A.3 Work, Energy & Power

What is the Principle of Conservation of Energy?

Energy cannot be created or destroyed; it can only be changed from one form to another. The total energy of a closed system always stays the same.

What is the relationship between Work Done and energy?

Work done by a force is exactly equal to the amount of energy transferred. If you do ten units of work on an object, you have transferred ten units of energy to it.

What is a Sankey Diagram?

A visual way to show energy transfers. The width of the arrows represents the amount of energy. The main arrow shows the total energy put in, which then splits into "useful" energy and "wasted" energy.

How do you calculate Work Done by a constant force in words?

Work equals the force applied times the distance moved times the cosine of the angle between the force and the direction of movement.

What is Mechanical Energy?

It is the total sum of an object's energy of motion (kinetic energy) and its stored energy (gravitational and elastic potential energy).

When is Total Mechanical Energy conserved?

It is conserved when there are no outside friction or air resistance forces acting on the system. In these cases, the sum of kinetic and potential energy never changes.

What is Kinetic Energy and how is it calculated in words?

It is the energy an object has because it is moving. It equals one-half times the mass times the speed squared.

What is Gravitational Potential Energy in words?

It is the energy an object stores because of its height. The change in this energy equals the mass times the strength of gravity times the change in height.

What is Elastic Potential Energy in words?

It is the energy stored in a stretched or squashed material, like a spring. It equals one-half times the spring constant times the change in length squared.

What is Power and how do you calculate it in words?

Power is the speed at which work is done or energy is transferred. It equals the work done divided by the time taken. For a moving object, it also equals the force times the velocity.

What is Efficiency in plain words?

It is a measure of how much useful energy you get out compared to what you put in. It equals the useful energy (or power) output divided by the total energy (or power) input.

What is Energy Density?

It is the amount of energy stored in a fuel source per unit of its volume or mass.

How do you find the work done by a Resultant Force?

The work done by the total resulting force on a system is exactly equal to the change in the total energy of that system.

What happens to Mechanical Energy when friction is present?

The total mechanical energy decreases because some of it is "lost" as heat. The amount lost is equal to the work done by the friction forces.