newton's first law-inertia!!!!!!!!! truth or fake?

Question

according to newton's1st law!
a body tries to be in motion untill and unless some external force is applied on it!
and a body tries to be in rest position until and unless any external force is applied!!!!!!!!!

so my question is,
suppose there is a hollow cube placed on a frictionless surface and a man is standing inside it.can that man move that cube?if he can, then isnt the law of INERTIA wrong?

note:
1)inside the cube there is sufficient friction so that man can move.
2)man do not have any tools with him.

Answer 1

Newton’s Laws are indeed valid (specifically for non-relativistic values, past this they can be extended using relativity).

But onto your question....

Yes, the man can move the cube, but not indefinitely unless the cube in of an infinite size. If the cube has some finite size, the cube can move, but then it will stop at some point for reasons I am about the explain.

Let’s consider the initial state of the man located in the center of the bottom 'wall' of the cube. Both the man and the cube are stationary with respect to the frictionless surface the cube sits on.
You said there was enough friction inside the cube for the man to move around. So what would happen if the man started to walk 'forward'? In order for the man to walk friction will exert a force on the man and the man will, in turn exert a force on the bottom wall of the cube. As he puts his foot down and then attempts to pull it back towards himself, friction exerts a force in the opposite direction of the motion of foot and pushes the man forward. By way of friction again, the man is exerting a force back onto the box that pushes the box backwards. This process is nicely summed up in Newton's 3rd law and the law of conservation of momentum. In order for the man to move forward, the box must start moving backward. If the man accelerates forward, the box accelerates backward. Overall, the net change in momentum is zero even though both the man's and the cube's momentums at any instant might not be zero.
So the man can defiantly start the cube in motion if he starts to move around inside the cube.

But if the cube is of a finite size...the man cannot move around forever in a straight line, he has to stop, turn around and change direction. In order to stop and/or change direction the max must accelerate. Whenever something accelerates there must be a net force. This net force is acting both on the man and, by taking into account what is causing this force (friction) also on the box too, in order to conserve momentum to be conserved.
Since our initial state was that the man and box were both stationary....whenever the man is stationary so must the box be, and whenever the man is moving "forward", the box must be moving "backward".

Answer 2

Yes, the man can move the cube but only so far. Once the man walks far enough to run into the interior wall of the cube, the cube will stop shifting. Note that the man will ALWAYS be in the same X-Y position the whole time, because he has no means of changing his position with the cube being on a FRICTIONLESS surface and him trapped inside. He cannot build momentum to shift his position with respect to the FRICTIONLESS surface. The key to this is to remember the frictionless part of it. There's nothing this guy can do inside this cube to change his position. When he exerts force on the floor of the cube in one direction, he will be moving the cube and not himself, until he hits a wall and has to stop.

Answer 3

Inertia is defined as an object's tendency to remain at rest or to keep moving unless acted upon by an outsided force which is also the definition of Newton's 1st Law. The Second Law deals with how force is equal to mass times acceleration. The third law says that any force exerted on an object will be pushed back by that object with an equal and opposite force.

Answer 4

read the WHOLE of what i write very very carefully....i know what doubt u r having and it is VERY imporatnt to eliminate it lest u will fall wrong afterwards

a body moves due to the disbalance of forces acting on it
do not take a cube---take a sphere......if the man provides his force as an external force which is wasted away when the body shows response as a constituent of energy spent....the body CAN move...
inertia says that a body continues to be in its initial status ,but IF something like external force is applied here,which certainly is being in this case:the body WILL move............mass is the same-everything is the same----except that there is a virually external force acting on the body......the body that is APPLYING the force ie,the man's body and the bodies which are MOVING ie,both the man's and the sphere's body ,...okay,if the mass of the man is very small ,okay if the inertia is very small...and the inertia of the body being moved is very large....

but intertia of a body is something with which we have to move about...it CANNOT be overcome...it remains to be there...its a status quo rather than something that can be neutralised to zero
we move WITH it...we CANNOT finish inertia
a body can resist a change in its state due to its mass, but if a force moves it ultimately ---it doesnt mean that it has OVERCOME inertia......F= ma ...mass has already been taken inot full consideration with force......if a body that is MOVING another body gives a great force...remember, i may be only 4kg by mass,...but STILL i can apply a force of say ANY newtons.....if the boy did the same=> then the body is CERTAINLY gonna move.....now is there any confusion? i think not!

Answer 5

Why so many exclamation marks? Anyway... In this example the man would be considered an external force and yes depending on how he moved he could move the cube. An external force is any force that does work onto an object.

Answer 6

The man can jump against an inside wall of the cube and it will move. Although he is inside the cube, he is still external to the wall of the cube, so the law holds. What's the problem? His jump energy goes down against the floor, not against a side wall.

Answer 7

He can because he can manipulate the friction in certain ways such that the cube will move. It still keeps true with newton's 1st law because he can put 100 n force one way and 500 n force the otherway if he knows what he's doing. Astronauts have to do this during spacewalks.

Answer 8

No because if he puts a force on one side of the cube there will be an equal force on the other side of the cube. No matter how hard he tried, the cube would remain stationary

Answer 9

Newton's law is not fake.

Classical physics is base on the Newton' three laws and they support each other.

I do not understand your whole statement regarding "so my question is".

First of all there is no such thing as "frictionless surface". There is always some friction, it is relatively less but it is there.

Answer 10

"unless some EXTERNAL force"
the man would provide and internal force :)