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# Work Power and Energy

## Introduction

### W = F•d ## (archer stretches her bowstring, weightlifter raising her barbell) ## (slowing down a car, speeding up an airplane) # I feel like I’m doing work, but I’m not ?!

## How much work is done? Hmm, how is it that no net work was done on the box in the last case?
Let’s break it down:
Man applies forward force:          That does + work on the box
But friction applies an opposing force: That does work on the box

Net force is the total of the forward and backward = they cancel out = add up to 0

Still not convinced?
Logically, friction force must be equal (and opposite) to forward force:
If the forward force were bigger then the man & box would pick up speed!
If the forward force were smaller then the man & box would slow down.

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## Consider an object is being lifted up at constant velocity (i.e. object is not accelerating.) ## Over what distance does this force act? The height of the lift. ## PE = mgh = (4 kg)(9.8 m/s/s)(5 m) = 196 joules

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## Question: A waiter carries a tray full of meals across the room – Is work being done? ## So the net work on the waiter equals zero, and the net work done on the tray must equal zero. The work-energy principles joule = unit of work – Newton x meter

## Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS3-1. Use algebraic expressions and the principle of energy conservation to calculate the change in energy of one component of a system when the change in energy of the
other component(s) of the system, as well as the total energy of the system including
any energy entering or leaving the system, is known

Massachusetts Science and Technology/Engineering Curriculum Framework
2. Conservation of Energy and Momentum
The laws of conservation of energy and momentum provide alternate approaches to predict and describe the movement of objects.
2.3 Describe both qualitatively and quantitatively how work can be expressed as a change in mechanical energy.
2.4 Describe both qualitatively and quantitatively the concept of power as work done per unit time.

SAT Physics Subject Test Learning Standards
Mechanics: Energy and momentum, such as potential and kinetic energy, work, power, impulse, and conservation laws

Learning Standards: Common Core Math
CCSS.MATH.CONTENT.7.EE.B.4 Use variables to represent quantities in a real-world or mathematical problem, and construct simple equations and inequalities to solve problems by reasoning about the quantities.
CCSS.MATH.CONTENT.8.EE.C.7 Solve linear equations in one variable
CCSS.MATH.CONTENT.HSA.SSE.B.3 Choose and produce an equivalent form of an expression to reveal and explain properties of the quantity represented by the expression. (including isolating a variable)
CCSS.MATH.CONTENT.HSA.CED.A.4 Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. For example, rearrange Ohm’s law V = IR to highlight resistance R.
http://www.corestandards.org/Math/