
Lecture Description
It is rather obvious that the greater the MASS - the greater the INERTIA of a body - the harder it is to put it into motion. That is: if a body of MASS M requires a FORCE F to give it "so much" motion - that is, so much acceleration - than a body of MASS 2M requires a FORCE 2F to give it the same motion. These truths are tied up in NEWTON'S SECOND LAW in the mathematical form F = ma. When now a body - held in the hand, say - is dropped to fall freely, the force urging it DOWNWARD is its weight. The motion it acquires - its acceleration downward - is now g. We call this the acceleration of gravity for short. Accordingly we write W = mg. Thus it is that F = ma and W = mg are analogous expressions.
A - We show two cars - a little one and a big one - masses M and 2M say. They are connected with a "spring" which pulls them together each with a force F. The mass M is urged to move faster than the mass 2M. Obviously. We thus write
Ma = F = mA
B - A mass M hangs on a scale. The scale reads the "weight" of M. If we accelerate the system upward the scale reads more. If we ac¬celerate the system downward the scale reads less. We write this F = Mg + Ma and F = Mg - Ma. A good question to ask is this: If we drop the whole thing and let it fall freely what will the scale read during the free fall? We find the answer by writing F = Mg - Ma and since a = g in this case the equation says F = Mg - Mg - which is ZERO. So the scale reads ZERO in free fall.
C - We show an array of DEMONSTRATIONS revealing Newton's Second Law:
1 - A tiny sphere falls at the same rate - with the same acceleration
as a BIG one. Although their masses are different the forces on them are different in
the same order. 'That is, if W = Mg so 2W = 2Mg.
2 - The cartoon showing two boys jumping on to a platform scale re-
veals Newton's Second Law. So cartoons often have good physics!
3 - Two men on the free ends of a rope over a pulley: If A climbs and
B just holds on B gets a FREE RIDE since whatever A does to the rope B feels.
4 - In the PARADOX OF FORCES a weight W pulls one way and another
equal weight W pulls the other way. What does the scale read? Not zero. Not 2W. It reads just W.
5 - When you ride an elevator Newton's Second Law acts in a very
clear way: Starting upward F = Mg + Ma - so your knees buckle. Or the bag in your hand feels heavier! Starting downward F = Mg - Ma - so your belly feels empty.
Thus it is that Newton's Laws of Motion play their roles in our everyday lives.
Course Index
- The Idea of the Center of Gravity
- Newton's First Law of Motion: Inertia
- Newton's Second Law of Motion: The Elevator Problem
- Newton's Third Law of Motion: Momentum
- Energy and Momentum
- Concerning Falling Bodies & Projectiles
- The Simple Pendulum and Other Oscillating Things
- Adventures with Bernoulli: Bernoulli's Principle
- Soap Bubbles and Soap Films
- Atmospheric Pressure
- Centrifugal Force and Other Strange Matters
- The Strange Behavior of Rolling Things
- Archimedes' Principle
- Pascal's Principle: The Properties of Liquids
- Levers, Inclines Planes, Geared-wheels and Other Machines
- The Ideas of Heat and Temperature
- Thermometric Properties and Processes
- How to Produce Heat Energy
- Thermal Expansion of Stuff: Solids
- Thermal Expansion of Stuff: Gases & Liquids
- The Strange Thermal Behavior of Ice and Water
- Heat Energy Transfer by Conduction
- Heat Energy Transfer by Convection
- Heat Energy Transfer by Radiation
- Evaporation, Boiling, Freezing: A Dramatic Adventure
- Miscellaneous Adventures in Heat
- The Drama in Real Cold Stuff: Liquid Nitrogen
- The Physics of Toys: Mechanical
- The Physics of Toys: Acoustic and Thermal
- Waves: Kinds of Properties
- Sound Waves: Sources of Sound & Pitch and Frequency
- Vibrating Bars and Strings: The Phenomenon of Beats
- Resonance: Forced Vibrations
- Sounding Pipes
- Vibrating Rods and Plates
- Miscellaneous Adventures in Sound
- Electrostatic Phenomena: Foundations of Electricity
- Electrostatic Toys, Part 1
- Electrostatic Toys, Part 2
- Adventures with Electric Charges
- Adventures in Magnetism
- Ways to "Produce" Electricity
- Properties and Effects of Electric Currents
- Adventures in Electromagnetism
- Further Adventures in Electromagnetism
- 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.