Newton's Third Law of Motion: Momentum 
Newton's Third Law of Motion: Momentum
by Prof. Miller
Video Lecture 4 of 46
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Date Added: February 5, 2015

Lecture Description

The only way I can walk forward is to have the Earth to push on! We discover this when we try to walk on sheer clear smooth ice! So - too - I can jump upward only by pushing downward on the Earth. Invoking Newton's Second Law and the Third Law we are led to say that if I go up the Earth must go down! and indeed this is so. But the Earth is so massive that ITS motion is not de¬tected. You see ME go up but you do not see the Earth go down - yet it most certainly does.
We show an array of DEMONSTRATIONS bearing on this Third Law.

A - A balloon attached to a little car is blown up. When the air comes out this way the car goes the other way.

B - A rocket goes best where "there is nothing" . The STUFF coming out be¬hind DOES NOT need the atmosphere to push against.

C - In the Case of The Two Cars (See Program *3) the force which the spring exerts on both cars is the same. We see that the little car of mass m acquires a higher velocity than the big car of mass M. Thus we can write mV - Mv which is to say that the momenta of both cars are the same. (Momenta is the plural of momentum).^

D - With the "Sputnik" demonstration the gas comes out one way and the vehicle goes the other way. We find the CO2 cartridge VERY COLD after the gas comes out! The gas, experiencing free expansion, drops in temperature. We shall learn more of this in later programs. You can discover this same thing by letting the HOT air out of a HOT tire on a HOT day. As the HOT air IN the tire emerges it feels COLD!

E - The elastic steel spheres on the track demonstrate CONSERVATION of LINEAR MOMENTUM. As many "go away" after the collision as collide before.
Toys can demonstrate in a pretty way the principles of Physics.
1 - A spinning propeller goes UP only because it PUSHES the air DOWN.
2 - The air in a blown-up balloon comes out one way; the balloon goes the
other way.
3 - Hero's Engine - known as early as the 5th century B.C. - shows the idea
of Newton's Third Law.
4 - To demonstrate that I PUSH BACKWARD when I WALK FORWARD I stand on a free-wheeling cart like a skate-board. As I move forward the cart goes backward. So does the Earth!

Course Index

  1. The Idea of the Center of Gravity
  2. Newton's First Law of Motion: Inertia
  3. Newton's Second Law of Motion: The Elevator Problem
  4. Newton's Third Law of Motion: Momentum
  5. Energy and Momentum
  6. Concerning Falling Bodies & Projectiles
  7. The Simple Pendulum and Other Oscillating Things
  8. Adventures with Bernoulli: Bernoulli's Principle
  9. Soap Bubbles and Soap Films
  10. Atmospheric Pressure
  11. Centrifugal Force and Other Strange Matters
  12. The Strange Behavior of Rolling Things
  13. Archimedes' Principle
  14. Pascal's Principle: The Properties of Liquids
  15. Levers, Inclines Planes, Geared-wheels and Other Machines
  16. The Ideas of Heat and Temperature
  17. Thermometric Properties and Processes
  18. How to Produce Heat Energy
  19. Thermal Expansion of Stuff: Solids
  20. Thermal Expansion of Stuff: Gases & Liquids
  21. The Strange Thermal Behavior of Ice and Water
  22. Heat Energy Transfer by Conduction
  23. Heat Energy Transfer by Convection
  24. Heat Energy Transfer by Radiation
  25. Evaporation, Boiling, Freezing: A Dramatic Adventure
  26. Miscellaneous Adventures in Heat
  27. The Drama in Real Cold Stuff: Liquid Nitrogen
  28. The Physics of Toys: Mechanical
  29. The Physics of Toys: Acoustic and Thermal
  30. Waves: Kinds of Properties
  31. Sound Waves: Sources of Sound & Pitch and Frequency
  32. Vibrating Bars and Strings: The Phenomenon of Beats
  33. Resonance: Forced Vibrations
  34. Sounding Pipes
  35. Vibrating Rods and Plates
  36. Miscellaneous Adventures in Sound
  37. Electrostatic Phenomena: Foundations of Electricity
  38. Electrostatic Toys, Part 1
  39. Electrostatic Toys, Part 2
  40. Adventures with Electric Charges
  41. Adventures in Magnetism
  42. Ways to "Produce" Electricity
  43. Properties and Effects of Electric Currents
  44. Adventures in Electromagnetism
  45. Further Adventures in Electromagnetism
  46. Miscellaneous and Wondrous Things in E&M

Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.


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