12/10 Physics question: When one lifts weights, s/he exerts more forces,
hence the pressure the ground s/he stands on receives more
pressure. Is that correct? My friend tried to tell me it is
rather because the object (the weights) are moving, it is the
acceration that translates into greater force hence greater
pressure. But shouldn't the object as a whole (the weights +
the weight lifter) be considered a still object since the area
touching the ground isn't moving? Any url reference to this
would be great.
\_ pressure is force per area, so if the are is fixed(the area
of the bottoms of your shoes if you are standing), then the
force is your weight plus the weight of the weights plus
the added force from the acceleration of the weights. this
added force is the mass of the weights plus your arms(measuresd
in slugs) times the acceleration you apply. if you don't bielive
\_ what do you mean measured in slugs? It doens't matter
what units you use. English units are confusing. Best
to use kg to measure mass, but slugs will work too.
\_ right. i was just making the point that slugs
are mass and pounds are force. IF you use kilograms,
you'd better use Newtons, which most people are
not familiar with.
that this aceleration adds force, accelerate the weights as
fast as you can, and feel the burn in the knees.
to keep the same force, but change preassure on the floor, just
change area: stand on tiptoes or sit down.
\_ You can also do the experiment by lifting dumbbell while standing
on a bathroom scale. The reading is roughly the same as the
force (although expressed in mass units) between your feet and
the ground when the scale is not there. Watch the reading
becomes higher after you grab the dumbbells and hold them steady.
Then watch the reading changing when you accelerate and
decelerate the dumbbells in the vertical direction.
\_ I understand f=ma. But ultimately, it's the weight lifter
that needs to exert more force. The acceration of the
weights is caused by the extra force the lifter has to put
out. My friend's argument is that the accerleration generates
more force, but isn't it really the oher way around in this
case? The fact of the weights moving at an accerlerated speed
and the greater pressure are both caused by the more force
exerted from the lifter, no?
\_ Don't confuse energy and force. I can slump on the sofa
like a big pig not spending a single calorie of energy,
but my fat butt would still be exerting a force on the
sofa. Remember Energy = Force x Distance Moved. I
am a lazy bum who don't like to move, and AnyForce x Zero
is still equal to Zero.
\_ hmm. I am more confused with weight v.s. force.
Isn't weight force in the first place? _/
Help me settle the bet here. My friend's arguement
is weight changes as objects are being lifted. But
my argument is it's the force that's changing, not
weight. Isn't weight simply the gravitational pull
on the mass? In this case, aren't both constant?
\_ What the bathroom scale usually measures is the
gravitational attraction (a force) between your mass
and the fat lump of mass we call the Earth. When you
hold onto two dumbbells like some dumb body builder
and stand on the bathroom scale, it is now measuring the
the gravitational attraction between "your mass
plus two dumbbells" and the Earth. When you are
further accelerating the two dumb bells upwards,
the bathroom scale is now measuring the above plus
an additional force exerted by the accelerating
dumbbells on your body. Remember "Action Reaction".
That's why when measuring yourself on a bathroom scale,
you try to be naked, so that there isn't any
extra mass, and you try not to jump up and down.
What your friend refers to as "weight" is what
is measured on the bathroom scale. What you
refer to as "weight" is the gravitational
attraction between your mass and the Earth's mass.
As seen above, the two of you are referring to
different things. The two are only one and the same
when you are naked and not jumping up and down.
I think I have been trolled. Oh well.
\_ Your friend's thinking is inherently flawed, but explain
it to him this way:
When you talk about a person's weight, you imply that
that person isn't moving. If he jumps up while on a
scale, his "weight" will increase as he jumps up.
This is cheating; you take weight when you're standing
still. So you can't be pushing dumbbells while taking
your weight.
Likewise, you can't raise your arms up and down. Ask
him if his weight is changing when he's pumping his
arms or jumping on the scale.
\_ conservation of angular momentum
\_ Huh?
\_ Check an elementary physics text. Or sit on a
computer chair, hold a spinning bicycle wheel and
twist it.
\_ Yeah I know what conservation of angular momentum is,
but what does that have to do with the questions in
this thread?
\_ the overall pressure's the same but your feet exert more.
\- if you have flies in a jar on a scale
the scale wil read the same whether they are
sitting on the bottom, the side or flying ...
but at lift off it will register more.
\- here is a fun problem: you have a uniform gold
chain of legnth l ... say 1meter ... with mass
m ... say 100 grams. you hold the chain l units
above a scale and drop it. so at t0 when you
release the chain, the scale reads 0 and at t1
then the chain is resting on the scale, it reads
m->100g ... plot f(t) between t0->t1 where f(t)
is what the scale reads. this might be sort of
unkind as an interview problem for a software
guy :-) --psb
\_ Birkett demonstrated and then explained
this in class once, but I forgot the
physical basis of it. Can you post it?
\_ typical physics h7a homework problem
\_ yeah, so typical.
\_ Ok, I give up. What is the solution?
I got a big complicated equation
( (7Mggtt)/(2l) ) which I don't think
is right. |
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