Mechanical Energy - Kinetic and Potential Energy

Mechanical Energy:
- What causes motion?
- Energy is the ability to cause change.
- Kinetic energy (KE) is the energy due to motion. All moving objects have kinetic energy.
- The kinetic energy of a moving object depends on two factors: the object’s speed and its mass.

A baseball has more mass than a tennis ball.
The more massive ball creates a larger dent because it has more kinetic energy than the less massive ball.
Kinetic energy increases as mass increases.
The relationship between mass and kinetic energy is a proportional, linear relationship.
KE ∝ M

Like mass, speed is a factor that affects kinetic energy.
The faster an object moves, the more kinetic energy it has.
When you increase the speed of an object, the kinetic energy increases by the square of the factor. This is a square, nonlinear relationship.
If pitchers throw the ball twice as fast, they quadruple its kinetic energy.
KE ∝ V^2

Potential energy is the energy due to interactions between objects or particles when distance changes.
The amount of potential energy an object has depends on the positions of objects or particles.
Elastic potential energy is energy stored in objects that are compressed or stretched, such as springs and rubber bands.

Open and closed systems.
In a closed system, the focus is just on the rubber band and the nickel. In an open system, the environment surrounding the objects is involved.
The farther back a rubber band is stretched, the farther a nickel will move. The rubber band contains more potential energy when it is stretched farther back. The larger the change in position between the objects, the greater the potential energy.

Some types of potential energy are due to fields created by objects. These fields are invisible to the eye and extend into the space surrounding the object.
Springs can contain elastic potential energy. The more you change its shape, the more potential energy the spring contains.
Gravitational potential energy, electric potential energy, and magnetic potential energy are examples of potential energy due to forces in fields.

Often, the focus is on the change in the potential energy of an object, rather than the potential energy of an object at a particular point in time.
Gravitational Potential Energy: Stored energy due to the interactions of objects in a gravitational field.
The farther away the object is from the center of the gravitational field, the more gravitational potential energy the object has in relation to the field.
The difference in gravitational potential energy depends on the object’s change in position and its mass.
The potential energy of the backpack increases as its height increases.
Gravitational potential energy and mass have a proportional relationship.

Mechanical Energy: The sum of the potential energy and the kinetic energy in a system.
All forms of energy are measured in joules (J).
When objects change motion, kinetic energy changes to potential energy and vice versa.
As the ball moves upward, its speed and kinetic energy decrease. However, the potential energy is increasing because the ball’s height is increasing. Kinetic energy is changing into potential energy.
At the ball’s highest point, the gravitational potential energy is at its greatest, and the ball’s kinetic energy is at its lowest.
As the ball drops, potential energy changes to kinetic energy.

The law of conservation of energy states that even though energy is always transferring, energy is not created and energy is not destroyed.

A change in an object’s motion is a sign that an energy transfer occurred between objects.
A force is a push or a pull. When a force is applied to an object, the object’s kinetic and potential energy can change. You can transfer this energy when you do work.
Work is the transfer of energy to an object by a force that makes the object move in the direction of the force.
Work depends on force and distance.

The direction of friction is in the opposite direction of the motion. Friction between a car’s tires and the road causes some of the kinetic energy of the tires to transform into thermal energy.
In every energy transformation and every energy transfer, some energy is transformed into thermal energy. This thermal energy is transferred to the surroundings. A change in temperature is another sign that an energy transfer has occurred.