PHY 185, EXPERIENCING
THE PHYSICAL WORLD, X - 2
MORE 1 & 2D MOTION, MASS AND INERTIA
Today's hands-on experiences include more examples of motion "without
friction" using the air table, a video of "Games Astronauts Play" and a
number of short demonstrations illustrating inertia, acceleration, and
the influence of mass on acceleration. Most of these demonstrations can
be done in at home to the great delight and edification of your roommate.
1. Motion 1 & 2D.
The air table is available for a wide variety of 1 & 2-D motion
investigations. We did the "monkey-shoot" last time that demonstrates the
independence of the horizontal and vertical components of projectile motion.
Feel free to repeat it. The investigation of the effects of forces on the
path of a moving object can also easily be shown by collisions between
air table pucks. Uniform and non-uniform circular motion can be observed
by attaching a peg to the center of the table and causing a puck to orbit
the peg by attaching a string between the puck and the peg. UCM if the
table is level. Non-uniform CM if the table is tilted.
2. Video - "GAMES ASTRONAUTS PLAY"
This video shows astronauts lifting barbells while in a "weightless"
condition. The bar bends even though the bar bells are "weightless" in
the free-falling Skylab environment. The bar bells still have inertia and
so resist acceleration.
3. WHAT FORCE DOES THE SPRING BALANCE RECORD?
A spring balance is held vertically by means of a string. A string
holding a mass is attached to the hook on the spring balance. As expected,
the spring balance records the weight of the mass. Now the balance is placed
on a force table with the strings attached to the balance draped over two
pulleys. Equal masses are attached to the ends of the string. What does
the spring balance read?
4. THE PHYSICS METHOD OF PUTTING AN OLIVE IN A MARTINI
A 3" x 5" card is placed on top of a glass (the martini). A coin (the
olive) is placed on the card. A quick flick of your finger knocks the card
from under the coin; the coin falls in the glass.
5. ANOTHER EXAMPLE OF INERTIA
A dollar bill is placed on top of a bottle standing upright on a table.
On top of the dollar bill and over the opening to the bottle is placed
a coin. A second bottle with neck down is balanced on top of the bottle
on the table. The dollar bill is yanked from between the bottles, the coin
falls in the lower bottle, and the two bottles remain balanced on top of
each other.
6. STILL ANOTHER INERTIA DEMONSTRATION
A stack of coins (actually washers) are placed on a table top. Using
a metal ruler the bottom coin is knocked from under the remainder of the
stack without disturbing the other coins.
7. THE TABLE CLOTH TRICK
A cup of water is placed on top of a piece of paper toweling on top
of the lecture table. The cloth is yanked from under the cup without spilling
the water. Things to investigate include changing the amount of water in
the cup and using a styrofoam cup rather than a heavy plastic cup or glass
beaker.
8. STUDENTS CAN PRODUCE ACCELERATIONS LARGER THAN g
With your hand drawn back toward your shoulder, place a coin (or a
stack of coins) on elbow. Quickly pull your elbow from under the coins
and catch the coins accelerating at 9.80 m/s2. Your hand must
accelerate at a rate greater than g in order to catch the coins. Perhaps
inertia is also a factor. Think about it. What other ways could you demonstrate
this (safely)?
9. HAMMER AND LEAD BLOCK
With a lead block held in your outstretched hand and a hammer in your
other hand take a whack at the block with the hammer. You experience very
little feeling in your hand. How does inertia play a role here?
10. FORCE AND ACCELERATION
Masses are placed on a suspended platform that accelerates when a
force is applied to the platform. Compare the accelerations produced by
the same force when different masses are placed on the platform.
11. FINALLY - THE LAST INERTIA DEMONSTRATION
Threads are attached to the ends of a 2" x 4" x 12" block of wood.
Hold one string so that the block is suspended vertically and pull on the
bottom thread as follows: 1) a quick yank breaks the bottom thread leaving
the top thread intact, & 2) a steady pull breaks the top thread leaving
the bottom thread intact. Why???
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Address comments and suggestions to:
marcumsd@muohio.edu
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