Before answering the question, let's remind the difference between kinetic energy and potential energy:
- Kinetic energy: it is the energy due to the motion of an object. It is given by
[tex]K=\frac{1}{2}mv^2[/tex]
where m is the mass and v is the speed: so, an object has kinetic energy whenever it is in motion
- Potential energy: it is the energy due to the position of an object. It can be of different types:
-- Gravitational potential energy: due to the height h of an object with respect to the ground
-- Elastic potential energy: energy stored by a compressed/stretched spring
Keeping in mind these definitions, let's now analyze each case:
1) a rock at the edge of a cliff... --> potential energy (because it has some height h above the ground, while it is at rest, so no kinetic energy)
2) a plane in flight at 30,000 feet... --> both potential and kinetic energy (because it has some height h above the ground and it is also moving)
3) water behind a dam... --> potential energy (because it has some height h above the ground, while it is at rest, so no kinetic energy)
4) water flowing downstream... --> both potential and kinetic energy (because it has some height h above the ground and it is also moving)
5) a car moving on a level road... --> kinetic energy (because it is moving, while it has no height h above the ground, so no potential energy)
6) a compressed spring... --> potential energy (elastic potential energy)
7) a child on a swing... --> both potential and kinetic energy (because it has some height h above the ground and it is also moving)
8) a bouncing ball... --> both potential and kinetic energy (because it has some height h above the ground and it is also moving)
A rock at the edge of a cliff
Explanation
As the rock is waiting on the edge of the cliff it has potential energy due to its position. This is the resting position of the object that is having the self-energy. However, due to the wind flow, heavy storm or any other movement, the object shall move from its place. As it moves it will automatically convert the potential energy into kinetic energy.
A plane in flight at 30,000 feet.
Explanation
As the plane takes off from the land into the air, it goes higher as per the requirement. Here the takeoff position of the object possesses mechanical energy, later this mechanical energy is converted into potential energy. Thus, when the plan takes a study position at the height of 30,000 feet it maintains having the potential energy.
Water behind a dam
Explanation
Another similar case of potential energy changes into kinetic energy. The water is at a resting position in the dam. The water possesses potential energy while it's in the dam without moving. However, as the water starts to flow due to the wind blow in the dam, the water from its resting position adopts the changes, the potential energy to kinetic energy automatically.
Water flowing downstream
Explanation
As the water is flowing downstream from a steady position to its moving positions, the energy changes as the movement take's place. As the natures pattern of movement of any object, the potential energy of the resting water converts into kinetic energy. It will have kinetic energy due to its motion from the resting position of the potential energy.
A car moving on a level road.
Explanation
When the car is at the resting motion it has potential energy. Any object that is at a resting pose will have potential energy. Though, as the object starts moving it will gain kinetic energy from potential energy. A moving car possesses kinetic energy due to its motion of the car from its resting position. Energy conversions are very common in objects from one place to another.
A compressed spring.
Explanation
A compressed spring possesses potential energy as it is not in a moving position. An object when at a steady posture it possesses potential energy which is also considered to be its self-energy. As the object starts its motion it automatically gains or converts the potential energy of the steady object into kinetic energy. It is a system of the nature of all the objects in the world.
A child on a swing.
Explanation
A child on a swing will have both form of energy, kinetic and potential energy. The part of swinging possesses potential energy and the kinetic energy is the last part when the swing is moving back and forth. Initially when the swing is not moving it will have potential energy. In addition to this, as the swing starts to move the swing gains kinetic energy.
A bouncing ball.
Explanation
A ball when is at the steady motion, it possesses potential energy because it does not have movement. However, as it starts to bounce it gain kinetic energy and then reverse back to have potential energy. As many time it bounces, it gains energy and converts back into another form of energy. This system goes on, until the ball stop bouncing and comes back at the steady position.
