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
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