Activity 04 — normal reaction
The last factor is how hard the two surfaces are pressed together. The force that measures that has a name: the normal reaction.
Normal reaction
A block resting on a table presses down on the table with a force equal to its weight. By the third law the table pushes back on the block with an equal and opposite force, at right angles to the surface. That upward push is the normal reaction, R.
On a horizontal table with nothing else pushing or pulling vertically, R is simply equal to the weight.
Apparatus
Three wooden blocks each of weight 20 N, and a Newton balance.
Method
- As in the previous experiment, stand one block on the table and find the force needed to start it moving — the limiting friction.
- Then take a second block, place it on top of the first, and find the limiting friction again in the same way.
- Then, as in figure 5.5, place the third block on top of the first two and find the limiting friction once more.
Figure to be added
Instead of stacking blocks on the first one, the experiment can equally well be done by placing various other weights on top of it.
Observation
The readings are not equal. As the weight increases the limiting friction increases with it.
| Blocks | Total weight | Normal reaction R |
|---|---|---|
| One | 20 N | 20 N |
| Two | 40 N | 40 N |
| Three | 60 N | 60 N |
Conclusion
When the weight of an object increases, the force it exerts on the table increases, and so does the equal and opposite force the table's surface exerts on it — the normal reaction. As the normal reaction between the two surfaces increases, the limiting friction increases too.