April 5 & 8, 2013Homework Due

Read & Study 10.1, 10.4, 10.6 - Do # 1-3, 8-10, 18-20 p. 147Page 1 of POGIL!

Tonight’s HW

Read & Study Ch 10 - Do # 4-7, 11-17 p. 147*Expect an open notes Ch 10 quiz either next class or the following class

Do Now 1. Take back your exit slips 2. Take a clicker3. On my desk…take a Do Now and

complete BOTH SIDES

Which would be a correct possible position of the CG of this demo?

A. B. C. D.

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A. B. C. D.

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Let’s review the Do Now…

• Get ready to make additions/ changes/corrections to your Do Now!

#1. When you carry a heavy load with one arm, like a suitcase, why do you tend to

hold your free arm out horizontally?

A. B. C.

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A. To shift your CG towards the suitcase and maintain balance

B. To shift your CG away from the suitcase and maintain balance

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#2. Why is it dangerous to roll open the top drawer of a fully loaded file cabinet

that is not secured to the floor?

A. B.

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A. Because the CG will be above the area of support and it will fall

B. Because the CG will extend beyond the area of support (will no longer be above the area of support) and it will fall

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#3. The CG is marked by the ‘x’. Which truck will fall over?

A. B. C. D. E.

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A. Truck AB. Truck BC. Truck CD. All of the trucks will fallE. None will fall

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#4. Why does a VERY pregnant woman lean backward when walking?

A. B. C.

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A. To shift her CG forward, allowing her to keep CG above area of support and maintain balance

B. To shift her CG backward, allowing her to keep CG above area of support and maintain balance

C. To look cool

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#5. Why does a hiker carrying a heavy backpack lean forward?

A. B. C.

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A. To shift her CG forward, allowing her to keep CG above area of support and maintain balance

B. To shift her CG backward, allowing her to keep CG above area of support and maintain balance

C. To look cool

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And now, a brief recap of the concepts you should have attained

by now…

The center of mass of an object is the point located at the object’s average position of mass.

Center of Mass

#6 - How does a heavy tail enable a monkey standing on a branch to

reach farther branches?

• Free response!

Location of the Center of MassFor a symmetrical object, such as a baseball, the center of mass is at the geometric center of the object.

For an irregularly shaped object, such as a baseball bat, the center of mass is toward the heavier end.

Center of Mass

The center of mass for each object is shown by the red dot.

Center of Mass

Objects not made of the same material throughout may have the center of mass quite far from the geometric center.

Consider a hollow ball half filled with lead. The center of mass would be located somewhere within the lead part.

The ball will always roll to a stop with its center of mass as low as possible.

Center of Mass

The center of mass of the toy is below its geometric center.

Center of Mass

The center of mass of the rotating wrench follows a straight-line path as it slides across a smooth surface.

Center of Mass

As an object slides across a surface, its center of mass follows a straight-line path.

A baseball thrown into the air follows a smooth parabolic path. A baseball bat thrown into the air does not follow a smooth path.

The bat wobbles about a special point. This point stays on a parabolic path, even though the rest of the bat does not.

This point, called the center of mass, is where all the mass of an object can be considered to be concentrated…it’s the AVERAGE

Center of Mass

The centers of mass of the baseball and of the spinning baseball bat each follow parabolic paths.

Center of Mass

The center of mass of the fireworks rocket and its fragments move along the same path before and after the explosion…. CG or CM always follows a smooth path!

Center of Mass

The block topples when the CG extends beyond its support base.

11.5 Torque and Center of Gravity

The Rule for TopplingIf the CG extends outside the area of support, an unbalanced torque exists, and the object will topple.

11.5 Torque and Center of Gravity

This “Londoner” double-decker bus is undergoing a tilt test.

So much of the weight of the vehicle is in the lower part that the bus can be tilted beyond 28° without toppling.

11.5 Torque and Center of Gravity

The Leaning Tower of Pisa does not topple because its CG does not extend beyond its base.

A vertical line below the CG falls inside the base, and so the Leaning Tower has stood for centuries.

If the tower leaned far enough that the CG extended beyond the base, an unbalanced torque would topple the tower.

11.5 Torque and Center of Gravity

On to the POGIL!

• CHECK IN AT STOP SIGNS!• We are NOT going over as a class – PLEASE

check in with a teacher as a group! • Stopping with

What part of an object follows a smooth path when the object is made to spin through the

air, or across a flat smooth surface?

A. B. C. D. E.

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A. The entire objectB. The exact middle of the

objectC. The top of the objectD. The bottom of the objectE. The center of gravity

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Why is the center of mass of a baseball bat not at its midpoint?

A. B. C. D.

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A. Because a baseball bat is perfectly symmetrical

B. Because one end of a baseball bat is more massive than the other, so the center of mass or CG is closer to the HEAVIER end

C. Because one end of a baseball bat is more massive than the other, so the center of mass or CG is closer to the LIGHTER end

D. This is silly - the CG of a baseball bat IS at the midpoint!

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A basketball thrown through the air does not wobble. What does this tell you about the

location of the CG?

1 2 3

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1. It IS in the exact geometric center of the basketball

2. It IS NOT in the exact geometric center of the basketball

3. Not enough information to say.

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A basketball with a weight attached to the side of it appears to wobble when it is thrown

through the air. What does this tell you about the location of the CG?

1 2 3

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1. It IS in the exact geometric center of the basketball

2. It IS NOT in the exact geometric center of the basketball

3. Not enough information to say.

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Suppose you want to balance the cut out object you made today. You should apply a

support force at the…

1 2 3 4

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1. Exact middle2. Right end3. Left end4. Center of gravity

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Any questions about…

• Page 1 of the POGIL from last class? (The HW that was due today)

• Any material from last class/ today?

On a scale of 1-5, how confident are you with the material from Chapter 10 so far?

1 2 3 4 5

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1. I have no clue what is going on2. I understand a few bits and pieces,

but other than that, I’m stuck3. Ehh..With more practice, I’ll be OK4. I’m OK with this – tonight’s HW

practice should clear up any issues for me!

5. So confident, I could teach it to someone else!

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