Work, Kinetic Energy, & Potential Energy

WORK: Work is done only when… Work Equation The amount of energy transferred by a force acting on a body through a distance Work is done only when… a force causes an object to move in the direction of the force. Work Equation work = force x distance W = F x d No work is being done on the tray

Work is measured in joules (J). 1 J = 1 N•m = 1 kg•m2/s2 1 joule is roughly equal to the energy required to lift a small apple 1 meter POWER: the rate at which work is done or energy is converted. power = work / time Measured in watts (W) 746 watts = 1 horsepower James Prescott Joule

ENERGY: the ABILITY to do work. When you do work on an object, you transfer energy to that object. Whenever work is done, energy is transformed or transferred to another system. Energy is also measured in joules. Energy is not a substance!

Energy Systems Scientist study energy systems. Boundaries define a system and are set by the observer. Systems may be open or closed. CLOSED SYSTEM: the flow of energy into and out of a system is small enough that it can be ignored OPEN SYSTEM: the system exchanges energy with the space that surrounds it.

KINETIC ENERGY (KE): Energy of a moving object due to the object’s motion. Kinetic energy depends on… Mass: more mass = more KE Speed: faster = more KE Kinetic Energy Equation

Potential energy is stored energy. POTENTIAL ENERGY: Energy that an object has because of the position, shape, or condition. Potential energy is stored energy. There are many types of potential energy that we will cover… Gravitational Elastic Chemical Electrical Magnetic Potential Energy + Kinetic Energy = Mechanical Energy

GRAVITATIONAL POTENTIAL ENERGY (GPE): Energy an object has due to its relative height Gravitational potential energy (GPE) depends on: Mass: more mass = more GPE Height: higher = more GPE Gravitational free-fall acceleration: 9.81m/s2 (same everywhere on Earth) Gravitational Potential Energy Equation GPE = mass (m) x free-fall acceleration (g) x height (h) GPE = mgh

Energy doesn’t appear out of nowhere. LAW OF CONSERVATION OF ENERGY: States that energy cannot be created or destroyed. Energy doesn’t appear out of nowhere. Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source. Energy is transformed to another type or transferred to another system/object. Energy doesn’t disappear, but it can be changed to another form.

Conservation of Energy Kinetic Energy (KE) Gravitational Potential Energy (GPE) Kinetic Energy + Potential Energy = Mechanical Energy

Energy Transfers and Transformations Example: GPE & KE Energy readily changes from one form to another. 12kg 7kg A) B) C) 7kg 5 m 5 m 3 m D) E) 7kg 5 m 7kg 5 m

Energy Transfers and Transformations B Which one of the spheres has the most gravitational potential energy? Which one of the spheres has the most kinetic energy? What is the gravitational potential energy of the system in A? What is the kinetic energy of the system in D? Which sphere has kinetic and gravitational potential energy present? D 343 J 343 J E