i have a question about newtons law.?

Question

for every force there is an equal and opposite force. i understand that. but what if there is a big metal box and it gets place on top of quicksand? what exactly happends there? is it that as it sinks the metal box still puts an equal and opposite force on the quick sand, although its sinking...wait..that woundlt make sense...im confused...

Answer 1

you can look at this problem as a object was put in a fluid (water, oil ... or sand) ... So, there is always a gravitational force pulling the object down, such as the metal box. The gravitational force will keep pulling untill there is another force from the sand pushing it back. The force from the sand is the boyance force which is the product of the sand density and the replaced volume (or the volume that sunk into the sand). So, you can see the boyance force depends on the replaced volume, and this volume has limit which is the volume of the object. If this boycance force is not big enough to overcome the gravitational force of the metal box, the metal box will keep sinking untill hit bottom ... if there is a bottom :)
Is that making any sense ?

Answer 2

The sinking metal box exerts equal and opposite force on the quick sand but the metal box is also experiencing equal and opposite gravitational force (weight) in the downward force due to earth.

The downward force (weight) on the box due to the earth is more than the upward force due to the quicksand , hence the resultant force on the box is downward and the box sinks due to the resultant downward force

Answer 3

There are still equal and opposite forces, but just because they're equal forces does not mean they have euqual RESULTS. I know it sounds confusing, but just think of it this way- because the quicksand's mass is bigger than the big metal box, the force on the quicksand seems much less noticeable than the force on the box. Trust me, its the smae force, but different results.

Hope thats not too confusing!

Answer 4

I have read all answers up till now and am not satisfied by any. In order to understand and not to confuse oneself one should learn to list the pair of equal and opposite pairs of forces which Newton's third law talks about. Remember any pair of equal and opposite force does not become the pair which Newton's law talks about. For example if A applies force on B and C applies equal and opposite force on D. They do not form the pair which Newton's law talks about.
In fact Newton's law States that any force anywhere is the result of interaction of two things at a time. and these two interacting things always exert equal and opposite forces on each other.

In the metal box sand case, metal box applies let us say force, 'a' on sand as result of the force of earth on metal box. The force which sand would apply on metal box will be equal and opposite to 'a'. So also the metal box would apply equal and opposite force on earth taken as a whole which can be supposed to act on the centre of earth as earth,s force on metal box is supposed to come from the centre of earth. This pair belongs to the pair of gravitational pair and the other pair is between object which you may call of friction basically pair of electromagnetic forces.

Answer 5

the force here is the gravitation force which pulls down the metal box.