Regents Physics Work and Energy

Energy and Work Energy is the ability to Work Work is the transfer of energy to an object when the object moves due to an application of a force W = Fd unit is Joules (J) Energy is also measured in Joules

When is Work Done? Work is only done when the direction of motion is in the direction of the force So we can rewrite the equation to: W = Fcos  d F

The F is important! F = Fg = force due to gravity on an object In this case, you are doing work against or with the force of gravity F = applied force = pushing or pulling something F = force of friction  doing work against friction

The direction is important The force must be in the direction of motion For example: A person holds a book and walks 2 m across the room. Is work being done against the force of gravity? No! Force on book Forces are at 90 degrees. No work is done! Your motion

Power  The Rate at Which Work is Done Work is done when a force moves an object in the direction of the force Work = Force x distance Power is the rate at which work is done Power = work (J) / time (s) Unit of Power is a Watt (W) = J/s P = Work / time = Fd/t = Fv

Forms of Energy Energy has many different forms. Here we discuss the various forms of energy! Forms of Energy Stored Energy and Energy of Motion

Forms of Energy Energy has many forms, including: Thermal Energy – heat, is the total kinetic energy possessed by the individual particles of an object Internal Energy – is the total of the potential and kinetic energies of an object Nuclear Energy – is the energy released by nuclear fission or fusion Electromagnetic Energy – is the energy associated with electric or magnetic fields

Stored Energy - Potential Energy The energy possessed by an object due to its position or condition If there is no energy loss due to friction, the work done to bring an object from its original position is equal to the object’s change in potential energy We can see this in observing changes in gravitational potential energy PE = mgh

Gravitational Potential Energy Objects gravitational potential energy as they are lifted to a distance above the Earth’s surface Work is done against gravity to lift the object As long as there is no loss due to friction, the change in potential energy is due only to change in height! PE = mgh

Work and Energy Relationship If there is no friction, all the work done in lifting an object to a new height is equal to the object’s increase in potential energy The change in potential energy depends only on the height, not on the path taken For example Work also = 98 J W = 98 J 10 Kg Vs. 10 Kg

Conservative Forces When work done against a force is independent of the path taken, the force is said to be a conservative force Gravitation is an example of this type of a force Notice no friction is involved

Nonconservative Forces Air resistance and friction are examples of nonconservative forces The work done against a nonconservative force is dependent upon the path taken Path A requires more work than Path B 10 Kg 1.0m A B

Nonconservative example Wf = Ffd Ff = ukFN FN gets larger as the angle gets smaller, so…A requires more work against friction than B W = 98 J Just to lift it

Energy of Motion - Kinetic Energy Energy associated with motion Kinetic energy is gained as potential energy is lost KE = 1/2mv2 M = mass in kilograms V = velocity in m/s KE = energy in joules

Conservation of Energy Just like momentum, energy is also conserved Energy cannot be created or destroyed, it can only be transferred! The sum of the changes in a closed system must be equal to zero We must consider energy conservation under “perfect” and reality like situations KE gained = potential energy lost! Click picture for demo!

Ideal Mechanical Systems The sum of the kinetic and potential energies in a system is called the total mechanical energy Ideal Mechanical System – is a closed system in which no friction or other nonconservative force acts The sum of the kinetic and potential energy changes is equal to zero Example: the pendulum Click above for demo!

Nonideal Mechanical Systems When a system is acted upon by a nonconservative force, such as friction, it is called a nonideal mechanical system The friction opposes the motion of two objects in contact with each other and moving relative to each other The frictional energy is converted into internal energy..an increase in temperature

Ideal vs. Nonideal NonIdeal Ideal KE = -PE ET = PE + KE + Q 1/2mv2 = mgh ET = mgh + 1/2mv2 + Q

Regents Physics Springs!!

Elastic Potential Energy Energy is stored in a spring when work is done stretching or compressing it This energy is called elastic potential energy

Compression / Elongation The compression or elongation of a spring is the change in spring length from it’s equilibrium position when a force is applied to it The compression (elongation) of the spring is directly proportional to the applied force…provided the elastic limit of the spring is not exceeded This gives us an equation!

Hooke’s Law Fs = kx The applied force on a spring is proportional to the distance the spring is displaced (x) and the spring constant (k) k is the spring constant and is the constant of proportionality between the applied force and the compression/elongation of the spring Unit is the Newton - meter

Springs Store Energy W = Fsx = ½ kx • x = ½ kx2 PEs = ½ kx2 Work done to compress/stretch a spring is equal to the stored potential energy..just like in gravitation! Thus… W = Fsx = ½ kx • x = ½ kx2 PEs = ½ kx2 Click for demo

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Use the following diagram to answer questions #5 - #7 Use the following diagram to answer questions #5 - #7. Neglect the effect of friction and air resistance. 5. As the object moves from point A to point D across the frictionless surface, the sum of its gravitational potential and kinetic energies a. decreases, only. b. decreases and then increases. c. increases and then decreases. d. remains the same.

6. The object will have a minimum gravitational potential energy at point a. A. b. B. c. C. d. D. e. E.

7. The object's kinetic energy at point C is less than its kinetic energy at point a. A only. b. A, D, and E. c. B only. d. D and E.