reading quiz. 1. what keeps a suction cup against the wall?

Reading quiz.

1. What keeps a suction cup against the wall?a. sticky rubber, b. electrical attraction between oppositely charged particles, c. the force of air pressure, d. the buoyancy force, e. magic

2. You remove a partially filled sealed container of food from the refrigerator and let it warm up. You notice that the lid bows out as it warms up. This is because: a. as the food warms it produces gases that increase the pressure inside.b. the pressure on the outside of the container decreases as the container warmsc. the air pressure in the room is higher than in the cold refrigeratord. as the gas inside the container warms up it increases the pressure.

3. A hot air balloon can lift more ona. a hot day, b. a cold day, c. there is no difference

ans. 1 c, 2. d, 3. b.

PAYLOAD(~3 Large Paper Clips)

#1Air #2

He #3AIR

How does F buoyancy compare for each balloon? A. F buoyancy for #1 > #2 > #3 B. F buoyancy for #2 > #1 > #3 C. F buoyancy for #1 = #2; & both larger than #3 D. F buoyancy for #2 > #3 > #1 E. F buoyancy for #1 = #3; & both smaller than # 2.

Correct answer is: C. F buoyancy for #1 = #2; & both larger than #3

#1Air


F buoyancy

WeightBalloon + Air+ string + paperclips

How does F buoyancy compare for each balloon?

1. F buoyancy = upward force from pressure of air surrounding object

2. The air surrounding object cannot tell what it is pushing on … could be a balloon, a person, a table, or just air … hits it the same regardless of what it is hitting.

3. F buoyancy upwards equals the weight of the air displaced by the object. So depends on volume of object and density of air.

?

F buoyancy

How does F buoyancy compare for each balloon?

1. F buoyancy = upward force from pressure of air surrounding object2. The air surrounding object cannot tell what it is pushing on … could be a

balloon, a person, a table, or just air … hits it the same regardless of what it is hitting.

3. F buoyancy upwards equals the weight of the air displaced by the object. So depends on volume of object and density of air.

Pocket of air

Why is #3 true? Because we know that if our object is just a pocket of air, then Fbuoyancy will exactly balance it’s weight and the pocket of air will feel no net force. F buoyancy = mass of air displaced by object*gravity

Weight of air

F buoyancy

F buoyancy = (density of air * volume of object) * g

#1Air


#2He

#3AIR

F buoyancyF buoyancy F buoyancy

WeightBalloon + He+ string

WeightBalloon + Air+ string

WeightBalloon + Air+ string + paperclips

Calculating F buoyancy for Balloon #1 :

density of air at sea level = 1.25 kg/m3 density of air in boulder = 1 kg/m3

Volume = 4/3 pi r3= 4/3 pi (0.15 m)3 = 0.014 m3

Fbuoyancy = Volume * density * g

= (0.014 m3)(1 kg/m3)(9.8 m/s2) = 0.14 N = 0.03 lbs

How to get Force net to be Up?

What if we make a sealed light-weight Al foil balloon… remove air inside volume/balloon? Could we carry our selves?

Nope… Squashed by atmosphere (just likeDrum!)

What could you do that could make this work?a. make out of really heavy metal to hold against air pressureb. fill with water so would not compress inc. fill with some other gas that is lighter than air.d. fill with aire. something else

BIG VOLUME BIG F buoyancy

How to get Force net to be Up?

What if we make a sealed light-weight Al foil balloon… remove air inside volume/balloon? Could we carry our selves?

Nope… Squashed by atmosphere (just likeDrum!)

What could you do that could make this work?c. fill with some other gas that is lighter than air.

BIG VOLUME BIG F buoyancy

light gas atoms

F buoyancy

F gravity on He

Air vs. Helium Balloon

AIR HELIUM

How does the pressure in each balloon compare? a. Pressure in Air > Pressure in He b. Pressure in Air < Pressure in Hec. Pressure in Air = Pressure in Helium

c. Pressure in Air = Pressure in Helium…. Both are equal to air pressure in the room! Pressure pushing in = pressure pushing out or else bag will collapse.

How do the number of He atoms compare to number of Air molecules in each balloon?

a. # Air Molecules > # He atoms b. # Air Molecules < # He atomsc. # Air Molecules = # He atoms

SAME VOLUMES

Same volume so same Fbuoyancy = upwards net force from pressure of air surrounding balloon.

Fbuoyancy

Weight = mg

Review: Air vs. Helium Balloon

AIR HELIUM

How do the number of He atoms compare to number of Air molecules in each balloon?

c. # Air Molecules = # He atoms…. Ideal Gas Law: P = k * (# molecules) * Temperature

Volume

Balloons have same volumes … same pressure … same temperature of gas …so have same # of gas particles.

Air molecules heavier, but slower He atoms lighter, but fasterApplet showed us that at same temperature

So we could use He Balloon to lift stuff! (Pressures balance)

little helium atoms, only 2 protons andtwo neutrons each. Much less mass.

fat air atoms, N- 14 neutrons & protonsO- 16 neutrons and protons

Same number of gas particles in each balloon. But weight of each He particle is less…

As the Helium balloon rises…

a. the volume of the balloon increasesb. the Fbuoyancy increasesc. the pressure inside the balloon increasesd. a and be. a, b, and c

BALLOON AT 42 KM ALTITUDE!

a. The volume of the balloon increases!!!

Pressure of surrounding air decreases

Balloon expands until pressure inside = pressure outside

P = k * (# molecules) * Temperature Volume

Same # He molecules inside, to decrease P, volume increases!

Fbuoyancy does not increase. Volume is much bigger, but density of air is much less Fbuoyancy is weight of displaced air:

= Volume of balloon x density of air around balloon x g

Volume increases but air density decreases!

What will happen if we heat a beaker of air, and then remove the stopper?

a. nothing, the air will just stay there but get hotter

b. there will be fewer molecules inside because some will be pushed out into the room

c. the molecules inside will become lighter because they are hotter

d. there will be fewer molecules inside because heating destroys some of them.

A look at hot air

Lower T, lower P outside

Higher T, higher P inside

Answer is b. Atoms push out into room untilpressure inside and out is the same

P= k (# molecules/Volume) T

lower higher

Hot air has fewer atoms in same volume, but same pressure.

If air inside balloon is heated so that it is 50 C hotter than before, how does the number of air molecules inside balloon change? What is ratio of number of air molecules of hotter balloon to number of air molecules of colder balloon?

a. # air hot / # air cold = 70 C / 20 Cb. # air hot / # air cold = 20 C / 70 C c. # air hot / # air cold = 293 K / 343 Kd. # air cold / # air hot = 343 K / 293 Ke. I do not really understand how to reason through this.

293 K1.25 kg/m3

293 K1.25 kg/m3

Air at 20 Celcius Air at 70 Celcius


Answer is c: # air hot / # air cold = 293 K / 343 K

293 K1.25 kg/m3

293 K1.25 kg/m3

Air at 20 Celcius Air at 70 Celcius


When heating: NOT CHANGING:

volume, pressure

CHANGING: temperature, # of molecules

P * V = (# molecules) * T k At 293 K: At 343 K:(# cold) * T_cold = (# hot) * T_hot

(# hot)/(#cold) = T_cold/T_hot

ALL CONSTANT DURING HEATING

weight = mg

Fbuoy

293 K1.25 kg/m3

So how do the numbers work out for making a real hot air balloon (at + 50 degrees)? Net force upwards = Fbuoy – Weight of air inside

Fbuoyancy= (density of air) (volume) (g) = (1.25 kg/m3) * (5 m3 ) (9.8 m/s2 )

Density of hot air: # hot air molecules = # air molecules cold x (293K /343 K) Volume Volume

Weight of air inside = density of hot air x volume x g= (1.25 kg/m3 * 293 K/343K) * 5 m3 * 9.8 m/s2 = 1 kg

So to lift just 1 kg would need a volume of 5 m3.

If I stick a balloon full of air into liquid nitrogen and wait for some time, then …. (Liquid Nitrogen temp = 77 K)

a. number of molecules inside balloon will be less b. pressure inside the balloon will be lower c. volume will decrease d. b and ce. a and b

P= k particle T

T down, P down initially so gets squished in by air around ituntil (# air molecules / Volume) or density of particles (particle) so big that P inside balloon = P in room.

With particle (number density) is so large, mass density is large also: Mass = 1.25 kg/m3 * (293 K / 77 K) ~ 3 times higher than air in roomFbuoy < < weight, drops like rock!

Fbuoy

Weight

A question from extra creditBottle filled with hot liquid, sealed and then allowed to cool

Bottle filled with hot liquid, sealed and then allowed to cool

Why does this happen as it cools? a. Walls of bottle collapsed due to heatb. Pressure of the air outside is higher

than pressure of air inside c. Liquid and air inside is pulling sides

in from the inside

b. Air inside cools.. Lowers pressure inside, so force of air pressure pushing out less than force of air pressure pushing in.

Walls cannot sustain pressure difference, Volume decreases until pressure inside pushing out equals pressure outside, or until walls can sustain pressure difference.

Suction Cup Strength?A B

The diameter of Suction Cup B is twice as big as the diameter of Suction Cup A. How much weight can each suction cup hold?

a. A and B can support the same amount of weightb. A can support about 2 x more weight than Bc. A can support about 4 x more weight than Bd. B can support about 2 x more weight than Ae. B can support about 4 x more weight than A

Suction Cup Strength?A B

The diameter of Suction Cup B is twice as big as the diameter of Suction Cup A. How much weight can each suction cup hold? b. B can support about 4 x more weight than A

Area of suction cup = pi * r2

Area of B is 4 times Area of A Force upwards from air molecules colliding with suction cup = Pressure * Area

Pushed manyair molecules out of here

Force up = Air pressure X Area

Force down = Mass of weight x gravity


#1He

#2He

#3AIR

What is net force on each balloon? A. #1 and #2 up, #3 down B. #1 and #2 down, #3 up C. #1 down, #2 up, #3 downD. #1 can’t tell, #2 up, #3 down E. #1 and #2 can’t tell, #3 downD (know from experience) or E (uncertainty in string weight)


#1He

#2He

#3AIR

How do F buoyancy compare on each balloon? D. #1 = #2 = #3. They are all the same! All have same Volume!

F buoyancy = upward force from pressure of air surrounding balloon

F buoyancyF buoyancy F buoyancy

WeightBalloon + He+ string

WeightBalloon + Air+ string

WeightBalloon + He+ string + paperclips

Calculate F buoyancy density of air at sea level = 1.25 kg/m3 density of air in boulder = 1 kg/m3

Volume = 4/3 pi r3= 4/3 pi (0.15 m)3 = 0.014 m3

Fbuoyancy = Volume * density * g = (0.014 m3)(1 kg/m3)(9.8 m/s2) = 0.14 N = 0.03 lbs

density of air at sea level 1.25 kg/m3, helium 0.175 kg/m3

Weight of He gas is small! = density of helium x volume x gravity

reading quiz. 1. what keeps a suction cup against the wall?

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