PHY 185,
EXPERIENCING THE PHYSICAL WORLD, X - 1
MOTION: 1 & 2D
Projectile motion is the dominant
theme of the premier meeting of the "toys" course. By now you've heard
several times from various sources that the horizontal and vertical components
of projectile motion are independent of one another. Today you watch a
couple of videos and play with some toys that will demonstrate to you by
direct observation that the horizontal and vertical components are indeed
independent. We prefer to do real experiments rather than watch videos
of classroom demonstrations. However, we can neither slow down time nor
reduce the local acceleration of gravity. The videos, on the other hand
contain slow motion sequences that effectively do just that.
1. SPRING GUN - TWO BALLS
FALLING
We use a spring-gun apparatus
that causes two balls to fall; one falls straight down, the other falls
in a parabolic trajectory. The point is to demonstrate the independence
of the horizontal and vertical components of projectile motion. Sorry,
no slo-mo, this one's in real time. We can't make g smaller, so we're stuck
with this demo going very quickly. That means that we must observe that
the components of the motion are independent in some way other than direct
visual observation. The fact that the balls hit the floor simultaneously
(your ears are the detectors!) shows that their vertical motions are independent
of their horizontal motions.
2. Video #1 - "PROJECTILE MOTION:
SIMULTANEOUS FALL"
This time our eyes are our
detectors. This video also shows two balls falling simultaneously using
the same apparatus as in the previous demonstration. Slow motion sequences
and stroboscopic photography show that the vertical motions of each ball
are identical while the one following the parabolic path exhibits a constant
horizontal velocity.
3. TWO COINS FALLING
This is the low cost version
of the previous two demonstrations. Use a finger to push a pair of coins
off the edge of a desk. Just nudge one coin off the edge while giving the
other a hefty horizontal velocity. Again, listen for them to hit the floor
simultaneously. This one's a cheapy that anyone can do - educate your roomate!
4. Video #2 - "PROJECTILE MOTION:
MONKEY AND HUNTER"
This one shows a more general
case than the first three. A projectile is fired toward an object (toy
monkey) that is suspended by an electromagnet. When the projectile leaves
the end of the air gun barrel, the electromagnet releases the monkey. Note
how the air gun is aimed toward the target. This time the projectile has
an initial velocity upwards rather than being projected horizontally. The
projectile hits the falling monkey, again showing the independence of the
perpendicular components of its motion - slow motion helps.
5. AIR TABLE VERSION - DILUTING
GRAVITY
This is the local version
of the previous video. We substitute air table pucks for the projectile
and the monkey and tilt the table slightly - i.e. we don't use all of g,
just a small portion of it - we "dilute" gravity. From watching the video,
how must the "hunter" aim at the "monkey"? When should the person that
releases the monkey let it go relative to the motion of the projectile
that is aimed at it? This one demonstrates the predictive aspect of physics.
Newton could guarantee that Halley's comet would return and you can guarantee
that the "monkey" will be hit if you aim at it (and that you'll miss it
if you don't aim at it). There are no accidents in nature.
6. WALK THIS WAY...
A SONAR (SOund Navigation
And Ranging) device that is used on some auto-focus cameras
is interfaced to a microcomputer. The "sonic ranger" can measure positions
as a function of time fairly accurately. We'll use the sonic ranger to
measure the position of a moving student for a few interesting cases.
This document last modifed on Tuesday, 01-Sep-1998 23:57:32 EDT
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Please address comments and suggestions to:
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