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Work, Energy & PowerWarm up

SogandKevin ANadyaCourtneyIlianKevin CJackDylanAlexa TaylorMarkKylieFind your assigned seatCatherineNickGarrettAlexMeghaJustynaChristopherDaniellePersiaReganMichaelConnor

MichelleJennyKatieBriannaDaltonLoganJulia

New Unit Work, Power & EnergyThese words mean very specific things in physics class. They have different definitions than when we use them in everyday language. energyDiscussion question: What types of energy have you learned about/heard of before?WorkEvery day definitionPhysics definitionA force does work on an object when the object undergoes a displacement. Forces can do positive or negative work on an object.Not all of the forces on the object do work.Beginning of the notes:EnergyEvery day definitionPhysics definition/usageIn this class we only have equations for:Kinetic EnergyGravitational potentialElastic potentialPowerEvery day definitionPhysics definitionThe rate at which work is done or the rate at which energy is expended.Work in PhysicsWork is the result of applying a force to displace an objectWork = Force DisplacementW = F d

Units of WorkW = F dFrom the equation, work is measured in Force units times Distance units.1 N 1 m = 1 JouleThe name was chosen to honor James Joule for his work in Physics.

Displacement Can Be HorizontalWork is done to overcome the force of friction caused by gravity making contact between the object and the surface.Direction of DisplacementFrWork is done by exerting a force equal to the weight of an object that lifts the object. FDisplacement Can Be Verticalr

Positive WorkFaPositive or Negative Work?Force and displacement in the same direction:Energy Added To SystemFfNegative WorkFfPositive or Negative Work?Energy Removed From SystemForce and displacement in the opposite direction:

FgF1Forces do work?If the box is moving horizontally?If the box is moving vertically?F2F3Fg = 100 NFa = 20 NSample ProblemsHow much work is done to drag this box 4.0 m across a floor?W = F r = 20 N x 4.0 m = 80 N m = 80 JF = 20 Nr = 4 mW = 20 N x 4 m x cos 35o = 65.5 J 35oWnet = Fnet r cos Work Done at an Angle

Energy that is stored by the force of gravity or by the force of elastic energyGravitational Ug = mghElastic Us = kx2Where k is the spring constant and x is the amount of compression/elongation

Potential EnergyAn increase in elevation means an increase in potential energy.A 5,000 kg boulder atop a 5 m cliff has:U = 5,000 kg x 9.8 m/s2 x 5 m = 245,000 J

Potential EnergyEnergy that is the result of an object (mass) in motion (velocity).K of a 1200 kg car going 20 m/s K = m v2 = 1200 kg (20 m/s)2K = 240,000 JK = m v2

Kinetic EnergyEnergy is always conserved, but mechanical energy is not always conserved.

If no energy is lost (to work done by friction) you can set the initial energy equal to the final energy.

Ei=Ef

Conservation of Energy

The potential energy at the top of a ski hill is converted to an equal amount of kinetic energy at the bottom of the hill.Conservation of EnergyTotal Mechanical EnergyEtotal = U + KEtotal = Ug + Us + Khttp://www.youtube.com/watch?v=mhIOylZMg6Q Conservation Example 1: If a 5 kg ball is dropped from a height of 30 m. How fast is it going when it hits the ground? (energy equations make this problem easier than when we used kinematics)Conservation Example 2: If a 5 kg ball is dropped from a height of 30 m. How fast is it going when it is at a height of 10 m? From the numbers and diagram below, determine the mass of the skier

Conservation Example 3: Conservation Example 4: A 5.0 kg box is pressed up against a spring (k=200 N/m) to compress it 10 cm. Once it is let go, on a frictionless surface, how fast does it slide when it leaves the spring?The potential energy of a pendulum at the end of one swing is converted to an equal amount of kinetic energy at the bottom of the swing.

Pendulumshttp://phet.colorado.edu/sims/pendulum-lab/pendulum-lab_en.html The potential energy at the top of a roller coaster lift hill is converted to an equal amount of kinetic energy at the bottom.

Conservation of EnergyThe rate at which work is done is called power. A unit of power is a unit of work divided by a unit of time. P = W / tThe unit of power is a J/s or a Watt.PowerAnother way of defining power is to break down work into force units. P = F r / tBut (r / t) is velocity so

(when velocity is constant)P = F vPowerKinetic EnergyKinetic energy is the energy of motion.What is the unit of energy?The kinetic energy of an object is equal to:K = mv2K = kinetic energy, mass = m and v = speedNote the units!kg m2/s2 = Nm = JKE Problem #1A 7.00 kg bowling ball moves at 3.00 m/s. How fast must a 2.45 g table-tennis ball move in order to have the same Kinetic Energy as the bowling ball?Work - Kinetic Energy TheoremThe net work done on an object is equal to its change in kinetic energy.W = KEW = KEf KEiW = mvf2 mvi2This is true if the only change on the object as a result of the work done on it is its speed.Work - Kinetic Energy Problem #1A 6.00 kg block is initially at rest.Then, it is pulled along a frictionless surface by arope with a constant tension of 12.0 N. Find the speed of the block after it has moved 3.00 m.

Work - Kinetic Energy Problem #2A 6.00 kg block is initially at rest.Then, it is pulled by a rope with constant tension of 12.0 N. The coefficient of kinetic friction, k, between the block and the surface it slides upon is 0.150.Find the speed of the block after it has moved 3.00 m.Hint: The work done on the block is reduced by Ffkd.Recall that Ffk = kFN.

How fast is it going as it hits the floor?What is the balls kinetic energy at this point?

If a 10 kg ball falls from rest from 3.0 m.

If the ball was 100 kg, how would this affect the kinetic energy & speed?

EA: 7.67 m/s

K= 294J

Follow up questionQ: if no energy is lost, how high will it bounce back up?EA: 3m37If a 0.5 kg soccer ball has 20.0 J of kinetic energy, how fast is it moving?

EA: 8.9 m/s 38A 12 kg cannonball is launched straight up at 15 m/s. Neglecting air resistance, what is its maximum height?

What type of energy does it have right after it leaves the cannon?What type of energy does it have at its max peak?What is its max peak? (neglecting air resistance)EA: a) kinetic b) potential c) -- m39Daniel wants to throw a ball so that it hits a Frisbee that is stuck 8.0 m up in a tree. How fast does Daniel need to throw the ball so that it reaches the Frisbee?

EA: 12.5 m/s 40If Rowan throws a ball down at 1.5 m/s, how much faster is it going after falling 3.5 meters?What is its total speed?If Maddy throws a ball down at 1.5 m/s, how much faster is it going after falling 3.5 meters?EA: 8.28 m/sQ: what is its total speed?EA: 9.78 m/s

41HomeworkHW set has 1-8Quiz 1 will be on #1-4 (next class)Quiz 2 will be on #5-8 (2 classes from now)