In this question, three acrobats are standing
on top of each other in a human tower. In
this first part, we are interested in the
forces acting on the top acrobat. There are
two vertical forces acting on him. The first
of these is his weight, which acts downwards,
and is equal to m, his mass, timesed by g,
the acceleration due to gravity. There is
also an upwards reaction force from the middle
acrobat. For this acrobat to be in equilibrium,
the resultant force on him needs to be equal
to zero. This means that the reaction force
and his weight must be equal and opposite.
We can now work out the reaction force acting
on the top acrobat given his mass.
