some thing's not clear about newton's 3rd law of motion,,?

Question

every action has an equal and opposite reaction,,

then why do the objects move?
in the sense,,if an object is moving in 3 N force,,then there is an equal and opposite force ,,so why does it continue to even move in any force,,???

if a rocket is moving upwards,,some force pushes it downward,,which is equal in magnitude,,and opposite in direction,,so why does it still moves upward???why does it even move??
why it doesnt stop without any motion??

it really seems to be unclear abt newton's 3rd law of motion,,even my physics teacher couldnt answer,,he was finding excuses to avoid this question,,
can u explain me the newton's 3rd law of motion clearly??

Answer 1

First of all if all the forces balance then there is not motion.
The thing that you are not taking into account is that F=ma. Meaning that in the case of the rocket, there are not enough forces (specifically gravity) to balance out the force of the rocket engine, so the object accelerates which in its self is a force (really it is mass*acceleration that makes the force).
Does that make more sense?

Answer 2

You have stated that

“A rocket is moving upwards, some force pushes it downward, which is equal in magnitude, and opposite in direction,”

Your statement is not correct.


The rocket moves upwards implies that some (external) force is acting on it in the UPWARD direction.

Any object will move with acceleration in the direction of the applied force.


In the rocket the force is given by the expanding gases of the rocket.

We are not considering the rocket and gas as one and the same object while explaining the motion of the rocket.

There are two objects one is the rocket and the other one is the gas inside the rocket.

The rocket moves up because the gas exerts a force on the rocket in the upward direction.

The gas moves down because the rocket exerts a force on the gas.

The acceleration of the rocket is determined by the force exerted by the gas divided by the mass of the rocket,

The acceleration of the gas is determined by the force exerted by the rocket on the gas divided by the mass of the gas.

The action force is acting on B, and then the reaction force is acting on some other object A.

Consider a book resting on a table.

The book is acted on by two forces which are exerted by two different objects.

One force is the pull of the earth on the book
The other one is the force exerted by the table on the book.

The resultant of these forces is zero if the book is at rest.
These two forces are acting on one and the same object namely the book.

Suppose the pull is greater than the force given by the table, (imagine a huge rock is placed on the table instead of book) then the object will pierce the table and will go down.

The two forces referred above are not the forces stated in Newton’s third law.

Now consider the pulling force exerted by the earth on the book. By Newton’s third laws the book pulls the earth toward it. ‘Whether the earth will move because of this force’ is a different question. This is the reaction force.

Considering other force, the book exerts a pressure on the table and as the reaction the table exerts a force on the book. ‘Whether the table will move because of the book’s force?’ is a different question.

As for as the book is concerned we are worried only about the forces acting on them and not about the reaction forces due to these action forces.

Thus the rockets motion is determined only by the force acting on it and we are not worried about the reaction force which acts on the gas.

Answer 3

To every action there is an equal and opposite reaction.
This has to do when two moving masses interact;there is a response which is borne called force ; Its an incident force and a reflected force which is equal in magnetude but opposite in direction.
In the same manner the law could also be stated as; to every Energy there is an equal and opposite reflected energy.
For example if Gravity was pushing you down ,the earth
would be pushing back with the same push, in the opposite
direction.
IN the case of the rocket we have a gravity power scenario; The rocket must use sufficient power to overcome the the power of gravity .Note gravity is maximum at the Earth surface and decrease as the distance above the earth increases. So Once the rocket has lifted it needs less and less power as it move up. The initial lift off is where the escape velocity has to be greater than the gravity velocity at the surface of the Earth.

Answer 4

Then you are cursed.

I don't suppose that they showed you an example of a man in a boat, motionless on a lake. The man jumps forward from the front of the boat. As he jumps forward, the boat moves backward with equal and opposite force.

The rocket moves upward, overcoming gravity by spewing out masses of high-velocity hot gasses from its boosters or thrusters (equal and opposite). Now, if the rocket engine only put out enough force to match the downward force of gravity, the the rocket would appear to hover.

The example you cite, the object has been imparted a 3 Newton force. It is true that to get the object going with a 3 Newton force, something had to act on it and react to it, but this example is NOT a system involving two objects, only the one.

Suppose I take a cue stick and strike a cue ball with a 3 Newton force. Somewhere inside my body, right down to my braced legs, I had to absorb the equal and opposite reaction to my striking the ball. But, having said that, the focus is now on the cue ball and what it is going to do to the rack of balls on the pool table when it collides.

Does this help?

Answer 5

Newton's third law means that if one object exerts a force on a second object, then the second object exerts the same force in the opposite direction on the first object. For example with the Rocket, it exerts a force on the exhaust downward, and that exhaust pushes the rocket upward. The reason the rocket can move is that when the exhaust pushes on the rocket, the counter force pushes on the exhaust, not on the rocket. The exhaust moves down and the rocket moves up.

Answer 6

In the rocket case, when the rocket push the air with a downward force, the air then give a reaction force upward, so it lifts the rocket! Other cases are pretty much the same, think about where these two forces are applied on.

By the way, I really doubt how much your physics teacher know about physics.

Answer 7

A rocket moves up because that is the equal and opposite reaction to the flames and exhaust moving down.
In your statement "if a rocket" you say a force is pushing it down. Well, gravity is pulling it down a bit, but there isn't any other force pushing it down. The exhaust moving down has to overcome the force of gravity on the rocket, and what is left is the rocket moving up.
Equal force includes the speed and mass of both sides of the equation.

Think of a pool table, if you hit a ball with the cue ball, the ball you hit goes one way and the cueball goes in the opposite direction.
There's a lot more physics involved, the direction and velocity of the balls also depends on the spin of the cueball, etc. But we won't get into that here.

Answer 8

The statement reads - ' To every action acting at a point. thers an equal and opposite reaction at dat point.'
ther is already force of gravity acting on the rocket which is balanced by the reaction force given by earth which inturn arises due to contact [had der been no contact dat rocket wud have fallen down due to force of gravity!] now in this state of equilibrium the chemical reactions taking place in the engine of the rocket thrusts the rocket to propel in the forward direction..now take the eg of a man shootin a bullet the gun exerts a force on the bullet..but the mass of bullet is too small as compared to the total mass of gun and the man holdin it. thus same force [rate of change of momentum] acts on the gun (along with the man) and the bullet but as u know that momentum is a product of mass and velocity thus bullet gains more velocity thus more displacement and the man less of each...: )

Answer 9

when something begins to move in the first place, there will be an acceleration in the opposite direction of his motion.....imagine a basketball player jumping to dunk the ball, he has to apply the force into the ground in the first place, thus providing a "thrust" upwards, and the opposite and equal force keeping him from jumping infinitely into space is caused by the "drag" of wind and gravitational force downwards. inertia keeps the object moving until another force stops it.

Answer 10

the force on the rocket is greater in the upward direction. If the force become greater thru gravity then the rocket slows to a stop, then falls to earth.

The force of something moving in linear fashion has the effect of gravity and friction on it. When this force is greater than the force of the car, the car will eventually stop.