Newton's third law and air resitance?

[scilearner]

Hello,

 

Few questions about these things.This is not homework!!

 

1. Is Newton's 3rd law caused by conservation of momentum?

2. If I drop a stone into a pond. The stone would exert a force on the water and the water would exert a force on the stone. So why doesn't the stone move up!!

3. Is air resistance caused by collisions with air particles. How come it takes time neutralize the force. I mean if I hit a air molecule with 10N force it would exert 10N back. So If I threw something at 10N why wouldn't it immediately stop after hitting an air molecule.

 

I know these questions demonstrate a very poor understanding but please help me so I can improve!!Thank you!!

[Bob_for_short]

Consider two interacting bodies. The Newton equations read:

 

M1*a1 = F12

 

M2*a2 = F21 = -F12

 

So the action and reaction appear in different equations. The firs body feels only the force from the other.

 

The other feels the force from the former. The third Newton law says these forces are equal and opposite.

 

Now, any force makes a body move according to the Newton equation. The timing is obtained from it.

[swansont]

2. If I drop a stone into a pond. The stone would exert a force on the water and the water would exert a force on the stone. So why doesn't the stone move up!!

 

Direction of acceleration is not the same as direction of motion. The force on the stone is upward, but the motion (velocity) is downward. What happens in this case is the stone slows down.

 

The stone could move up, if it were to strike something massive enough — things do tend to bounce when they strike solid things. If you analyze a collision, you will find that a less massive object will recoil when it strikes a more massive object in an elastic collision.

[Sisyphus]

You also have to look at all the forces on the stone. Gravity is exerted downwards, water resistance (and buoyancy) is exerted upwards. Gravity is greater, so it sinks. However, each of these forces has an equal and opposite reaction on something else. In the case of gravity, the rock exerts the same force on the Earth as the Earth exerts on the rock, and so the whole Earth moves very very very very slightly towards it as it falls.

[scilearner]

Thanks a lot for the answers so far

 

When I hit a ball onto a concrete floor why does the ball move back. Is it due to absense of resistance forces and the ball moves back as the reaction force.Is this a clear example of newton's third law.

 

Also in the stone question if a stone falls at 10N (due to gravity). Does the earth under the water move towards the stone as the reaction force. Why does it only move a bit and how come the ball doesn't move up because of this.

What do you exactly mean by earth moving up.

 

If I throw a ball at 10 N. Why wouldn't it hit an air molecule at 10N and then cause neutralization of the force. Why is however in real air resistance less than 10 N

 

Thanks a lot for the help so far

[J.C.MacSwell]

Thanks a lot for the answers so far

 

When I hit a ball onto a concrete floor why does the ball move back. Is it due to absense of resistance forces and the ball moves back as the reaction force.Is this a clear example of newton's third law.

 

Also in the stone question if a stone falls at 10N (due to gravity). Does the earth under the water move towards the stone as the reaction force. Why does it only move a bit and how come the ball doesn't move up because of this.

What do you exactly mean by earth moving up.

 

If I throw a ball at 10 N. Why wouldn't it hit an air molecule at 10N and then cause neutralization of the force. Why is however in real air resistance less than 10 N

 

Thanks a lot for the help so far

 

You accelerate a ball with the force of 10 N. For how long do you maintain this force? At what velocity do you release it?

 

Why do you assume the ball will collide with the molecule with a force of 10 N? Can the molecule exert the same force as your hand did in the opposite direction? Can it maintain it for the same length of time?

[Sisyphus]

Also in the stone question if a stone falls at 10N (due to gravity). Does the earth under the water move towards the stone as the reaction force. Why does it only move a bit and how come the ball doesn't move up because of this.

What do you exactly mean by earth moving up.

 

The force exerted by the Earth on the stone is equal to the force exerted by the stone on the Earth: 10N. However, 10N can accelerate the stone a lot (9.8m/s^2, in fact), but 10N isn't anywhere near enough to noticeably accelerate the whole Earth. Does that clear it up?

[scilearner]

The force exerted by the Earth on the stone is equal to the force exerted by the stone on the Earth: 10N. However, 10N can accelerate the stone a lot (9.8m/s^2, in fact), but 10N isn't anywhere near enough to noticeably accelerate the whole Earth. Does that clear it up?

 

Yes it did Thanks

 

I'm only confused with the ball question now.

 

If I throw a ball at 10 N and someone stands on the way wouldn't he get hit by 10N in the absenses of resistance forces.

 

Now I'm comparing this person to an air molecule. So if the airmolecule get hit with 10 N it would exert 10 N back(air resistance force) neutralizing the force. That is my question

 

Thanks a lot for help so far

[Sisyphus]

Newtons are a measure of force, not kinetic energy. The ball doesn't "have" 10N. You apply 10N to it for a certain amount of time, and that accelerates it a certain amount to give it a certain velocity and hence a certain amount of kinetic energy.

 

Anyway, no. The forces that a particle of air and the ball exert on one another are indeed equal, but there's no reason they would be equal to the force you exerted on the ball earlier. It depends on a lot of factors (their relative velocity, their masses, their respective elasticities), but what forces were exerted on them in the past is not one of of those factors.

[Spyman]

Kinetic energy is not the same as Force.

 

The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity.

http://en.wikipedia.org/wiki/Kinetic_energy

 

In physics, mechanical work is the amount of energy transferred by a force acting through a distance.

http://en.wikipedia.org/wiki/Work_(physics)

 

In physics, a force is any agent that causes a change in the motion of a free body, or that causes stress in a fixed body. It can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform.

http://en.wikipedia.org/wiki/Force

 

You can use the force of 10N to accelerate a ball during some time and it will then have a certain kinetic energy depending on the time and the mass of the ball.

 

The air molecule or the person would be hit with the kinetic energy and some of the kinetic energy would transfer over to them during the collision. As they gain kinetic energy the ball is loosing the same amount but depending on the difference in mass a small ball is not going to accelerate a human to very high speed and cause the ball to bounce back, while a tiny air molecule is being pushed aside easily by the ball and is hardly able to lower the speed of the ball at all.

 

In both cases they are affecting each other with equal forces, the human/molecule is accelerated with the same force as the ball is deccelerated with. But since the human has more mass than the molecule the influence and thus the force is greater than with the molecule.

Edited October 2, 2009 by Spyman

[scilearner]

Thank you everybody for your replies. I seriously have a major misunderstanding. I'll do this step by step

 

1. If I apply a force of 10 N to a ball inside a vaccum my understanding is ball would keep travelling at 10 N unless acted upon by an external force. Isn't this newton's first law? If I throw 10 N force for 1 m where does the force dissapear in the absence of resistance forces. Where does it go? At this point does the object reach terminal velocity?

 

2. When I hit an air molecule is it less than 10 N force?

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Ok I understand the answer for first question.

 

2. When I hit an air molecule is it less than 10 N force? Why is Russ Waters equation show 500 N. Thanks for your example and response though it was helpful.

 

This is what I have trouble with. How did it become 500 N. How long does it take a ball to decelerate when hit by an air molecule. Wait it is impossible for the ball to decelerate when hit by one air molecule right it must be hit by heaps of air molecules. Did I get it. Did you get a decelerating time for the 500N question because it was a person who got hit. An air molecule would move away with any speed but a cluster of air molecules would take more. Or this is simply pushing a large object and small object right. A large object needs more force. Am I right?

Edited October 3, 2009 by scilearner

[Bignose]

Thank you everybody for your replies. I seriously have a major misunderstanding. I'll do this step by step

 

1. If I apply a force of 10 N to a ball inside a vaccum my understanding is ball would keep travelling at 10 N unless acted upon by an external force. Isn't this newton's first law?

 

Here is a significant part of your misunderstanding.

 

"ball would keep traveling at 10 N" is a meaningless statement because of the unit error.

 

Newtons, N, are a unit of force. In basic units, a newton is a mass*length/time^2.

 

Your statement indicated that what is needed is something with the units of speed or velocity, length per time, such as m/s or miles per hour.

 

mass*length/time^2 will never ever be the same as length/time.

 

In physics, it is very important to get all the units right. As an example, if I asked you "how many strawberries do you have" and you replied "16 bananas" -- there was a significant unit error there. Answering in bananas does not in any way answer my question about strawberries.

 

In the same way, an object cannot "keep traveling at 10N" because N is NOT a unit of velocity or speed. Anymore than an object can keep traveling at "10 strawberries" or "10 kilograms" or "10 dollars". These are all meaningless statements because of the unit error.