April 25 & 28

CG & Stability – Day 3 – Review & Quiz

Do NowTake 1 & Label!

Yours may look a little different!

Coin Balancing on Bill Trick

• https://www.youtube.com/watch?v=qcOyN9aFdr4 (:15 – 1:10)

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

by now…

You should know how to…

• Predict whether or not an object will topple (based on whether or not the CG is aligned with the area of support)

• Determine what type of equilibrium an object experiences based on the location of its CG (stable, unstable, neutral)

Lowering the CG of an ObjectThe CG of an object tends to take the lowest position available.

Stability

StabilityThe CG of an object has a tendency to take the lowest position available.

a. A table tennis ball is placed at the bottom of a container of dried beans.

StabilityThe CG of an object has a tendency to take the lowest position available.

a. A table tennis ball is placed at the bottom of a container of dried beans.b. When the container is shaken from side to side, the ball is nudged to the

top.

Try this with Popcorn!

StabilityThe CG of the glass of water is affected by the position of the table tennis ball.

a. The CG is higher when the ball is anchored to the bottom.

StabilityThe CG of the glass of water is affected by the position of the table tennis ball.

a. The CG is higher when the ball is anchored to the bottom. b. The CG is lower when the ball floats.

Stuff tends to take the LOWEST CG = Most Stable!

• If the object weighs less than an equal volume of water, the object is forced to the surface. The CG of the whole system will be lowered because the heavier water occupies the lower space.

• If the object is heavier than an equal volume of water, it will be more dense than water and sink. The CG of the whole system is lowered.

• If the object weighs the same as an equal volume of water, the CG of the system is unchanged whether the object rises or sinks.

Stuff tends to take the LOWEST CG = Most Stable!

• If the object weighs less than an equal volume of water, the object is forced to the surface. The CG of the whole system will be lowered because the heavier water occupies the lower space.

• If the object is heavier than an equal volume of water, it will be more dense than water and sink. The CG of the whole system is lowered.

• If the object weighs the same as an equal volume of water, the CG of the system is unchanged whether the object rises or sinks.

Both of these objects are WEIGHTED towards the bottom.

They are STABLE ---

This explains why they always right themselves!

Why doesn’t the Seattle Space Needle topple??DISCUSS!

The Seattle Space Needle is so “deeply rooted” that its center of mass is actually below ground level. • Its foundation is 30’ deep & 120’

across. • The foundation weighs 2000

pounds MORE than the rest of the structure!

It cannot fall over intact because falling would not lower its CG at all. If the structure were to tilt intact onto the ground, its CG would be raised!

http://www.urbanistdispatch.com/2014/01/the-space-needle/

Any questions about…

• Any material from last 2 classes?

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.

http://www.britishpathe.com/video/bus-tilt-tests

Torque and Center of Gravity

For everyday objects, the center of gravity is the same as the center of mass.

Center of Gravity

• Center of mass is often called center of gravity, the average position of all the particles of weight that make up an object.

• For almost all objects on and near Earth, these terms are interchangeable.

• There can be a small difference between center of gravity and center of mass when an object is large enough for gravity to vary from one part to another.

• The center of gravity of the Willis Tower in Chicago is about 1 mm below its center of mass because the lower stories are pulled a little more strongly by Earth’s gravity than the upper stories.

Center of Gravity

WobblingIf you threw a wrench through the air, or slid it across a table, you’d see it wobble about its center of gravity.

The sun itself wobbles off-center.• As the planets orbit the sun, the center of gravity of the

solar system can lie outside the massive sun.

• Astronomers look for similar wobbles in nearby stars—the wobble is an indication of a star with a planetary system. (aka how they find other solar systems!)

Center of Gravity:

From now until 20 minutes before the bell…You should be working on…–Ch. 10 Review worksheet–Reviewing Ch. 10 HW with peers–Ch 10, p. 148-49 #24, 26, 31, 33, 37–Finishing anything that might be missing

or confusing on your CH 10 POGILS.

Quiz • Normal Quiz procedures apply• Cell phones / iPods out and SILENT• Can use your notes but not your book• No calculators needed• No talking until all quizzes are in• No extra credit • Good luck • When you are done…– you can get back your Ch 9 & 12 test & make

corrections…remember, you WILL see these questions again on your final (in LESS THAN 2 months!)

– You can start your HW for tonight!