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AP Physics C: Mechanics. Work & Energy. What is Energy?. The ability of a physical system to cause change. What is its definition?. Energy can come in many forms and is transferrable. It is a conserved quantity in the universe. What are its properties?.

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what is energy
What is Energy?

The ability of a physical system to cause change

What is its definition?

Energy can come in many forms and is transferrable. It is a conserved quantity in the universe.

What are its properties?

Chemical, electrical, nuclear, thermodynamic, light, mechanical, kinetic, potential, elastic, etc.

What forms can it be in?

what is work
What is Work?

The amount of energy transferred by a force acting through a displacement

What is its definition?

Work (Energy) is a scalar quantity.

What are its properties?

How can it be calculated?



The area under an Force-displacement graph


Work is an energy transfer.

If energy is added to the system then W is positive.

If energy is transferred from the system then W is negative.


Work is a scalar.

The NET work on an object is the sum of the work done by each force acting on it.


Units of work

1 erg=10-7J

work energy theorem
Work-Energy Theorem

The work done by a net force in displacing a particle equals the change in kinetic energy of the particle. Stress on the word particle. This is a drastic idealization.

mechanical potential energy
Mechanical Potential Energy
  • Gravitational
  • Elastic
conservation of energy
Conservation of Energy

This is always true, but mechanical energy is only conserved when only conservative forces are present.

Work done by a conservative force is path independent

Forces such as friction and air drag are non-conservative. They result in energy dissipated as heat.


The time rate at which work is being done.

How do we calculate it?

If a net force acts on a particle and does work W over an interval of time Δt, then the average power is:


The time rate of energy transfer.

Instantaneous Power


Watt are the units?


Note: A kilowatt-hour is often used by power companies. What is it a measure of?

potential energy and conservative forces
Potential Energy and Conservative Forces

Potential energy is only associated with conservative forces. If an object or field does work on some external object, energy is transferred from the object or field to the external object:

The energy transferred as work decreases the potential energy of the system from which it came.

potential energy and conservative forces1
Potential Energy and Conservative Forces

The conservative force equals the negative (partial) derivative of the potential energy with respect to x.

The negative sign means that the final potential energy of the system is lower than the initial as the conservative force does work.