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| Author |
Message |
dang
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physics
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Aug 9 06:17 UTC 1994 |
enter questions about physics here, and i or someone else will do our best
to help you
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| 85 responses total. |
dang
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response 1 of 85:
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Aug 9 06:20 UTC 1994 |
i get to ask the first question of the conf! here goes. if speed is the
dirivitive of distance, and v' is acceleration, what is a', or the rate of
change of acceleration? this has been bugging me, as i know there is such
a thing. i use it in my car all the time. i accelerati, and i accelerate
faster and faster , until i reach a high ehough rpm, and then shift and
start over. so, what is the dirivitive of acceleration, or the rate of
change of acceleration?
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rcurl
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response 2 of 85:
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Aug 9 06:36 UTC 1994 |
The expression "accelerate faster and faster" may not mean that the
acceleration is increasing, though the speed is. What you are asking about
is the first derivative of acceleration, which is the second derivative of
velocity (and the third derivative of position with respect to time).
If you record your speed as you accelerate, you can estimate the
acceleration. Say, plot the speed vs. time. If the line is straight, your
acceleration is constant. If it is concave downward, the acceleration is
decreasing, even though the speed is increasing. It is possible that the
curve would be "S" shaped, which could mean that as the speed starts to
increase, the engine efficiency increases, so initially the acceleration
increases, but eventually friction will take over, and the acceleration
will decrease. (It might be easier to measure the times at which you are
going 10, 20, 30, etc mph, and plot that.)
The rate of increase in acceleration will also be felt as in increasing
pressure of the seat on your back - or, how far you are pushed back into
the seat. If you feel a constant force, your acceleration is constant. I
have very definitely noticed the "S" shaped pattern of acceleration when
taking off in an airplan, where engine efficient does increase very
dramatically as speed starts to increase.
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dang
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response 3 of 85:
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Aug 9 06:43 UTC 1994 |
i realize all this. i just finished physics mechanics, and we had many
hours of this kind of thing. what i was asking, if i was unclear, is what
the first dirivitive of acceleration is *called*, other than the rate of
change of acceleration. (one defination of dirivitive)
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rcurl
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response 4 of 85:
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Aug 9 06:54 UTC 1994 |
I do not recall ever hearing a separate *name* for the acceleration of the
acceleration.
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aruba
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response 5 of 85:
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Aug 10 03:15 UTC 1994 |
I have heard a' referred to as the *jerk*, and have been told that it is
really high jerk which hurts you when you're in a car accident, not high
speed or high decelleration.
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rcurl
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response 6 of 85:
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Aug 10 06:51 UTC 1994 |
Can you cite a source for that? Its not in any reference I have at hand.
It is a very descriptive term for a', though.
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aruba
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response 7 of 85:
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Aug 10 13:52 UTC 1994 |
My source was a math TA peer of mine. I'll write to him and see if he
knows a book it's in.
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dang
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response 8 of 85:
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Aug 11 03:33 UTC 1994 |
sounds about right. i was under the impression that it was wiplash, or in
other words, inertia. still, it could quite conceivably be both.
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rcurl
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response 9 of 85:
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Aug 11 06:18 UTC 1994 |
Inertia is a well, and anciently, defined physical concept. Wiplash
involves differences of acceleration of components of a flexibly coupled
body. However, the term "jerk" has some drawbacks. For example, someone in
a test vehicle might say, "We sure have a lot of jerks here."
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srw
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response 10 of 85:
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Aug 11 06:28 UTC 1994 |
I, too, have heard "jerk" used to describe da/dt.
I find it difficult to believe that da/dt is relevant to collision injuries.
The actual injury is done by a (not da/dt and not v), although the amount of
'a' is closely related to the 'v' you were doing before the collision,
since the collision reduces v to 0 in the same short delta-t.
So for collision purposes, v, a, and da/dt are kinda proportional (t
being roughly constant, though not quite, depending on car crumple factors).
The force on the body parts is the injury inducement factor, and
f=ma and m is constant, so I'd say the injury was proportional to a.
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rcurl
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response 11 of 85:
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Aug 11 06:35 UTC 1994 |
Going from a low rate of deceleration to a high one causes differential
displacement of body parts, depending on their mass, elasticity, etc.
It may be that a *sudden* differential displacement may cause damage
that a slower differential displacement would not, perhaps because
of the ability of the body to respond at a finite speed (reaction time).
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aruba
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response 12 of 85:
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Aug 11 12:18 UTC 1994 |
We probably should try not to get too sidetracked here. This is the tutoring
conference after all. But since no one's asking questions ...
Re #10: I can imagine that if I were to put a wieght on top of someone,
then gradually increase its mass, the mass could get quite big before any
damage would be done. Thus the force, when increasing slowly, is more
tolerable. If however you were to increase the force quickly, by adding a
whole lot of weight all at once, that would do some damage, I think. I'm
just speculating here, I haven't tried it. :)
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aruba
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response 13 of 85:
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Aug 12 03:38 UTC 1994 |
I asked my ex-peer and he said he heard the term "jerk" from a physics TA
he knew, and he doesn't have a reference for it. :(
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dang
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response 14 of 85:
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Aug 12 03:57 UTC 1994 |
ask a simple question... :)
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rcurl
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response 15 of 85:
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Aug 12 04:04 UTC 1994 |
I look upon this item as a room full of physics tutors. We are just
chatting, waiting for a tutee to come in. We need to advertise
tutoring, I guess, and tell people that have questions to just
interrupt. Re #13: instructors generally hope the students don't
hear when they mumble "jerk" ;->.
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dang
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response 16 of 85:
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Aug 13 00:19 UTC 1994 |
this is true. *HEY!! IF YOU HAVE A QUESTION, BUTT IN!! WE'RE JUST
PLAYING AROUND UNTIL SOMEONE ASKS A QUESTION!!!* there. i hope that
helps. also, i'll go remention it in agora.
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popcorn
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response 17 of 85:
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Aug 14 17:06 UTC 1994 |
This response has been erased.
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aruba
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response 18 of 85:
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Aug 21 08:09 UTC 1994 |
I suppose that's an alternate definition. :)
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carl
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response 19 of 85:
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Aug 22 01:37 UTC 1994 |
Ok. I've got a question to get ya'll going.
What are the parts of the unified theory and what's missing at
this point?
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dang
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response 20 of 85:
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Aug 23 02:42 UTC 1994 |
definately gravity, and i think time too. what else?
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carl
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response 21 of 85:
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Aug 27 21:42 UTC 1994 |
I know there's seven parts, let me see if I can remember most:
-Mass/energy
-Gravity
-Electromagnetism
-Strong nuclear force
-Weak nuclear force
I remember reading about this in Steven Hawkings' book. That was a
while ago and I don't recall all the details.
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dang
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response 22 of 85:
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Sep 9 20:33 UTC 1994 |
Ditto on Steven Hawking.
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