If the kinetic and potential energy in a system are equal, then the potential energy increases. What happens as
result?
Total energy increases.
Stored energy decreases.
Energy of motion decreases,
© Total energy secreases.

Total energy increases.

Explanation: Total energy is the sum of all different types of energies a body can have. A molecule can have Kinetic energy due to it's random motion, potential energy due to it's bonding with other molecules, rotational kinetic energy due to it's rotation. In this case, total energy is the sum of all these.

a. 4.5secs

Explanation:

From the question, the equation describing the height is given by

at the point when the rock hit the ground, the height,h will be zero.

Hence we can have

hence the rock will hit the ground in 4.5secs