh) 1.0 Pearls 2.6 – 3.0 Aluminium 2.7 3 Iron 8.0 푚푎푠푠 = 2.6 × 96,000 푐푚 = 249,600 푔푟푎푚푠 Sink in water Silver 10.4 Lead 11.3 Gold 19.7 푚푎푠푠 = 250 푘푔 , 푎 푤푒푔ℎ푡 표푓 550 푝표푢푛푑푠 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 • The parsec is the distance to Doing the Kessel Run 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 × 1012푘푚 • Slow light pulses • 31,000,000,000,000 푘푚 • 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://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html https://youtu.be/ip3-4vhVnIE?t=634 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, nucleus with negatively charged and the fluid can flow electrons moving around it • The nucleus has protons • A gas is a collection of atoms or (positively charged) and molecules which are very weakly 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 22/10/2019 Plasma: The 4th State of Matter Spacecraft Banking Whilst Turning • The atoms are ionized, so there is a charged • Aircraft need to do this to turn in the atmosphere “gas” • In space there is no atmosphere, so the spacecraft • https://youtu.be/EDD360PgDRg does not need to bank, just fire an engine in the • How do you keep the plasma in one place? appropriate direction • Magnetic fields: but they would interfere with nearby plasma charges! X-wing: https://youtu.be/ntG28o6DhcY What they should do: Babylon 5 Star Fury https://youtu.be/nanfQ0mviaU 9:33 AM 43 9:33 AM 44 43 44 • The lift force from the wing is partly directed to the centre of the turn Giant Creatures • This pushes the aircraft round in a circle • Only works if there is air flowing over the • Them! wings • Giant ants (1954) • Useless in space • https://youtu.be/KPw4U_IhWV4 where there is no air • The chitin exoskeleton of an insect is not strong enough when you scale up in size 9:33 AM 45 9:33 AM 46 45 46 Giant Humans Hydrostatics • https://www.youtube.com/watch?v=h_ZDlJ61erY • Pressure increases as you go further down in a fluid • Blood Pressure in the legs • Due to the weight of the fluid above you exerting an extra • Heat Regulation pressure 푃 = 푃푠푢푟푓푎푐푒 + 휌푔푑 Density of fluid Acceleration due Depth below the surface 9:33 AM 47 9:33 AM to gravity 48 47 48 8 22/10/2019 • Apply this to the human body • Assume no fluid flow (small The Incredible Shrinking Man correction needed for flow) https://www.youtube.com/watch?v=VnDfrveCADo Measure blood pressure at the shoulder • What happens to the mass? Similar height to the heart.