Regents Physics

Work and Energy

Energy and Work

Energy is the ability to Work Work is the transfer of energy to an object,

or transformation of energy in 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 dF

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

Your motion

Forces are at90 degrees.No work is done!

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 = mgh

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 = mgh

Energy of Motion - Kinetic Energy Energy associated with

motion Kinetic energy is

gained as potential energy is lost

KE = 1/2mv2

M = mass in kilogramsV = velocity in m/sKE = 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

Ideal NonIdeal

KE = -PE

1/2mv2 = mgh

ET = PE + KE + Q

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 springUnit is the Newton - meter

Springs Store Energy

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

#1

#2

#3

#4

#5

#6

answers

1) B 2) A takes more work 3) A 4) A 5) A 6) 1000J 7) D 8) B

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.