PLC Activity #13
Fluids
Due: see website for due date
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
Problem 1: Trap liquid in a drinking straw
It is well known that you can trap liquid in a drinking straw by placing the tip of your
finger over the top while the straw is in the liquid, and then lifting it out. The liquid runs
out when you release your finger.
a. Draw and label the forces acting on the trapped liquid. Is the fluid in equilibrium or
nonequilibrium?
b. Is the gas pressure inside the straw, between the liquid and your finger, greater than,
less than, or equal to the atmospheric pressure? Explain, basing your explanation on
your answer to part (a).
c. If your answer to part (b) was "greater" or "less," how did the pressure change from
the atmospheric pressure that was present when you placed your finger over the top
of the straw?
Problem 2: Ball in an air stream
Carefully observe a beach ball in an air stream from a blower.
a. Draw and label the forces acting on the trapped ball in the air stream. Is the ball
roughly in equilibrium or nonequilibrium?
To help with this, (i) move the ball slightly to the side of the air stream and release it;
what happens to the ball? (ii) What happens to the ball when the air stream is moved
slowly sideways or tilted at an angle?
b. Is the gas pressure at the top of the ball greater than, less than, or equal to the
atmospheric pressure? Explain, basing your explanation on your answer to part (a).
Problem 3
A liquid with negligible viscosity flows through the pipe shown. This
is an overhead view.
a. Rank in order, from largest to smallest, the flow speeds v1 to v4
at points 1 to 4. Explain.
b. Rank in order, from largest to smallest, the pressures p1 to p4
at points 1 to 4. Explain.