Conceptual Physics

http://mrlafazia.com/PHYS111/lessons/201011Fall/PHYS111_06.ppt

THURSDAY September 9th, 2010

LESSON GOALS:• Present “Work-Energy Theorem” concept• Provide and Explain formulas for Kinetic Energy, Gravitational Potential Energy, and Elastic Potential Energy• Demonstrate 2-Energy Transformation (GPE to KE)• Assign HMWK #3

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INTRODUCTION to

Work & Energy

(“The Work/Energy Theorem”)

Applied Force

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INTRODUCTION to

Work & Energy

Applied Force

Displacement

WORK is defined as the Applied Force times the Displacement

W = F . d

F = 2 N

d = 3 m

Work = 2 N * 3 m Work = 6 N.m Work = 6 Joules

2/14

Further thoughts on WORK

If a Force is Applied, but no displacement or movement results, then NO Work is done on the Object.

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Further thoughts on WORK

The person might be expending ENERGY, but NO WORK is being done ON the object.

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If the box had never moved, how much work would have been done?

Think:

W=Fxd=(4N)(0m)=0N.m=0J

Discuss: What could have kept the box from moving?

“Does this mean that no Energy was expended??”

Answer: No, it simply means that even though Energy was expended or used, no work was done on the object. In order for work to be done on an object, there has to be a force applied and there has to be a displacement parallel to the direction of the force.

4/14

Further thoughts on WORK

The person might be exerting a Force, but NO WORK is being done ON the object, since it is not moving

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Further thoughts on WORK

The person might be exerting a Force, but NO WORK is being done ON the object, since it is not moving

We can just as easily replace the person with a metal pole, which exerts a force but expends no energy and does no work!

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Further thoughts on WORK

If the person is carrying the box horizontally, exerting a Force upward…

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Further thoughts on WORK

No work is done on the box since the force is not in the direction of the motion…

displacement

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Further thoughts on WORK

A force perpendicular to the displacement direction does no work on the object!

displacement

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Further thoughts on WORK

Even though the person is expending energy.

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Work and Energy are Conserved

If we use a Lever to move a Box upward… we exert a Force downward…

F = 10 N

The Load we are lifting is 40 N…

40 N

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Work and Energy are Conserved

The Box rises 0.5 m…

F = 10 N

40 N

0.5 m

The Force was applied through a distance of 2.0 m…

2.0 m So work Input is 10 N x 2.0 m = 20 J

and work Output is 40 N x 0.5 m = 20 J

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Work Input = 20 J Work Output = 20 J

Work Input = Work Output

It is plain that the Energy to do Work is conserved: “You never get more Work out of something than the amount of energy you put in.”

Think: What was the efficiency of this simple machine?

… Is this possible? Why/Why not?

… What was most likely the case, then?

Conservation of Energy (or Work)

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Any Questions???

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Some Useful Energy FormulaeKinetic Energy:

½·m·v2 (½mv2)

Gravitational Potential Energy:

m·g·h (mgh)

Elastic Potential Energy:

½·k·x2 (½kx2)

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For all of these, …

m = mass of objectv = speed of objectg = acceleration due to gravity (9.8 m/s2)h = height object has the potential to fallk = elastic constant of materialx = distance material is stretched or compressed

2-Energy Transformation

11/14

As a class, view and discuss the following Interactive Physics demonstrations:

7.1 (lifting object)8.1 (pendulum)

8.2 (roller coaster)8.3 (snowboarder)

Recommended Reading:Sections 5.7 – 5.16

(Same old same old + the rest)

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HMWK #3

P. 99 #’s 11, 13; 14

P. 101#’s 43, 44; 50.

P. 103#’s 11, 13; 14.

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Looking Ahead:

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• Next class we will explore Efficiency and the Work/Energy Theorem using a Hydraulic Jack.• Additionally, we will look at Energy values and multi-Energy transformation in a simple (in design, not Physics) roller-coaster example.