The 3rd law of motion is confusing.

[krah]

This is not a big philosophical question or anything, I'm just a little confused in my physics class. A good question arose in the text book, and I don't know how to answer it. Newton's 3rd law, as everyone knows, is "For every action there is an equal but opposite reaction."

If you are pulling a wagon, then the force you use pulling on the wagon would be equal to the force of the wagon pulling on you, right? (I know this doesn't sound like it makes sense, it may not.) So then you could say the force of you on the ground is what allows you to overcome the force of the wagon and pull it, but wouldn't the force you exert against the ground have an equal but opposite reaction itself and result in the ground exerting the same force against you which means you would just stay in place? Damn. It's only after typing it that I realize just how confused I am...somebody please explain to me why we're not all stuck in one position forever.

[filburt1]

Pulling a wagon:

Gravity is a force acting down
Earth is a force acting back up balancing gravity
Pulling is a force...pulling
Friction resists the pulling but not equal and opposite so the wagon moves.

[Richard Simmons]

It means if you get fit with Richard Simmons you will be Super Slim.

[Stefan Raab]

Raab is gonna force feed you somdething wicked if you don't simma down!

[Richard Simmons]

Mm..what are you going to feed me?

Just make sure it's super low in fat!

[crptcblade]

Boy, I hope you two super powers don't get into some sort of war that brings down the world.

[krah]

I'm glad to see that Richard Simmons is back and is more Slim and homosexual than ever.

Filburt, you seem to be the only one trying to help (which I appreciate) so what about this: If Mr. Simmons thrusted Mr. Simmons Jr. (which is also Super Slim) into his boyfriend's rear entrance, the "action" would be the thrusting of his hips from point A to point B. What would be the "equal but opposite reaction?" Note- this question would also apply to non-disgusting example such as a person walking from point A to point B, but that just wouldn't be Super Fun (and in Super Capital Letters) now would it Richie?

[crptcblade]

Originally posted by krah
I'm glad to see that Richard Simmons is back and is more Slim and homosexual than ever.

Filburt, you seem to be the only one trying to help (which I appreciate) so what about this: If Mr. Simmons thrusted Mr. Simmons Jr. (which is also Super Slim) into his boyfriend's rear entrance, the "action" would be the thrusting of his hips from point A to point B. What would be the "equal but opposite reaction?" Note- this question would also apply to non-disgusting example such as a person walking from point A to point B, but that just wouldn't be Super Fun (and in Super Capital Letters) now would it Richie?

the reaction would be his boyfriend's rear pushing back.

[filburt1]

Please tell me you've done vectors in Physics, krah, or is that the next chapter? I'm taking physics right now.

[kovan]

when my right testical pulls down the left after scratching myself
that is law of gravity.. and in turn can be law of motion as one is effected by the others physical state, and motion

[filburt1]

Perhaps a slightly...cleaner...analogy next time?

[kovan]

that is the cleanest i could come up with
here is the dirty version

When i rub my right ball, the left one wants some rubbin too...

nah lets not get into that

[filburt1]

Originally posted by kovan
nah lets not get into that

Agreed.

[kedaman]

there's a thread going on in math forums which explains how force and acceleration is tied together.
http://www.vbforums.com/showthread.p...hreadid=112781
For a mass with constant movement, the sum of all forces is 0, that is what newton's third law says actually. If the movement changes, then then there is acceleration due to a force inbalance. Have a look at the thread.

[scsa20]

what goes up, must come down

[Gary.Lowe]

Originally posted by scsa20
what goes up, must come down

Super Fresco makes it easy by Graham & Brown

[krah]

Yes, we just starting doing vectors. I still don't get it though. If for every action there is an equal but opposite reaction then how does anything move? Am I interpreting this rule incorrectly?

[kedaman]

That doesn't apply for accelerating bodies. If a mass accelerates in a direction, there is a resultant force F=ma applying on that body.

Note: A body that move uppwards doesn't nessarily accelerate uppward. If you throw up a tennis ball, it will be accelerating downwards due to gravity while in air. Velocity will slowly decrease due to this and then alternate and go negative. The ball starts to fall downwards. In space where there is no friction or gravity, if you jump you will move in that direction with same speed until you hit something or goes into orbit of an asteroid or something.

[error40501]

Originally posted by krah
Yes, we just starting doing vectors. I still don't get it though. If for every action there is an equal but opposite reaction then how does anything move? Am I interpreting this rule incorrectly?

Think of yourself on ice, wearing ice skates, and you push against a tree at the side of the pond, then (we can assume zero friction) you'll zoom off across the ice. Your ACTION of pushing on the tree, has the REACTION of the tree pushing you. You skate off, the tree stays still.

(My son learned this the hard way a year ago, when he was standing on the jetty at the boat club, pushed a (much) larger lad, and actually fell backwards into the water himself.)

It is in fact this Action/Reaction which makes rockets fly. People often wrongly think it's the jet stream or whatever it's call exiting the back of the engine, pushing against the air which causes the rocket to move. It's not: it's the reaction of the inside of the engine on the 'front' of the jetstream that pushes the engine (and so the vehicle) forward, with the stream being pushed out the back. Indeed, given no atmosphere to speak of in outer space, there is nothing for the 'back' of the jetstream to push on; it has to be an interaction between the 'front' of the stream and the engine.

[CiberTHuG]

Newtonian physics is flawed.

There is no equal and oppisite reactions. Everything has an elasticity. When you think you are seeing equal and oppisite reactions you are really seeing

1) a single action going in the direction oppisite that which you were expecting

or

2) secondary actions generated from the transfer of energy into an elastic object.

[thinktank]

Courtesy : Ya. Perelman Physics for Entertainment 2
Mir Publishers, Moscow

According to the law, for every action there is always an equal and opposite reaction. Consequently, When a Horse pulls at a cart, it means the cart is pulling at the horse with the same force. In that case the cart should stay where it is, shouldn't it ? Neverthless it moves. Why don't these forces offset each other since they are equal ?

That is the usual argument raised when this law comes up. Does this mean the law is wrong ? Of course not. It is just that we don't understand it correctly. The forces do not offset each other simply because, they are applied to different bodies. One is applied to the cart and the other to the horse. The forces are certainly equal; but do equal forces always produce the same action? Do equal forces impart an equal acceleration to all bodies? Does not the action of a force on a body depend on the body itself? And on the value of the reaction which the body offers to the force? Once you think about it you will realise immediately why the horse pulls the cart along even though the cart is pulling the horse back with the same force.

The force acting on the cart and the force acting on the horse are of equal magnitude at every moment, but since the cart freely moves on it's wheels, while the horse pushes away from the ground, the cart rolls in the direction in which the horse is pulling it. Furthermore one must realise that if the cart did not react to the horse's motive power we would be able to dispense with the horse entirely, as the slightest push would already start the cart rolling. We need the horse to overcome the cart's reaction.

Perhaps you would grasp this point more easily were the law expressed not so laconically as it usually is - "Action is equal to reaction" - But as : "The force of the reacting body is equal to the force of the acting body." After all it is only the forces that are of equal magnitude: the actions of the forces if understood - as they are usually understood - as the translation of a body, are on the other hand different as a rule, because the forces are applied to different bodies.

Even in falling, every body strictly obeys the law of reaction. An apple falls, because it is attracted by the earth's gravity. However, the apple itself attracts the whole planet with exactly the same force. Strictly speaking, the apple and the earth fall towards each other, but their speeds of falling are different. The equal forces of mutual attraction impart to the apple an attraction of 10 m/sec^2 (approx.) while to the earth they impart an acceleration which is as many times less as many times the earth's mass is more than the mass of the apple. Naturally, the earth's mass is an incredible number of times greater than that of the apple. No Wonder the earth's movement is so infinitesimally small that for all practical purposes it can be considered as non-existent. Now you know why we say that the apple falls on earth, instead of saying that the apple and the earth fall on each other.

[krah]

Thanks kedaman, error40501, and thinktank. It makes sense to me now. CiberTHuG, that's great if you want to explain or something, but all I know is it won't be like that on my test.

[CiberTHuG]

You know what, I'm a little bored, I'll explain it...

In elementary school they used the roller skates and wall example to explain the third law. The example states, if you are wearing a pair of skates and you push a wall, the oppisite reaction will push you away from the wall.

This is grossly wrong. First, it implies that your initial push had a direction. First off, your arm doesn't create a directional force. When you "push off" of a wall, your arm is extending between you and the wall. You have a much lower moment of inertia, so you move. The wall doesn't.

With the horse and cart example mentioned above. The cart never pulls on the horse. The horse pulls on the cart. The horse is strong enough to overcome the carts moment of inertia. 'Nuff said.

There is the example of the falling ball. If the ball falls and hits the ground, it will bounce. It is bouncing because the impact creates an action, and the reaction pushes the ball away from the ground. This is flawed, too. The ball and the ground will deform apon impact, and when they reform they will push against each other. This goes back to you and your skates. The ball looses that fight and bounces. This is not the third law, this is elasticity.

If the third law exists, I've never seen an example of it.

Any example presented to me can be explained with elasticity, conservation of energy, inertia, and the fact that most things don't actually create a directed force, they just appear to.

Hope that makes sense.