AP PHYSICS REVIEW Energy

Work  Work is when a force is applied to an object to move it a distance.  W = Fd cos( Ɵ )  Work can be done by many forces including gravity, friction etc…  Ɵ is the angle between the direction of the force and the direction of the displacement.  If the displacement and force are at 90 degrees. The work done by that force is 0.  Units of work are a Nm or a Joule (J)  A Joule is a unit of work and also a unit of any kind of energy.  Work is a vector and can be negative

Power  Power is the rate at which work is performed or the rate at which energy is change.  P = W/t or P=Fd/t since d/t =v, P=Fv  The units of power are Watts (W) or J/s or Nm/s  Power is a scalar quantity and does not have a direction. However it can be negative if the work done is negative. It would simply imply that the work or energy is decreasing at a certain rate.

Work Energy Theorem  W = ΔKE  Kinetic energy is the energy an object has because of its motion.  Work done on an object can change its motion and therefore change its kinetic energy.  If friction is doing work, it can decrease the object KE. Likewise if a thruster does work, it can increase the KE.  KE is a scalar and cannot be negative but ΔKE can be negative if the energy is decreasing.

Potential Energy  PE is energy stored based on an objects position, or chemical makeup.  Gravitational PE is the energy stored in a an object based on its height above a reference point. Typically that reference point is the ground.  GPE = mgh  Elastic PE is the energy stored from compressing or stretching a spring or other flexible object.

Conservation of Energy  Assuming there are no external forces that do work to decrease the energy (non conservative forces), the total KE and PE initially will equal the total final KE and PE  Keep in mind that there is mass in each part of this equation. They may cancel out.

Non Conservative Forces  If friction or air resistance is present (non conservative forces), the final mechanical energy will not be equal to the initial mechanical energy.  KEi + PEi + W = KEf + PEf  The work in this case will most likely be negative and therefore decrease our initial energy.  This can also be written as: