# Question #75e1d

Dec 10, 2014

All three! Each law tells us something about why the picture does not move.

#### Explanation:

Let's start with what we can observe directly. The picture does not move. So, Newton's first law tells us that it will remain at rest, unless it's state of motion is disturbed by some force (it could be due to some different force being applied ou due to the change in forces that are currenty acting over the picture).

Newton's second law tells us that the resultant force $F$ (the sum of all forces) acting on the picture is equal to the product of the mass $m$ of the picture by it's acceleration $a$, or

$F = m a$

Since the picture is at rest, it's velocity does not change and is, in fact, equal to $0$. Therefore, it's acceleration is $0$. So, the resultant force acting over it must also be equal to $0$.

Lastly, Newton's third law tells us that for every force acting over an object $A$ that is due to an object $B$, there is a force of equal magnitude and oposite direction acting over the object $B$ that is due to $A$.

For the case of the picture, gravity pulls the picture down. So, because the picture is attached to the wall, it must exert a (contact) force equal to it's weight on the wall, acting downwards. By Newton's third law, the wall must exert a force of the same magnitude (equal to the weight of the picture) pointing upwards, wich compensates the weight of the picture. So, we have the result that the resulting force acting on the picure is $0$, a result that is consistent with the one obtained using Newton's second law, but that addresses the interaction of the picture with other bodies.