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Unit 3 Work, Energy & Power

Serway Chapters 7 & 8Glencoe Chapter 10 & 11

Unit 3 Section 2 Energy and the Work-Energy Theorem

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Unit 3 Section 2 Lesson 1 FEB 1Kinetic Energy and the Work-Energy Theorem

KE ObjectivesUnit 3 Section 2 Lesson 1 Kinetic Energy Objectives:

▫ Show understanding of the Physics concept of Kinetic Energy▫ Correctly identify Kinetic Energy from given situations▫ Recall and show understanding of the formula to calculate Kinetic

Energy▫ Solve related problems involving Change in Kinetic Energy

▫Do NOW: If you placed a 10.0 Kg object 1.5 m from the fulcrum of an ideal lever with an IMA of 4.0 what would the mass of the object needed to keep the lever balanced?

▫ If you had to place a 3.0 kg mass to balance the lever what is the efficiency of the lever?

▫ Unit 3 Section 2 Lesson 1 HOMEWORK: ▫ Serway PAGE:209 : #’s 27, 30, 31, 33, 34, 37, 40, 41▫ Glencoe Page 278 – 279: #’s 71, 72, 78▫ Glencoe Page 283: #’s 1 – 7 all

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Energy – Quick Re-cap

•Energy is the capacity to do WORK•SI Unit: Joule (J)•Many forms•Common ones:

▫Kinetic▫Potential▫ Electric▫ Chemical▫ Solar▫ Nuclear

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Kinetic Energy (KE)• A form of energy that a body in motion possess.• Examples:

▫ Bullet shot out from pistol

▫ Helicopter flying at 120km/h

• A body a rest, will NOT possess any KE!• The amount of KE of a moving body depends on:

▫ Mass of body (kg)

▫ Velocity (m/s)

• When either mass or velocity of moving body is increased, KE will also increase.• Kinetic Energy = ½ (Mass)(Velocity)2 = ½ (mv2) [ J ] = [kg] [m/s]2

• Mass Doubles Kinetic Energy Doubles• Velocity Doubles Kinetic Energy QUADRUPLES

Mass = m

Velocity, V

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Examples of KE• Find the KE of an empty van of mass 1000kg moving at

2m/s.

• Find the KE of van when it is loaded with goods to give a total mass of 2000kg, and moving at 2m/s.

• Find KE of unloaded van when it speeds up to 4m/s.

KE of van at 2m/s = ½ x 1000 x (2)2

= 2000 J = 2 kJ

KE of van at 2m/s = ½ x 2000 x (2)2

= 4000 J = 4 kJ

KE of van at 2m/s = ½ x 1000 x (4)2

= 8000 J = 8 kJ

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Work – Kinetic Energy Theorem• Serway page 195

• W = ∫ F dx ∫ max dx

• Work = ΔKE = (½ mv2)final – (½ mv2)initial

• If you push a 6.0 Kg object from rest along a frictionless surface with a force of 12.0 N what will the velocity be after 3 meters?

• Romac loads a refrigerator for Purple onto his truck using a ramp. He claims that less work is required to load the truck if the ramp is lengthened. Is he correct?

• Serway page 196 Exp 7.7 – 7.8

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Unit 3 Section 2 Lesson 2Potential Energy

PE ObjectivesUnit 3 Section 2 Lesson 2 Potential Energy Objectives:

▫ Show understanding of the Physics concept of Potential Energy▫ Correctly identify Potential Energy from given situations▫ Recall and show understanding of the formula to calculate Potential Energy▫ Show understanding of the relationship between Potential and Kinetic Energy

▫Do NOW: If you accelerate a 4.0 kg object from rest at 2.0 m/s2 for 5.00 sec, what is the kinetic energy of the object? How much work was done on the object?

▫ Unit 3 Section 2 Lesson 1 HOMEWORK: ▫ Glencoe Page 307: #’s 58 – 72 ALL

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Potential Energy• Potential energy is the energy possessed by an object as a result of its

POSITION or CONDITION.

•Gravitational PE {GPE}▫ In Physics, ground level is normally assumed to be at ZERO GPE.▫ Any object that is at ground level has ZERO GPE.▫ If object is lifted a certain height above ground, its GPE has increased.▫ Elastic PE (not in syllabus)

“Slinky” … when stretched or compressed Spring … when stretched or compressed Rubber band … when stretched Balloon with air … when compressed

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Gravitational PE• Can be calculated with:GPE = mass gravitational height above

acceleration ground level = mgh

Units: [J] =[kg] * [m/s2] * [m]

SI Units of GPE : Joule [J] Ground,0 GPE

Distance fromground, h

Object on top ofbuilding, of mass, mg

earth

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Example of GPE•You lifted your 5.0 Kg book bag to the top of your

table. What can you say about the GPE of your bag?▫Zero, increase, decrease

•Lift the same bag on the Moon. What happens to GPE?▫Zero, increase, decrease

•Will the GPE be the same on Earth and Moon?▫Same, less on Moon, more on Moon?

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Examples of GPE•You lifted a set of books of mass 3.0 kg, for

2.0m. What is the GPE gained by the books? Take g=10m/s2.

•Find the work done by you to lift the books.

ΔUg = GPE = mgh= 3 10 2= 60 J

Work done, W = F d (F = weight of books) = (m g) d = 3 x 10 x 2 = 60 J (Note: same as GPE)

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Unit 3 Section 2 Lesson 2 PE {ΔUg} In Class 290/1

• You Lift a 7.30 kg bowling ball from a storage rack and hold it up to your shoulder preparing to roll it down the lane for a STRIKE! The storage rack is 0.610 m above the floor and your shoulder is 1.12 m above the floor. • How much work did you do lifting the ball from the rack to your shoulder?

• Nikita drops a 1.6 kg brick from the mansion roof 6.7 meters to the ground.▫ What was the change in potential energy?

• A Warehouse worker picks up a 10.15 kg box from the floor and places it on a shipping table 1.15 meters above the ground. As he wraps the box for shipping, he slides it down the 3.5 m table to the end at a constant velocity in 21.0 seconds. He then lowers the box back to the floor . ▫ Ignoring friction, what was the total energy change of the box?

• Nikita drops a 1.6 kg brick from the mansion roof 6.7 meters to the ground.▫ If all the potential energy was converted into kinetic energy, what was the velocity of

the brick (no friction) when it hit the ground?

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Labs

•Hooks LawAtwood’s MachineWork Energy and the Ramp

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Atwood’s Machine

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Hooks Law

•Hooks Law PHET