Nice questions. Let me start with the easy last one. The laws apply to any animal including humans just as much as they apply to anything else, including a sack of rice or a ball of clay.
The first law says that anything not subject to outside influence will continue moving at the same velocity.
The second says that objects can influence each others motion via forces, with accelerations given by a
/m, where m is the object's mass.
The third says that if object A exerts force F
on B, B exerts force exactly -F
(opposite direction) on A.
Here's one way they tie together. Say you look at an object like that sack of rice. You could also think of it not as one object but a collection of many- the individual grains. They are exerting forces on each other. The third law says that if you add up all the forces between these parts, you have to get zero. So there's no net
force resulting from the interactions of all those parts. Therefore, using a
/m we get a
=0. And that's the first law! (Here by 'a
' we mean specifically the acceleration of the center of mass of that collection of grains, the same as the apparent acceleration of the whole bag.)
(published on 01/26/09)