Forces in Fluids

Bernoulli’s Principle

Terms• Bernoulli’s Principle~ as the speed of a

moving fluid increases, its pressure decreases

• Lift~ an upward force on an object (such as a wing) caused by differences in pressure above and below the object

• Thrust~ the forward force produced by an airplane’s engines; thrust opposes drag

• Drag~ the force that opposes or restricts motion in a fluid; drag opposes thrust

Fluid Pressure Decreases as Speed Increases

• Shower curtain example– The faster the water moves the less pressure

it exerts– Creates less pressure in shower than outside

shower– The shower curtain is pushed closer toward

the stream of water

• Science in a Sink– Table tennis ball hangs on a string and held

next to a moving stream of water– Higher air pressure pushes the ball into the

faster moving (lower pressure) water

It’s a Bird! It’s a Plane! It’s Bernoulli’s Principle

• Wing shape plays a role in helping objects fly

• Faster moving air above the wing causes less pressure than the slower moving air below the wing

• Causes an upward force called lift

Wing Shape Creates Differences in Air Speed

Thrust and Wing Size Determine Lift

• Amount of lift created due to– The size of the wing– Speed air travels around wing– Thrust

• Relationship between wing size, thrust, and speed– Smaller wings require more thrust and

speed• Ex. jet

– Larger wings require less thrust and speed• Ex. glider

Thrust and Wing Size Determine Lift

• Bernoulli’s Principle is for the Birds– Birds with larger wingspans (hawk)

need to flap less to stay in the air– Birds with smaller wingspans (sparrow)

need to flap much more often

Drag Opposes Motion in Fluids• Strong winds

– Air drags on your clothing and body– Makes it more difficult to move forward

• Lift reduced when turbulence causes drag

• Response to turbulence– Flaps change wing shape and increase

lift– Birds can adjust wing feathers to

increase lift

Wings Are Not Always Required