Newton's third law example

1. Dec 21, 2003

MiniTank

hi...my teacher has offered an optional project of creating something that used newton's third law.. ive been trying to think of something but i dont have any ideas... i have to make something .. and umm it cant be a fan on wheel blowing against a wall cuz thats the example in the book.. thanks

2. Dec 21, 2003

enigma

Staff Emeritus
A model rocket.

Rocket pushes fuel down, fuel pushes rocket up.

3. Dec 21, 2003

MiniTank

a rocket would work...but how would i make it? any simpler ideas?

4. Dec 21, 2003

enigma

Staff Emeritus
They've got kits. If you're allowed, just go to a hobby shop and pick one up. They don't run very expensive.

Otherwise, you could look into water rockets. Put water in a bottle, turn it upside down, and use compressed air to raise the pressure in the bottle.

Stand back, pull the plug, and watch it go!

(Yes... I am an aerospace geek)

5. Dec 21, 2003

MiniTank

we cant buy kits.. and idono how well it would work with a bottle

6. Dec 21, 2003

enigma

Staff Emeritus
Hrmm... A balloon with a straw taped to it could be a simple 'rocket'. Just run string through the straw, and tie it to two places in the room to give the balloon a direction.

It's kind of simplistic, though. I don't know how complicated your teach wants it.

If that won't work either, I'll open up the floor for someone else to hog the thread...

7. Dec 21, 2003

MiniTank

pffft who needs direction...im gonna do that.. but just incase my teachers a prick..any other suggestions?

8. Dec 21, 2003

faust9

Make a cartesian diver out of a plastic soda bottle. The simple Cartesian diver shows Pascals principle, the Ideal Gas Law, and Newtons third law all in one inexpensive package (A soda bottle tap water and an eye dropper are all you need).

Good luck.

9. Dec 21, 2003

MiniTank

cartesian driver? whats that?

10. Dec 21, 2003

faust9

Here:

http://littleshop.physics.colostate.edu/Cart.html [Broken]
http://www.fatlion.com/science/cartesian.html
http://www.fortbragg.k12.ca.us/AG/cartesian.htm [Broken]

Last edited by a moderator: May 1, 2017
11. Dec 21, 2003

Emmsey Square

You know what else is Newton's third law? Hitting a ball with a ping-pong racket. For every action, there is an equal and opposite reaction. Oh, even better! Two pool balls (for best example, use the cue ball and the 8 ball). Hit the eight ball with the cue ball. The action is the two balls hitting eachother, and the reaction is the transference of momentum from the cue ball to the 8 ball, and the 8 ball moving.

12. Dec 21, 2003

MiniTank

how does the catesian driver show newtons third law?:s...you push on the bottle..and the packet sinks[?] and i cant use eightballs..it has to be like a mechanism almost...like the rocket ship..or catesian driver.. still dont see how this shows the third law

13. Dec 21, 2003

faust9

Newtons third law is that of action and reaction. Squeezing the bottle is the action and the bottle pushing back is the reaction. Additionally, squeezing the bottle caustes the volume of the bottle to decreases which causes the pressure of the fluid to increase. Again action reaction. The fluid pressure increase causes the gas bubble within the diver to compress due to Pascals law AND action reaction. Finally, The compressing of the gas bubble within the diver changes the divers buoyancy thus causing it to sink. Action Reaction.

To find examples of Newton's third law, just look around and say to yourself "Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first".

Every time you move this law applies.

Well, hope this helped, Good luck.

14. Dec 21, 2003

MiniTank

oh ok thanks..cuz i got confused when on the sites it was talking about buoyancy and stuff...thanks

15. Dec 22, 2003

Staff: Mentor

Well, yeah... but you don't need a cartesian diver... just squeeze an empty bottle.
Where's the "action-reaction"?
Of course, whenever a force is exerted Newton's third law is working. But I don't see how a cartesian diver makes an especially good illustration of that.
Now this is good advice. This is a much better statement of Newton's third law than the vague and misleading "action-reaction" statement.
It even applies if you don't move.

16. Dec 22, 2003

MiniTank

i know how it works..its just i need something that represents it..like something i can make...

17. Dec 22, 2003

faust9

Now Doc is all this nit picking necessary?

I dare say simply squeezing a bottle, though a true example of Newtons third law, wouldn't fit the boundries of MiniTanks' first post which called for a project.

Now to address the second part, the mere fact that the diver is moving is a prime example of action reaction.

Squeezing the bottle is the external force applied to the system and the systems response is to apply a force of equal magnitude but in opposite direction. You can stop there, but you'd be missing the point of the cartesian diver.

Look at the diver as a system we see that the buoyant forces push up while the force gravity pulls down on the diver. The action-reaction here is by increasing the mass of the diver as a system without affecting it volume, the weight of the diver will overcome the force of buoyancy thus causing the diver to sink. The initial action which caused the diver to sinke was the influx of fluid which compressed the gas bubble in the divers head.

The fluid applied a force to the gas bubble, and the gas bubble compressed to a point where it could apply a force equal in magnitude but opposite in direction to that of the fluid. Squeezing the bottle is the action and the bottle, as a system, pushes back. This very same thing occurs within the diver. The fluid pushes on the gas bubble, and the gas bubble pushes back.

The force of buoyancy pushes up against the weight of the diver. As the weight of the diver goes up, it begins to sink because the weight eventually overcomes the buoyant force.

The Cartesian diver shows that there are no isolated forces. The internal forces of the bottle system are countered by reaction forces in other parts of the system.

18. Dec 22, 2003

deda

you can demonstrate Newton the 3rd with a simple lever but...
if you don't want to confuse your teacher and the classmates remember to put the weights on the same distance from the equilibrium (fixed) point.

19. Dec 22, 2003

Staff: Mentor

I don't consider it nit picking! If one is going to use the cartesian diver to illustrate Newton's third law, one had better be prepared to clearly identify the bodies and forces involved. I don't think the diver lends itself to that.

Part of my "nit picking" is that I dislike the old-fashioned "action-reaction" form of the third law. A cleaner formulation is: whenever body A exerts a force on body B, then body B will exert and equal and opposite force on body A.
I think the diver is better used to illustrate bouyancy and gas compression, not Newton's third. Of course, any force can be used to illustrate the third law, if you wish. I just think the diver is too subtle.
So... what are the "action-reaction" pairs? Surely not the bouyant force and the weight! They act on the same body. If you wish to call the bouyant force (water pressing on the diver) the "action", then the "reaction" is the diver pressing back on the water.
True, but not that interesting.
Newton's Third??
I really don't mean to be a nit picker, but I just don't think using the diver---while a great thing to play with---is going to be easy to explain using Newton's third. (It's equivalent to trying to explain bouyancy using Newton's third.)

How about something simple? Make a wooden paddle boat with a rubber band and some flat sticks. This one's easy to "explain": the boat pushes the water, the water pushes the boat!

Or how about a wind up car? The wheels push the ground, the ground pushes back, making the car move. You can even change the surface, make it slippery and show how hard it is to get the car to move since the wheels can't push as hard against the ground.

Or how about a "rocket launcher"? Attach a rubber band "gun" to something with wheels. Fire the "rocket"--a pen, stick, whatever--and observe the recoil.

Come on, MiniTank... use that noggin'!

20. Dec 22, 2003

Staff: Mentor

OK, I'll bite. How does that illustrate Newton's third law?