ENERGY & ENERGY TRANSFORMATIONS. Energy (E) The capacity to do work (more to come in a couple slides)

ENERGY & ENERGY TRANSFORMATIONS

Energy (E)

•The capacity to do work(more to come in a couple slides)

•All forms of energy can be classified into one of two broad categories:

Kinetic Energy(energy of motion)

Potential Energy(stored energy)

Forms of Energy•Use the internet to prepare a definition for the

following types of energy and classify them as kinetic or potential:

ChemicalGravitationalKineticSoundElastic

ElectricalNuclearThermalRadiant

Energy Transformation Equations•Energy cannot be created or destroyed,

only transformed from one type to another

•Energy transformation equations show how energy changes throughout a process

•Energy transformation equation for a microwave:

electrical radiant thermalArrows indicate a transformation

Create Energy Transformation Equations for: (simple)•A child jumps on a trampoline

•A portable CD player operates with a battery

•An incandescent light bulb is switched on

•A rollercoaster climbs and descends the first hill

•A person playing an electric organ presses a key and we hear the note played

Create Energy Transformation Equations for: (complicated)•A nuclear core heats up water to the

boiling point which turns a generator•Spring with a mass attached is pulled

down and then released (moves up and down until it comes to a rest on it’s own)

•A match is struck against a matchbox and ignites

Work (W)•The energy transferred to an object by an applied force over a measured distance

•WORK is ENERGY TRANSFERRED

You can think of energy as …

• The ability to do work• Work waiting to happen

The Work-Energy Equivalence

•In general, doing work gives an object energy AND an object that has energy can do work.

•Both Energy and Work have the unit Joule (with a capital “J” = N.m)

The following conditions must be met in order for work to be done:

1.) A force must be exerted on an object

2.) The object must be displaced (moved) by the force

3.) At least part of the force must be in the same direction as the displacement

Examples

•A person lifting a box from the floor to the table

•A person pushing a box along the floor from one spot to another

What is Work?

•In Physics, work has a very specific definition

•Is this work?▫Video

Work

•Energy transferred to an object over a distance

•WORK IS ENERGY GAINED OR LOST

W=FΔdWhere:

W – Work (J)F – Magnitude of Force (N)d – distance (m)

W = F∆dImportant Notes:•Work is scalar – directions are ignored for F and d

•This equation is used for 1D problems only

•Work is measured in N∙m = J (Joule)

ExampleA worker pulls a heavy cart with a force of 40N [E] for 5.0m [E]. What is the work done by the person?

W = F∆d = (40N)(5.0m) = 2.0 x 102 J

Positive Work

•When the force is in the same direction as the displacement

•"+" work = force tends to increase the energy of the object

Negative Work

•If the force is opposite to the direction of the displacement (i.e. friction)

•"-" work = force tends to decrease the energy of the object

Example

A 42 kg child runs and then slides 7.8m along an iced over pond. The coefficient of kinetic friction between the child's boots and the ice is 0.0050.

a.) What is the force of kinetic friction acting on the child?

b.) Calculate the work done by the kinetic friction

Solutiona.) FK = μKFN

FN = Fg = mg=(42kg)(9.8m/s2)= 4.1 x 102N

FK = (0.0050)(4.1 x 102N) = 2.1N

b.) W = F∆d = (2.1N)(7.8m)

= 16 J

Work Done Against Gravity•Lifting an object to a higher position means you are working against gravity which is exerting a downwards force on that object

•If the velocity of the object being lifted is constant, than Fnet acting on the object is 0. Therefore,

FA = Fg

ExampleA 23kg box is lifted 1.2m from the floor to a

desk with no acceleration. Determine the work done on the box.

FA = Fg = mg = (23kg)(9.8m/s2) = 225N

W = F∆d = (225N)(1.2m) = 2.7 x 102J

Who thinks they can do zero work?•Hold a textbook at arms length

Zero Work (ZERO Energy Transferred)

•Exerting a force but 0 displacement▫a student holding another

student on their shoulders•Force is 0 but displacement

occurs▫a puck on an air table (no

friction)•Displacement is perpendicular

to the force▫a javelin thrower runs with

the javelin above their head

Fa

Challenge Question

Who will do more work?A.) 2 people lifting a piano into the back of

a truck

OR

B.) 2 people pushing the same piano up a ramp to the back of the same truck

Homework!

•Create two questions with full solutions