
Aim: to show that Forces can be added in the same
way as Displacements 
Displacements can be added by drawing a scale
diagram (either a triangle or a parallelogram). 
See Adding Vectors 
The aim of this experiment is to see if forces can
be added using the same method. 
If forces can be added in the same way as
displacements, we are probably justified in assuming
that all vector quantities can be added this way. 

Method 
Hold (or fix) three spring balances (also called
dynamometers) as shown in the diagram below. 

If the point O is stationary, then the three forces
are in equilibrium. 
This means that force A is balancing the combined
effects of forces B and C. 
Similarly, force B balances the combined effects of
forces A and C (and force C balances the combined
effects of forces A and B). 

1. 
Accurately mark on the paper the positions
of points O, A, B and C. 

Write beside A, B and C the magnitudes of
the forces (readings of the spring balances). 
2. 
Draw lines joining O to A, O to B and O to
C, so that your diagram records the
directions in which the forces were acting
as well as the magnitudes of the three
forces. 
3. 
Repeat the procedure with forces of
different magnitudes pulling along different
directions. 


Analysis of the results 
Using a suitable scale, add arrows to the diagram
representing the three forces. 
For example, if your scale is:
1cm represents 0.1Newtons,
a force of magnitude 0.8N
would be represented by an arrow
8cm long. 

On the diagram, do the vector addition F_{A}
+ F_{B} as shown in the example
below. 



Is the result of this addition equal but opposite to
the force F_{C} ? 

Note that each set of results allows you to attempt
to verify the principle three times. 