1. Sinking and Floating: Buoyancy
Lec 10: Liquids Part 1
Balloons: Pressure
Atmospheric Pressure
Sinking and Floating: Buoyancy
Hydraulic Lift

2. Air and Air Pressure Force Area Pressure = Pascals [Pa]
Pressure The force exerted by the particles as the bounce off the walls of a container.
Question: What holds the suction cup?
After you press an elastic suction cup against a smooth wall, it bends back and a small “empty” space is created between the cup and the wall. What keeps the suction cup against the wall?

3. Atmospheric Pressure Force Area Pressure =
Question: How much is the pressure of the Earth’s atmosphere at sea level?
1 Atm = 101, 300 Pa
Question: The burden on your shoulders.
How heavy is the Earth’s atmosphere sitting on top of your shoulders?
??? N
Force
Area
Pressure =
101, 300 Pa
(0.5 m) x (0.1 m)

4. The Atmosphere Pressure depends on density and temperature
Density decreases with altitude. And so does pressure.
Question: Where is the atmospheric pressure less:
In the mountains
At the sea
On the bottom of the ocean
It doesn’t matter, it is all the same

5. The Atmosphere
Question: Which picture corresponds to a higher altitude?
(a)
(b)
(c)

6. The Atmosphere
Question: If you could take all the air out from inside an opened plastic bottle, its sides would
(a) Remain where they are
(b) Bow inward slightly
(c) Bow outward slightly
(d) Crush inward extensively

7. Sinking and Floating Density Water density Specific gravity =
Question: Which object will sink when submerged in water?
(a) wood
(b) metal
(c) ice
(d) Balloon filled with air
(d) Balloon filled with concrete
Specific Gravity
Density
Water density
Specific gravity =

8. Puzzles
P1. If you pull your soup box from the fridge and leave it on the table to warm up, you frequently find the lid will bow outward and may even pop off. What is the reason for that?
P2. Which object will float in air?
(a) Balloon filled with helium; (b) balloon filled with carbon dioxide; (c) balloon filled with sand
P3. Can you draw what forces act on the hot air balloon?
P4. An object weighs 10 N in air and 8 N in water. What is the buoyancy force? Will it sink or float?

9. Water Towers
Incompressible Fluid: Once a cup of water is full to the brim, you cannot squeeze in any extra
Pressure changes with depth: The deeper you under the fluid surface, the more fluid “sits” on top of you, the higher the pressure

10. Water level, Pressure, and the Weather
Water seeks its own level:
Different fluids:
Which one is denser?
Water
Oil
Both are same

11. Water level, Pressure, and the Weather
High Pressure: no changes
Decreasing Pressure: weather changes expected
Barometers: What is going to be the weather?

12. Barometer CHECK YOUR NEIGHBOR
Why don't barometers use water instead of mercury?
Water cannot be used because it does not exert pressure.
Water cannot be used because it sticks to the glass.
Water can be used but the barometer will be too tall.
None of the above.
C. Water can be used but the barometer will be too tall.
© 2015 Pearson Education, Inc. 12

13. Hydraulic Lift force area FORCE AREA =
Pascal’s Principle: A change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container
force
area
FORCE
AREA =

14. Puzzles
P5. If you dive to a depth of 20 m below the water surface, what is going to be the pressure compared to the pressure at the surface level?
P6. A hydraulic lift for automobiles is an example of a force multiplied by hydraulic press. Pressure is exerted on the fluids in the small cylinder that results in a large force that lifts a car. If the lift’s cylinder is 30 cm in diameter and the small cylinder diameter is 1.5 cm, what force do you need to exert in order to lift your car (approx kg)?