ENERGY ENERGY TRANSFORMATIONS Energy E The capacity to

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: Chemical Gravitational Kinetic Sound Elastic Electrical Nuclear Thermal Radiant

Energy Transformation Equations • Energy cannot be created or destroyed, only transformed from one type to another • Energy transformation equations show energy changes throughout a process • Energy transformation equation for a microwave: electrical radiant thermal Arrows 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=FΔd Where: W – Work (J) F – Magnitude of Force (N) d – distance (m)

W = F∆d Important Notes: • Work is scalar – directions are ignored for F and d • This equation is used for 1 D problems only • Work is measured in N∙m = J (Joule)

Example A worker pulls a heavy cart with a force of 40 N [E] for 5. 0 m [E]. What is the work done by the person?

W = F∆d = (40 N)(5. 0 m) 2 = 2. 0 x 10 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. 8 m 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

Solution a. ) FK = μKFN FN = Fg = mg =(42 kg)(9. 8 m/s 2) = 4. 1 x 102 N FK = (0. 0050)(4. 1 x 102 N) = 2. 1 N

b. ) W = F∆d = (2. 1 N)(7. 8 m) = 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 = F g

Example A 23 kg box is lifted 1. 2 m from the floor to a desk with no acceleration. Determine the work done on the box.

FA = Fg = mg = (23 kg)(9. 8 m/s 2) = 225 N W = F∆d = (225 N)(1. 2 m) = 2. 7 x 102 J

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