# B Forces-General Doubts

1. Dec 4, 2017

### Jon Drake

Suppose there is an extended block lying on a frictionless surface. The surface will exert a normal force on block. Will the normal force act at one particular point on the block, or will it act along the entire length of block?

2. Dec 4, 2017

### CWatters

The Normal Force is a simplified model of how the block and surface interact. In practice it depends how flat the block and surfaces are. A warped piece of wood might only make contact at say three corners. A very soft rubber block would make a more uniform contact.

Depending on the exact set up and what you are trying to calculate you might need to model and/or position the normal force differently, for example as either:

a distributed force or
acting at a point
acting on a line
acting at a number of individual points.

For most simple problems such as block sliding down a slope we assume a single force acting through the centre of mass. Any complication due to the block or surface not being exactly flat is hidden in the coefficient of friction (which is typically found by experiment rather than calculated).

For something like a box being tipped over we might model the normal force as acting along one edge (which would be the case just after it starts to tip).

If you have a particular problem in mind perhaps post details.

3. Dec 4, 2017

### CWatters

PS You mention the frictionless case. The key thing to understand with frictionless surfaces is that any force between them must be perpendicular/normal to the surface.