The complete sentence is as follows:
When a loose brick is resting on a wall, it has potential energy. When the brick is pushed off the wall and is falling down, the amount of potential energy is decreasing while the amount of kinetic energy is increasing.
Explanation:
- Potential energy is the energy related to the position of an object: in particular, gravitational potential energy is given by
where m is the mass of the object, g is the gravitational acceleration, and h is the height of the object above the ground.
- Kinetic energy is the energy related to the motion of an object, and it is given by
where m is the mass and v the speed of the object.
In the example, we see that:
- at the beginning, the brick is at rest on the wall, so it only has potential energy due to its height above the ground (while the kinetic energy is zero, since it is at rest, so speed v is zero)
- as it falls down, the potential energy decreases, because the height h decreases, while the kinetic energy increases, because the speed of the object increases as it falls down.
