Home » Physics » Work Power and Energy

# 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.

_________________________

## PE = mgh = (4 kg)(9.8 m/s/s)(5 m) = 196 joules

________________________________________________________________________

## 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

NGSS

HS-PS3-1. Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

[Clarification Statement: Emphasis is on explaining the meaning of mathematical expressions used in the model.]

[Assessment Boundary: Assessment is limited to basic algebraic expressions or computations; to systems of two or three components; and to thermal energy, kinetic energy, and/or the energies in gravitational, magnetic, or electric fields.]

DCI – Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms.

DCI

 Conservation of energy means that the total change of energy in any system is always equal to the total energy transferred into or out of the system.
 Energy cannot be created or destroyed, but it can be transported from one place to another and transferred between systems.
 Mathematical expressions, which quantify how the stored energy in a system depends on its configuration (e.g., relative positions of charged particles, compression of a spring) and how kinetic energy depends on mass and speed, allow the concept of conservation of energy to be used to predict and describe system behavior.
 The availability of energy limits what can occur in any system.

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/