IB Physics II IA

# The Effect of Changes in Air Velocity and Air Pressure on a Falling Ping Pong ball with Different Masses

By Maaz Chaudhry

This investigation was inspired and modeled after a phenomena observed with balls in moving air(specifically ping pong balls, as they are light in mass). While in a laminar airflow, the ball is being hit with two different air velocity. At the top of the ball, the air is moving faster than the air at the bottom, which is important, as this gives a positive air pressure upwards, since the air at the bottom is pushing upwards. Theoretically, with correct air velocities, it would be possible to keep the ball indefinitely in the air, by using this upward air pressure created by the difference in air velocities to negate the force of gravity.

This project is different; instead of lifting the ball into the air, the project will be looking at how this upward pressure slows the acceleration downwards to give the ball more time to go farther horizontally, and how this is affected when the mass of the ball is changed. The ball would have the same horizontal velocity each time, but when the bottom air velocity is smaller and there is upward force, there is less total force downwards on the ball (force of gravity is lessened), taking slightly more time, allowing for more horizontal distance to be traveled (horizontal force, velocity and distance are independent of vertical forces, velocities and distance). The project looks at then how changing the mass of the ping-pong ball affects this phenomena through consequently increasing the force of gravity on the ball.

For this experiment, a setup was necessary, in order to manipulate air velocity. This diagram illustrates the setup used in this project. Setup Procedure:

The first step was to find an ideal table or surface. I used my kitchen island as the surface for the project. Underneath the ledge, I put a stool, then a box on top of this stool. On top of the box I put the first fan. This fan was centered; on top of the ledge, I placed and secured two boxes, creating an air channel. This air channel is extremely necessary, as it keeps the ball going straight and allows distance to be measured correctly; otherwise, the data may not be as reliable as it may go in a direction off-kilter. I placed a tape measure to measure the distance out from the bottom of the island. I measured and marked the line on top of the island where the ball would be placed. Lastly, I placed the other fan at the other end of the island and centered it.

Data Gathering Procedure:

I placed my ping-pong ball on the starting line, then activated the lower fan to the first lower speed. I placed my recording device (my phone) on the edge of the ledge, to accurately record the distance. I then activated the fan on top to the first, slower speed, and released my hold on the ball. It went forward and fell, and I proceeded to examine the footage to record the distance traveled. I repeated this process for five trials; then I proceeded to do the same process five times for the top fan going at the second, faster speed. After repeating these trials five times, I proceeded to puncture the ping-pong ball and fill it with carefully measured amounts of water, using a graduated cylinder. After adding 1 mL water each time, I repeated the trials and recorded the data.

-          Boxes

-          Stool

-          Two Identical Fans, capable of two speeds

-          Ping-pong ball

-          Tape, strong enough to seal water into ping-pong ball

-          Water

-          Tape Measure or measuring instrument

-          Ideal table or surface

The independent variable in the project is the mass of the ball, which will be manipulated through adding water to the ping-pong ball through a hole. Initially the ball was weighed on a scale, where the mass was determined at 2.75 grams. It was put through the experiment, then punctured and filled and drained accordingly to adjust the total mass. The amount of mass taken out by puncturing (under a hundredth of a gram) was replaced by a small amount of tape to seal the water in the ball. The dependent variable was the distance traveled horizontally.

The raw data gathered was in inches, as this was the measurement of the tape measure. I first converted the data to metric, in centimeters.

Raw Converted Data, with the speed constant (speed of bottom fan at slower speed, speed of the top fan at higher speed) and the mass changing: To Data text . : . Excel

The graph made initially showed the downward correlation of the data: After this, I calculated the averages of the data: The Converted Data for the drop with the both speeds the same, to compare with the other data in order to illustrate the effect of the air pressure:

 Top one, Bottom One Trial 1 58.89625 Trial 2 57.785 Trial 3 57.30875 Trial 4 54.9275 Trial 5 55.40375

The data illustrates the difference caused by the change in speed; specific focus is put on comparing both this data and the data from the first Independent Variable tested, where water was not added but the speed was different. Due to the difference in speed causing an upward air pressure, the ball was able to traverse more distance when there were differing speeds.

The average data yielded an accurate graph with the average line of linear regression: As can be observed from the graph, as the mass of the ball increased, the distance traveled by the ball decreased. The R value calculated by the line of linear regression calculates a downward trend. Since a increase in mass increases the force of gravity on the ball, there was more force pushing the ball down. As was shown by the earlier data, the upward air pressure served to help the ball travel farther; the increasing force of gravity as the mass increased served to shorten the distance, as the total force downwards increased.

The effect of the change in mass on the distance traveled was much the same as hypothesized; the distance shortened. The reason for this was that the upward force of air pressure, caused by the slower air below the ball pushing upwards, which had been lessening the total force downwards on the ball and allowing it to go further, was being less effective on the total force on the ball as the force of gravity increased due to the increase in mass. One of the issues inherent with the project was that at higher speeds, the airflow was liable to become more turbulent than laminar, which would severely affect the results as it would go in several different directions. The project would be more stronger if I had made a better air tunnel, such as a tube; this would allow for better air flow and focus the air better, as well as making the results more standard and reliable. This project was extremely interesting, as it displayed air pressure and air velocity affecting the ping-pong ball in a way that I would not have initially expected before hearing of the phenomena. It even has real-life connections, as it could affect results in any situation where there is a ball in moving air, from a windy football game to golf to a ping-pong match in a hot room with many fans.

Some Related Web Sites containing useful information.

This site is useful since it explains Bernoulli’s principle of moving fluids and pressure and provides some useful formula of the forces involved and different applications of Bernoulli. Although viscosity and fluids traveling from higher to lower isn’t relevant much to this specific project, it gives a good explanation of the forces involved with Bernoulli and of Bernoulli’s Principle.

This article discusses the specific application of Bernoulli’s principle in regards to a ping pong ball in the moving fluid(air).

Good source on the basics of Projectile Motion and the forces involved.

Khan Academy on the Conservation of Energy. Good Article on conservation of Energy.

5. Stokes law – Relates drag force to velocity