The equation of motion of a pendulum is:
where 
it its length and 
is the gravitational acceleration. Notice that the mass is absent from the equation! This is quite hard to solve, but for <em>small</em> angles (
), we can use:
Additionally, let us define:
We can now write:
The solution to this differential equation is:
where 
and 
are constants to be determined using the initial conditions. Notice that they will not have any influence on the period, since it is given simply by:
This justifies that the period depends only on the pendulum's length.
The feel of weight comes due to the normal reaction force given by the support. Hence, the condition of weightlessness is when the normal reaction force becomes zero. So, during free fall there is no support which can provide the normal reaction. Hence, the bungee jumper feels weightless as she falls towards the earth because of the lack of support force that balances gravity.
Hence, the answer is 3.
All sources of energy on the planet (excluding nuclear reactions) are ultimately attributable to the sun. 'Fossil fuels' are derived from what once were living organisms, and all sources of food begin with primary producers (plants and algae which utilize photosynthesis).
