15073 Experiment Pack - Simple Machines - info

STE 15073 Experiment Pack – Simple Machines

INTRODUCTION

This Experiment Pack includes everything needed to provide students with a practical understanding of simple mechanical force measurements, levers, pulleys and moments. It includes an 11 page Instructions and Teaching Guide.

TOPICS COVERED

1. THE DYNAMOMETER

2. EQUILIBRIUM OF A ROD WITH CENTRAL FULCRUM

3. FIRST KIND OF LEVER

4. SECOND KIND OF LEVER

5. THIRD KIND OF LEVER

6. FIXED PULLEY

7. MOBILE PULLEY

8. SIMPLE TACKLE

9. INCLINED PLANE

EQUIPMENT LIST

EXAMPLE EXPERIMENT

(all worksheets have necessary diagrams)

2. EQUILIBRIUM OF A ROD WITH CENTRAL FULCRUM

Arrange the device showed in fig. 4 and put one of the two mass holders at the left end of the rod loading it with a 10g mass. In this way you will have a 30g total weight as the mass holder weights 20g.

On the right arm of the rod arrange the other mass holder changing the attaching hole and the masses each time, in order to reach the equilibrium.

From this experiment it is clear that you can reach the equilibrium of the rod when the products of forces F1 and F2 and the length of their arms b1 and b2 are the same. It means that the forces are inversely proportional to the length of their arms:

F1.b1 = F2.b2                  (1)

And also  F1/F2 = b2/b1     (1a)

The product of the force times the distance from the pivot or fulcrum is known as a moment.

When the arrangement above is in equilibrium then the two moments in equation (1) are equal. This equality is an expression of the principle of moments.

It is this fact that allows us to vary the distances from a fulcrum at which forces and loads (resistances) are applied in order to obtain useful levers, so as, for example, to move or lift heavy loads.

When a lever (or pulley system) is arranged to increase the force that can be applied there is said to be a mechanical advantage.

An important historical example of the development and use of the principle of moments was achieved by Sir Christopher Wren in the design of the Sheldonian Theatre in Oxford in 1664-1668. This ceiling was at the time a great innovation – the largest flat unsupported ceiling anywhere.