Newton's third law is: For every action, there is an equal and opposite
For an educational display I want to make a demonstration aparatus. I was
thinking of a platform to stand on - and a pad on the wall to push against.
The pad on the wall could be something like a bathroom scale - I then need
something to measure the opposing reaction of the platform.
Obviously - low friction. Idealy giving a digital display to compare with
the reading on the scales.
Any ideas for hardware?
Yes, but you need to find a way to make the scales register the *sideways*
force which is a reaction to pushing against a wall, rather than measuring
your weight (downwards). Maybe a spring balance - the sort that is one tube
within another, joined by a spring that measures the force between them and
one extends with respect to the other. Mount the platform on supports that
can move sideways easily and attach the spring balance to the edge of the
platform and to a fixed point on the same wall as the scales that measure
your push. As you push, the wall-mounted scales measure your pushing force
and the balance measures the equal and opposite pulling force of the
platform on which you are standing.
I am pretty sure he meant gluing the scales to the wall and pressing against
it while standing on an intergalactic friction-free skate board.
Well, that's what I would have done. Always the simplist solutions me. :)
Mageia 5.1 for x86_64, Kernel:4.4.114-desktop-1.mga5
KDE version 4.14.5 on an AMD Phenom II X4 Black edition.
ISTM in dealing with forces exerted by arms and feet it'd be a lot
easier to work (sic) vertically rather than horizontally. Eg have the
person stand on one bathroom scale and push upwards on another? The
latter mounted on an a column adjustable for height?
That said, I've never been convinced that essentially static
demonstrations are good. IMHO it's hard to beat marbles - though I've
no idea if they'd pass a modern risk assessment.
reply-to address is (intended to be) valid
Small kiddies toy trolley with a wheel at each corner. Spring based
sucker gun mounted on top of it and fire. You may need to add extra
weight to the sucker so that the trolley recoils a decent amount.
Another good demo is two super balls a small one and a large one.
You can also do it with two the same size for reference.
Balance the small one in contact with the bigger one and drop the two
together onto a hard floor well away from any windows.
The smaller mass gets to share half the momentum on recoil and bounces
high. It is a memorable demonstration. Practice to avoid causing damage.
It is also why it is a bad idea for a car to pick a fight with an HGV.
Another nice one is the lemonade bottle rocket which graphically
demonstrates how throwing mass out the back move it forwards.
Dont know about equal and opposite, maybe entropy would be a better
By the time you have accounted for the numerous losses, equality would
need to be proven with a computer.
An air gun might be a simpler thing to provide an illustation of
action/ reaction, or better still a shotgun :-)
It would still be a nightmare demonstrating the equal and opposite
Tennis balls with a variable mass [oil/ water filled] and a large tray
with a cm layer of flour could provide a demonstration of momentum.
Better still buy a Newtons cradle
I think I would find this a bit boring. Were you thinking to have the
first scale at hand level and the other at foot level? In that case you
will have to find a way of reacting the moment as well, I can see all
sorts of falling over hazard.
You could have the victim stand on a skateboard and two bathroom or
perhaps kitchen scales at the same level across something like a doorway
and show that to stay stable you need equal and opposite pushes.
For momentum demonstrations, it is hard to beat a Newton's cradle using
first one then two balls.
GPS only uses relativity in the weak field limit as a perturbation.
Moving clocks run slower and clocks further out of the gravitational
field run quicker and if you are trying for <10m accuracy (30ns time
differences) on the ground the corrections are not negligible.
Amusingly the engineers insisted on having the clocks in the first GPS
satellites capable of Newtonian clock rates because they were too thick
to properly understand relativity (its badly taught in EE courses).
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