Potential energy and kinetic energy confusion

[roxas]

okay this question is confusing like everything i currently know about p.e and k.e.
okay so the maximum energy of a rollercoaster is 4000j. the mass of the trolly is 20kg, and the acceleration due to gravity is 10m/s^2(dunnoe where's the squared button on the keyboard) okay so i am supposed to calculate the velocity at this point. my teacher gave the working for this answer and he minus the potential energy from the total energy to find the k.e, and this is where i got confused. isn't the potential energy lossed =the gain of K.E? why did he minus the P.E from the T.E? help! my physics practical is a day away and i can't afford to be confuzzled.

by the way i didnt clarify this wif my teacher as i was daydreaming in class and just copied down the answer. thx

 

[radou]

The total energy consists of kinetic and potential energy, as you know already. Since total energy is conserved, maximizing kinetic energy must minimize potential energy, and vice versa. So, an object has its maximal kinetic energy when its potential energy is minimal. The same holds for maximal potential energy.