Notes – Pressure & Buoyant Force Chapter 3, Lesson 2

Pressure in a Fluid A fluid is a material that can flow and has no definite shape. Liquids and gases are fluids. Fluids can exert forces.

Pressure in a Fluid Pressure is the force per unit of area applied on the surface of an object.

Pressure in a Fluid Pressure depends on the area over which a force is exerted. If the area increases, pressure decreases.

Pressure in a Fluid Pressure depends on the size of the force. If the force increases, pressure increases.

Pressure in a Fluid The equation for pressure is: P = F ÷ A Pressure = force (newtons) area (meters squared)

Pressure in a Fluid The SI unit of pressure is the pascal, abbreviated Pa. A pressure of 1 Pa is equal to a force of 1 N applied over an area of 1 m2 1 Pa = 1 N / m2

Pressure in a Fluid For any liquid in a container of any shape, the pressure depends only on the height of the column of liquid above the surface where you measure the pressure.

Pressure in a Fluid The deeper underwater you go, the greater the pressure is on you. This is because as you go deeper, the column of water above you gets taller.

Pressure in a Fluid On April 10, 1963, the USS Thresher nuclear-powered attack submarine was lost at sea during deep-diving tests. All 129 aboard died. (8400 ft / 2600 m below sea level).

Pressure in a Fluid In a fluid, pressure is exerted on all points on the outside of an object in a direction perpendicular to the surface of the object at that point.

Pressure in a Fluid A gas, like any fluid, exerts a pressure on an object. That pressure depends on the height of the gas above the object.

Pressure in a Fluid Atmospheric pressure is the force exerted per unit area by air particles. As you climb, or increase your elevation, atmospheric pressure decreases. mb = millibar (100 Pa)

Pressure in a Fluid Atmospheric pressure at sea level is caused by a large weight of gas, but is difficult to notice because there is an equal, internal pressure pushing out from the inside of your body.

Buoyant Force The buoyant force is the net upward force exerted on an object in a fluid by the surrounding fluid. It acts against gravity. Buoyant force is the DIFFERENCE between the pressure above and the pressure below.

Buoyant Force In a fluid, there is no net horizontal force on an object caused by pressure from the fluid. No net horizontal force (equals 0). Only vertical forces

Buoyant Force In a fluid, pressure increases with depth. The force of the fluid pushing up on the bottom of an object suspended in water is always greater than the force of the fluid pushing down on the top of the same object.

Buoyant Force The horizontal forces cancel (no net force). Water pressure at the top surface is less than water pressure at the bottom surface. Thus, the fish swims around happily in the water. The boxfish experiences a buoyant force resulting from increasing pressure at increasing depth.

Buoyant Force The size of the buoyant force does NOT depend on depth.

Buoyant Force The buoyant force does NOT change as an object goes deeper because it’s a DIFFERENCE between the pressure above and the pressure below. Both those increase so the buoyant force stays the same.

Buoyant Force 5 N 10 N 10 N 15 N

Buoyant Force At the fish goes deeper, the force on its back increases. However, the force on its belly also increases to match and keep the fish suspended in the water. The net buoyant force is always the same, no matter what the depth.

Buoyant Force Archimedes’ Principle states that the buoyant force on an object is equal to the weight of the fluid the object displaces. This is why objects feel lighter in water!

Pressure is the ____ per unit area. A mass B volume C force D pascal 3.2 Pressure and the Buoyant Forces Pressure is the ____ per unit area. A mass B volume C force D pascal

Which direction does the buoyancy force act on an object? A upward 3.2 Pressure and the Buoyant Forces Which direction does the buoyancy force act on an object? A upward B downward C to the left of the object D to the right of the object

Where is the pressure the greatest on an object in a fluid? A top 3.2 Pressure and the Buoyant Forces Where is the pressure the greatest on an object in a fluid? A top B bottom C sides D none of the above

What does the weight of the fluid displaced by an object held under water equal? A the normal force on the object B the mass of the object C the buoyant force on the object D the volume of the object

Which exerts the greatest pressure at the bottom of the column? A a column of water 5 m high B a column of water 3 m high C a column of water 1 m high D cannot be determined

A column of fluid 5 m high weighs 100 N. The column covers an area of 0.25 m2. What is the pressure exerted by the fluid at the top of the column? A 25 pascals B 20 pascals C 5 pascals D 0 pascals

C upward and equal to the weight of water displaced by the boat SCI 8.c A toy boat weighs 120 N. If the boat floats in water, which describes the buoyant force acting on the boat? A 60 N upward B 240 N upward C upward and equal to the weight of water displaced by the boat D upward and equal to the total volume of the boat

3-2 Vocab – Pressure & Buoyant Force text p. 140-148 fluid area pressure atmospheric pressure buoyant buoyant force Archimedes’ principle