# Isaac Newton

Newton's third law is: For every action, there is an equal and opposite reaction.
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?
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On Tuesday, 24 April 2018 17:33:29 UTC+1, DerbyBorn wrote:

The obvious choice is bathroom scales. Old spring scales would be far better for this than modern digital things.
NT
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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.
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On Tuesday, 24 April 2018 17:56:50 UTC+1, NY wrote:

If how to do that isn't utterly obvious you need a new brain.
NT
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On Tue, 24 Apr 2018 17:02:03 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

unless bathroom scales come with integrators, I don't think his brain should be dispensed with too quickly.
The real difficulty comes with the words "equal and opposite".
AB
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snipped-for-privacy@gmail.com wrote:

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. :)
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On 24/04/2018 17:33, DerbyBorn wrote:

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.
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Robin
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On 24/04/2018 17:33, DerbyBorn wrote:

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.
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Martin Brown
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wrote:

Dont know about equal and opposite, maybe entropy would be a better demo.
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 though.
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.
AB
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On 24/04/2018 17:33, DerbyBorn wrote:

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.
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Why not get one of those old ball bearing swing executive toys. Lift one bearing and let go, one bearing moves on the other side, two and two move etc. Simples. Brian
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On Wed, 25 Apr 2018 07:37:22 +0100, "Brian Gaff"

Er! last I heard that was a Newtons Cradle.
AB
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On Wed, 25 Apr 2018 07:37:22 +0100, Brian Gaff wrote:

Newton's Cradle. It's already been mentioned... Twice! ... In two posts... *both* immediately prior to yours! :-)
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Johnny B Good

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On Wed, 25 Apr 2018 17:53:41 GMT, Johnny B Good wrote:

But how does that demo Newton's third 'law'? Not to mention that Newton's form of classical laws of motion are pretty obsolete these days.
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No they aren't.
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nor repeal legislation.
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On Thu, 26 Apr 2018 16:28:57 +0100, Tim Streater

+1
Although ISTR they get a little shaky close to the speed of light, but are good for everything else, which is most things.
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Chris

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On Thu, 26 Apr 2018 16:45:17 +0100, Chris Hogg wrote:

They don't work for relativity or for quantum mechanics, so the Euler-Lagrange form of motion laws are more generally useful/findamental - as are Hamilton's.
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wrote:

But how many day-to-day engineering projects involve relativity or quantum mechanics?
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Chris

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On 26/04/2018 18:54, Chris Hogg wrote:

Anything that uses GPS, so most construction projects these days.
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On 26/04/2018 23:04, dennis@home wrote:

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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Martin Brown
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