What are some examples of Newton's third law in action?

[MiniTank]

hi...my teacher has offered an optional project of creating something that used Newton's third law.. I've been trying to think of something but i don't have any ideas... i have to make something .. and umm it can't be a fan on wheel blowing against a wall because that's the example in the book.. thanks

 

[enigma]

A model rocket.

Rocket pushes fuel down, fuel pushes rocket up.

 

[MiniTank]

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

 

[enigma]

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)

 

[MiniTank]

we can't buy kits.. and idono how well it would work with a bottle

 

[enigma]

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...

 

[MiniTank]

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

 

[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.

 

[MiniTank]

cartesian driver? what's that?

 

[faust9]

Here:

http://physics.about.com/cs/airandfluidexp/a/040703a.htm
http://littleshop.physics.colostate.edu/Cart.html
http://www.fatlion.com/science/cartesian.html
http://www.fortbragg.k12.ca.us/AG/cartesian.htm

 

[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 each other, and the reaction is the transference of momentum from the cue ball to the 8 ball, and the 8 ball moving.

 

[MiniTank]

how does the catesian driver show Newtons third law?:s...you push on the bottle..and the packet sinks[?] and i can't use eightballs..it has to be like a mechanism almost...like the rocket ship..or catesian driver.. still don't see how this shows the third law

 

[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.

 

[MiniTank]

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

 

[Doc Al]

Originally posted by faust9
Newtons third law is that of action and reaction. Squeezing the bottle is the action and the bottle pushing back is the reaction.

Well, yeah... but you don't need a cartesian diver... just squeeze an empty bottle.

Additionally, squeezing the bottle caustes the volume of the bottle to decreases which causes the pressure of the fluid to increase. Again action reaction.

Where's the "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.

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.

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".

Now this is good advice. This is a much better statement of Newton's third law than the vague and misleading "action-reaction" statement.

Every time you move this law applies.

It even applies if you don't move.

 

[MiniTank]

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

 

[faust9]

Now Doc is all this nit picking necessary?

Well, yeah... but you don't need a cartesian diver... just squeeze an empty bottle.

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.

hi...my teacher has offered an optional project of creating something that used Newton's third law.. I've been trying to think of something but i don't have any ideas... i have to make something .. and umm it can't be a fan on wheel blowing against a wall because that's the example in the book.. thanks

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.

 

[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.

 

[Doc Al]

Originally posted by faust9
Now Doc is all this nit picking necessary?

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.

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

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.

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.

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.

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.

True, but not that interesting.

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.

Newton's Third??

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.

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'!

 

[Doc Al]

Originally posted by 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.

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

 

[Raiden]

Simple. The action is placing the weights. The reaction is the scale moving. Almost anything can be represented by Newton's third law. It's just harder for you because your teacher put a whole lot of restrictions on this thing.

 

[deda]

Originally posted by Doc Al
OK, I'll bite. How does that illustrate Newton's third law?

Well as 1st we asuume that weight1=weight2 and distance1=distance2 and that the Earth pools them both down.
The weights are on the oposite side of the equilibrium point so their forces by the Earth's pool should be equal and oposite in order the lever to remain horihontal and stady.
Later you can shift one weight closer to the equilibrium and that eventually breaks the balance in a sense that the lever is not horizontal anymore. You can explain that from the point when you make distance1<>distance2 the Archimed's relation for lever takes over the control cause at that time it is:
m_1=\frac{d_2}{d_1}m_2

 

[Doc Al]

Originally posted by deda
Well as 1st we asuume that weight1=weight2 and distance1=distance2 and that the Earth pools them both down.
The weights are on the oposite side of the equilibrium point so their forces by the Earth's pool should be equal and oposite in order the lever to remain horihontal and stady.
Later you can shift one weight closer to the equilibrium and that eventually breaks the balance in a sense that the lever is not horizontal anymore. You can explain that from the point when you make distance1<>distance2 the Archimed's relation for lever takes over the control cause at that time it is:
m_1=\frac{d_2}{d_1}m_2

All true, but note that you did not refer to Newton's Third law even once.

I hope you are not suggesting that equilibrium of a balanced lever is due to Newton's Third law!

 

[deda]

Originally posted by deda
forces by the Earth's pool should be equal and oposite in order the lever to remain horihontal and stady.

This actually means that Fa=-Fr. If it were Fa=Fr then the lever will spin in direction of Fa and Fr cause they represent same rotational direction.

I'm not that good at explaining my self even in my own native language. If it wasn't so, by now, I would have reached very far. Maybe you should examine the lever's functionality by your self.