Presentation on theme: "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."— Presentation transcript:
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: