Final answer:
A brick on the ground has no gravitational potential energy because it is at the reference level where GPE is zero. A 1 kg brick at a height of 4 m has a GPE of 39.2 joules, but once it falls to the ground, its GPE is 0 joules, being converted to kinetic energy during the fall.
Step-by-step explanation:
Gravitational Potential Energy
A brick resting on the ground would have no gravitational potential energy (GPE) because GPE is dependent on the height above a reference point. When we choose the ground as the reference level where GPE is zero, any object on the ground would have a height of zero and thus its GPE would also be zero. However, if a brick with a mass of 1 kg is placed at the top of a 4 m high roof, we can calculate its GPE using the formula GPE = mgh, where m is the mass, g is the acceleration due to gravity (9.8 m/s²), and h is the height above the reference level.
The GPE at the top would be:
GPE = 1 kg * 9.8 m/s² * 4 m = 39.2 joules
Once the brick falls to the ground, its GPE would be:
GPE = 1 kg * 9.8 m/s² * 0 m = 0 joules
The loss of GPE as the brick falls is transferred to the brick as kinetic energy, assuming no air resistance or other forces are acting on it. Energy conservation tells us that the change in kinetic energy of an object is equal to the negative change in its potential energy. When the brick hits the ground, the GPE has been converted to motion (and ultimately, upon impact, to other forms of energy such as heat and sound).