how do i make a free swinging pendulum?
im makeing it in wood shop and machine shop, i just need to know if there is any certain way i need to make it to make it work. it's going to be used like a timer to flip a light switch. i need to know who to put together the frictionless pivot, and how massive i need to make the swinging bob.
Bob can be any weight, the critical part is the length. 0.248 m will give you a 1 second period. But the bob has to be a lot heavier than the rest of the mechanism, like the rod and pivot, to minimize their source of error.
a lot depends on the accuracy you need. Good pendulum clocks have lots of mechanisms built into them to compensate for the changes in the length of the pendulum with temperature. The rod (between bob and pivot) should be metal so that it has a known coefficient of expansion.
also important is how you impart some energy to the pendulum so compensate for the normal slowdown due to friction and air resistance.
The reference has more info on these topics.
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Novel Plyometrics and Jump Training
Here Mel Siff gives us some insight into some of ingenius work back in South Africa regarding Plyometrics (or powermetrics as he liked to call them) - this is from the group that he formed in 2000 which is still going strong - health.groups.yahoo.com/group/Supertraining/
Drop jumps and various standing drills are so widely used as a means of
plyometric training that we often seem to forget that they classically may be
done in many other ways, such as using the common children’s swing that we
all used to play upon in public parks. With the latter, you simply arrange a
swing near a wall so that you can swing to different heights and strike the
wall with your feet to provide the rebound. For those who have our
textbook, “Supertraining” (Siff & Verkhoshansky 1999), see p275 for diagrams
of this exercise.
In this way you do not have to buy a big supply of costly “plyo” boxes - all
that you need is some rope, wood and something to hang your swing from and
you have a very controllable plyometric training device. With a little
ingenuity, you can even arrange a wooden wall at hand height that will allow
you to shove off with the hands, as well. (”Supertraining” p275 for
diagrams.)
Of course, you can make these devices more sophisticated and technical so
that you can carry out accurate biomechanical measurements with them. For
those who have our textbook, “Supertraining”, page 219, you will see a
diagram of a special plyometric bench that my senior engineering students
constructed at the University of the Witwatersrand (South Africa) in 1992.
The same students also constructed various upper and lower body swings with
built in load cells or force plates to measure the forces exerted during
impact.
Quite recently I came across the following research article that carried out
much the same sort of work that my engineers had been doing. I think that
many of you will find the results interesting.
Fowler NE, Lees A & Reilly T Changes in stature following plyometric
drop-jump and pendulum exercises. Ergonomics 1997 Dec; 40(12):1279-86
The aim of this study was to compare the changes in stature following the
performance of plyometric exercises using drop-jumps and a pendulum swing.
Eight male participants aged 21.7 +/- 1.8 years with experience of plyometric
training gave their informed consent to act as subjects. Participants
undertook two exercise regimens and a 15-min standing test in a random order.
The exercises entailed the performance of 50 drop-jumps from a height of 0.28
m or 50 pendulum rebounds (off a wall).
Participants were instructed to perform maximal jumps or rebounds using a
‘bounce’ style. Measurements of stature were performed after a 20-min period
of standing (pre-exercise), 2-min after exercise (post-exercise) and after a
20-min standing recovery (recovery). Back pain and muscle soreness were
assessed using an analogue-visual scale, at each of the above times and also
24 hours and 36 hours after the test. Peak torque during isokinetic knee
extension at 1.04 rad per sec was measured immediately before and after the
exercise bouts, to assess the degree of muscular fatigue.
Ground/wall reaction force data were recorded using a Kistler force platform
mounted in the floor for drop-jumps and vertically on the rebound wall for
pendulum exercises. Drop-jumps resulted in the greatest change in stature
(-2.71 mm), compared to pendulum exercises (-1.77 mm) and standing (-0.39mm).
Both exercise regimens resulted in a significant decrease in stature when
compared to the standing condition. Drop-jumps resulted in significantly
greater peak impact forces (p
3.2 x body weight, pendulum = 2.6 x body weight).
The two exercise conditions both invoked a small degree of muscle soreness
but there were no significant differences between either. Both exercise
regimens resulted in a non-significant decrease in peak torque, indicating a
similar degree of muscular fatigue. Based on the lower in height and lower
peak forces, it can be concluded that pendulum exercises pose a lower injury
potential to the lower back than drop-jumps performed from a height of 28 cm.
What does this mean for the coach? Well, first of all, it stresses that
there is a very real place for use of the PLYOMETRIC SWING as a safe and
effective form of explosive training. Secondly, it shows that the
*potential* for injury is greater with drop jumps, because of the greater
impact forces exerted on the whole body.
However, it needs to be pointed out that the body can adapt to such stresses,
but this potential for injury is a good reason why any drop jumps should be
prescribed carefully and intelligently, especially among novices whose
musculoskeletal system has not adequately adapted to the impulsive loading.
Regarding the experiment, it should also be pointed out that, if 50
repetitions are to be done, then it is essential that they be done in limited
sets of a few repetitions at a time, not like some sort of endurance
exercise. In the USA many coaches tend to prescribe far too many plyometric
repetitions at a time. More is not necessarily better. In the case of
plyometrics, quality is definitely more important than quantity.
About the Author
Dr Mel Siff
Author of Supertraining & Facts and Fallacies of Fitness
http://www.melsiff.com
