The great roman poet, lucretius wrote, "things cannot be born from nothing, cannot when begottend brought back to nothing." this statement became an accepted theory as a result of the work of the chemist, lavoisier in the 18th century. this is the law of conservation of matter. similarly, the law of conservation of energy is an important law in physics. can you discuss the validity of the conservation of energy in terms of potential (gravitational or elastic) and kinetic energy by giving real-life applications

Explanation:

When a swing is at its maximum height, the potential energy, associated with the height, is maximum. At this point its speed is zero, so the kinetic energy is zero. As the swing descends, having less height, the potential energy decreases and the kinetic energy increases. When it passes through the position with the minimum height, the potential energy is minimal and the kinetic energy is maximum.

This is true in the ideal case, in which there is no friction due to air resistance.  When friction is taken into account, implies that the swing stops gradually, since its energy is converted into thermal energy

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.