# Contrary movement

1. Aug 30, 2008

### sedaw

When 2 objects placed one on each other and one of them pulled, the second will move in a contrary way.

there is a way to proof it with newtons law ???

tnx .

2. Aug 30, 2008

### tiny-tim

Hi sedaw!

I'm not sure what you mean … can you be more specific?

Do you mean that, for example, if a man is standing on a table on wheels, and he walks forward, then the table will move backward?

That's because of Newton's first law … no external forces are acting, so the centre of mass stays at the same (stationary) velocity … in other words, it stays where it is.

Alternatively, use Newton's third law … for the man to walk, there must be a force on him from the table, and so there must be an opposite force from him on the table, which makes the table move the opposite way.

Or use Newton's second law …

Or am I misunderstanding the question?

3. Aug 30, 2008

### sedaw

http://img516.imageshack.us/img516/754/3232ej8.jpg [Broken]

Hello !

i`m hope it will be clear now...

if the force F will work on the object A and make em move to the right side the object B will move or aspire to move to the left.

why is that ?

TNX

Last edited by a moderator: May 3, 2017
4. Aug 30, 2008

### tiny-tim

No … if there is friction, B will move to the right (only very slightly, if you pull hard enough);

if there is no friction, B will remain stationary.

5. Aug 30, 2008

### sedaw

so the friction cause to the movment ?

6. Aug 30, 2008

### tiny-tim

Yup!

The friction causes the movement.​

7. Aug 30, 2008

### HallsofIvy

Staff Emeritus
And, notice that the friction causes B to move in the same direction as A, not "contrary". If there is little or no friction, B's movement may be less than that of A so B moves back relative to A, not relative to the floor.