Terminal Velocity of a Soap Bubble

TerminalVelocity of a Soap Bubble

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OBJECTIVE

This experiment wasperformed to prove that terminalvelocity of a soap bubble is generally reached immediately afterreleasing of a soap bubble.

BACKGROUND

Whenan object falls, the force of gravity causes the object to increaseits speed as predicted by Isaac Newton. As the speed increases, theair drag force increases as well until the air drag force is equal tothe gravity force, and there is no net force acting on the object. Ifthese two forces are at equilibrium, the object will no longerincrease the speed nor will it decrease its speed: Nevertheless itwill continue to fall at a constant velocity, called the&nbspterminalvelocity. Newton`ssecond law states that the summation of the forces on a body areequal to the mass of the body times its acceleration. Sincethe air drag force is highly dependent on the size and shape of theobject, objects that have a large surface area displays much lowerterminal velocity than objects with a smaller surface area. Theobject weight affects the air drag force on the object and alsoaffects the terminal velocity. However, this is not expressed in aflat piece of paper that falls more slowly than the same paper whencrumpled. Soap bubbles attain the terminal speed immediately afterrelease of the bubble. A bubble solution comprises ofglycerin, soap, and water. The glycerin happens to make the bubblemore durable. Soap bubbles attain a spherical shape due to surfacetension. The air inside a bubble is under compression.

Duringthis experiment, the assumption that the bubbles should remainspherical throughout their trajectory is made.

REQUIREMENTS

  1. Ruler (100 cm =1 m)

  2. Stop Watch (s)

  3. bubble solution (store bought)

  4. Plastic drinking straw

  5. Air-filled Balloon

  6. Beaker.

  7. Marker pen

PROCEDURE

  1. Pour the liquid bubble solution in a beaker.

  2. Using the marker, mark the ruler at 10 cm intervals from 0 cm to 50 cm and place it in an upright position.

  3. Dip the plastic drinking straw into the bubble solution.

  4. Remove the drinking straw from the beaker

  5. Gently blow air using your mouth through the opposite side of the straw, a bubble should form at the other end at which the straw was dipped in the soapy solution. Make sure the bubble is released near the top of the upright ruler.

  6. When the bubble is formed and finally detaches itself off the straw, immediately start the stopwatch, when the bubble reaches the 20 cm mark, stop the timer and record the time taken.

  7. Repeat the procedure again and this round, start the stopwatch when the bubble reaches the 20 cm mark and stop the timer when the bubble reaches the 40 cm mark. Repeat this experiment at 40 cm to 60 cm, 60 cm to 80 cm, and finally from 80 cm to 100 cm. (This will end up giving you five different results)

  8. Perform the experiment again. This time using an inflated balloon instead of a soap bubble. Repeat the procedure five times and record all the data in a separate table.

RESULTS

Table1. Procedure I- using soap Bubble

Experiment

Time (s)

Distance (m)

Velocity (m/s)

1

0.40 s

20 cm

0.495 m/s

2

0.43 s

20 cm

0.465 m/s

3

0.44 s

20 cm

0.455 m/s

4

0.44 s

20 cm

0.455 m/s

5

0.44 s

20 cm

0.455 m/s

Table2. Procedure II- Using an air-filled balloon

Experiment

Time (s)

Distance (m)

Velocity (m/s)

1

0.34 s

20 cm

0.589 m/s

2

0.32 s

20 cm

0.613 m/s

3

0.31 s

20 cm

0.644 m/s

4

0.31 s

20 cm

0.644 m/s

5

0.31 s

20 cm

0.644 m/s

Velocity is derived from the time taken to cover the distance. V=d/t

v- Velocity

d- Distance

t- Time

The distance for this experiment is kept at a constant distance of 20cm to evaluate the speed of a soap bubble and balloon at eachinterval.

DISCUSSION

The existence of abubble is a demonstration of surface tension acting on the water filmenclosing the air trapped inside. Soap increases the surface tensionand elasticity making the bubbles more durable. Glycerin also happensto make the bubble more durable in order to sustain the experimentbeing undertaken. A bubble filled with air will slowly fall to thefloor because the weight of the film. The air in the bubble cancelsthe weight of the air displaced. The bubbles in this experimentcontain air hence it slowly falls. As demonstrated above in Table1,the velocity immediately after release is the highest of them all.The size of the bubbles produced for this experiment is equal or atleast almost equal. The initial velocity of the bubble immediatelyafter release is higher than the velocity recorded afterwards. Thebubble experience an upthrust from the surrounding air fluid but theweight of the thin soapy water film keeps it falling. The speed ofthe bubble decreases to a constant speed.

Table2 showsthe acceleration of the balloon increasing at first due to gravitythat is the only force acting on the balloon. This is explained byNewton’s second law that states that the net force acting on theobject is directly proportional to the acceleration. With time, theair resistance begins to act on the balloon and hence decreases itsacceleration, and the terminal is attained. Acceleration is directlyproportional to the net force. This time the net force is equal togravity minus air resistance. This causes deceleration because thenet force is low

Both compares such that the soap bubble attains its terminal velocityalmost immediately after release while the balloon reaches itsterminal speed after a while. The soap bubble is lighter than thelatex balloon inflated with air. Hence, its downward speed isaffected more than that of the balloon.

CONCLUSION

In this labexperiment, we learn how a balloon and a soap bubble with differencesin surface area and mass affect the terminal velocity of an object. This lab was very interesting because I was able to prove thehypothesis practically that terminalvelocity for a soap bubble is reached immediately after its release.The experiment was successful because it gave theoretical results. However the one trial per range might have given skewed results, weshould be able to recognize the minimal errors and ignore them whendetermining the trends in the terminal velocities. Overall I enjoyedthis lab, and I learned a lot about terminal velocity of a soapbubble.

Reference

Sprott, J. (2006). Physics demonstration: A sourcebook forteachers of physics. Madison: University of Wisconsin Press.pg122.

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