# Why F=ma?

1. May 14, 2012

### Phyman1109

Why does a body accelerate when it experiences a net force? Is there any change in molecular state of the body when it experiences a force?

2. May 14, 2012

### Andrew Mason

We perceive a force as something that causes a body to change its motion.

If we accept that all inertial frames of reference (frames of reference that are not subject to forces) are equivalent (eg. we cannot tell by the way bodies behave inside an enclosed railway car whether the railway car is moving on a smooth straight track or stopped), it can be shown that Force must be equal to mass x acceleration, where mass is the number of unit masses contained in a body and acceleration is the time rate of change of the time rate of change of position.

AM

3. May 14, 2012

### meldraft

The simple explanation is that this is what we observe in nature.

The more complex explanation is that solid bodies (to keep it simple) are made up of arrays of molecules. If you apply a force at a point (say on one molecule), it will move closer to a neighboring molecule. If the combination of force you are applying and object that you are trying to move is not such that threatens the structure of the object, at some deformation you will stop displacing that molecule and start moving the entire object.

You can visualize this very roughly if you attach a piece of rubber to an object and try to pull it. The rubber will deform up to a point, and then you will start pulling the entire object.