Energy conservation — FAQ

In any real-world energy conversion, some energy is always dissipated or 'lost' to the surroundings in non-useful forms, most commonly as heat due to friction or air resistance, and sometimes as sound. This means useful output energy will always be less than total input energy.

You use $W = Fd$ when the force applied is exactly in the same direction as the displacement. You use $W = Fd \cos \theta$ when the force and displacement are at an angle $\theta$ to each other, as it correctly accounts for only the component of the force acting along the direction of motion. For Cambridge AS Physics, you must be prepared to use both depending on the problem.

Yes, GPE can be negative if your chosen reference point for zero potential energy is above the object's current position. Since only the change in GPE is physically significant, the absolute value doesn't matter as long as your reference point is consistent throughout the problem.