Measurement of Air Drag as Physics Experiment Enrichment at Senior High School Laboratory Using the Air Track Apparatus

Received: January 3, 2020 Accepted: April 6, 2020 Published: April 30, 2020 Linear air track is often used in physics learning for linear motion experiments because it can reduce friction between objects with trajectories. However, the use of air tracks for motion experiments in schools often does not care about aspects of air drag, so the purpose of this study is to calculate the air friction contained in the air track and as an offer of enrichment experiments at senior high school. The research method used is an experimental method that uses a set of air track experimental devices consisting of trajectors, carts, blower, and time counters with light sensors. Cart objects with a mass of 120.02 gram is given the initial velocity variation 12.272 cm/s, 16.286 cm/s and 24.599 cm/s. Then the time recorded when the cart crosses the distance of 10 cm to 110 cm at intervals of 10 cm. This experiment is conducted in the Integrated Science Laboratory, Faculty of Mathematics and Natural Sciences, Universitas Negeri Semarang. The second Newton law has been derived to obtain a special exponential function, so the relation between distance and time is obtained. The non-linear relation between distance and time shows the effect of air drag. Then, fitting the graph of the distance and time relation so that the air drag constants obtained are (10.6 ± 0.1) gram/s, (10.6 ± 0.2) gram/s, and (11.1 ± 0.2) gram/s. The results of the air drag constants obtained can be additional data as a factor affecting experiments using linear air track and can be enrichment experiments at senior high school laboratory.


INTRODUCTION
In the beginning, the experimental motion of objects in schools often used a ticker timer, but there were many obstacles (Prima et al., 2016;Marinho & Paulucci, 2016), so now physics experiments using linear air track apparatus have been widely offered for physics learning in school laboratories. Students can easily carry out dynamics and kinematics experiments using air tracks because the position, speed, and acceleration of objects can be easily measured and observed. After all, it is assisted by a photo gate-based time counter device (Saphet et al., 2002). There is another technology to observe the motion of objects. It is a video tracker (Yusuf, 2016), but the video tracker is less practical because it has to process image data first.
Mechanical experiments without using the air track should also consider friction between objects with trajectory (Minkin, & Sikes, 2017). One of the advantages of air track is that the system is considered ideal because friction is very small (considered non-existent) due to air exhaled between objects and trajectories. But actually, this system is not ideal because there is still air drag that affects it. In advanced experiments can be introduced to students about the effects of air drag in motion. In this study, we will measure the air drag produced by the air track apparatus system in a linear motion experiment.
Several studies using air track apparatus have been carried out including experiment of harmonic oscillator damped (Hauko et al., 2018;Hinrichsen & Larnder, 2018;Hinrichsen, 2018), one dimension collision of two objects experiment (Saphet et al., 2017), Investigation of magnetic damping (Oyelade, 2019). Also, some of the classical mechanical studies experiments have been concern the effects of air drag including modeling the effects of the air intake on Ping-Pong ball motion (Thuecks & Demas, 2019), cart experiments on an inclined plane (Amato & Williams, 2010), projectile motion experiments (Blanco, 2018;Mohazzabi, 2018), research of aerodynamics in the classroom and ballpark (Cross, 2012), freefall motion experiment (Mohazzabi, 2011), juggling a spinning Ping-Pong ball experiment (Widenhorn, 2016), Teaching kinematic graphs in an undergraduate course (Vaara & Sasaki, 2019), coupled oscillators (Case et al., 2017), research on how mechanical energy vanish into thin air (Moreno, 2018), Measuring the coefficient of restitution (González et al, 2017), Previously, a study to measure air drag in air track apparatus used the concept of an inclined plane, so that gravity force which is parallel to the track affects the motion of objects (Mungan, 2012). This requires that we look for the value of gravity acceleration first. In this study, only using a simple linear motion system where objects are given a certain initial speed so that it is easier to experiment. The time counter in this experiment uses sensitive light sensors that are directly connected to a computer also makes it easy for us to record the time when objects cross certain distances that we have set. Also, the study of two objects collisions using air track apparatus ignores the role of air drag (Saphet et al., 2017). So that study on measuring air drag in air tracks apparatus is very important to do because information about air drag can be an improvement in physics experiments in school. The Effect of water drag is also used in free-fall motion experiments (Sibert et al., 2019), and projectile motion experiments (Azhikannickal, 2019) From the literature study above, the advantage of this research is that the measurement of air drag only uses the straight-motion experimental method, so that it will be simpler if it is done by students than the measurement of air resistance by other studies such as Mungan (2012), which uses the inclined plane of the experiment. This experiment also uses a time counter with a sensitive light sensor and is directly related to the computer so that data processing is faster. This study also corrected previous research in the use of air tracks that ignored air drag factors.

Theoretical Explanation
The easiest explanation of motion is to use Newton's Law. Newton's first law explains the inertia of an object (Halliday et al., 2010). If an object initially has velocity v, then the speed will be constant when there is no force acting on the object. Suppose that in a laboratory experiment, the cart moves linearly with a low velocity on the air track. The cart will get an air drag force so that velocity will decrease over time. For low velocity, the air drag force Fair is proportional to velocity v (equation 1) (Merci, 2016).
= − (1) where c is air drag constant, which depends on the dimension of an object and air density.
Equation 4 shows that velocity is not constant, but it will decrease over time. We know that velocity v(t) is the first derivation of position s(t), so the integration result from equation 4, will be expressed in equation 5,

Place and Time of Research
This research was conducted in the Integrated Science Laboratory, Faculty of Mathematics and Natural Sciences, Universitas Negeri Semarang, from July to November 2019. The diagram of the experiment is shown in Figure 1.

Materials and Apparatus
Experiments used 1 set of air tracks apparatus consisting of 1.5 m trajectories, the cart with a mass of 120.02 grams, and a blower to drain air on the trajectories, a light sensor-based time counter. The schematic series of devices can be seen in Figure 1.

Experiment Procedure
The diagram of the experiment is shown in Figure 2. At first, the cart was moving at a certain velocity. The initial speed was measured based on the time counter readable results when the cart position s = 0. In this study the three initial velocity variations used are 12.272 cm/s, 16.286 cm/s and 24.599 cm/s. The cart moved along the air track with the hypothesis the velocity decreased with time due to the air drag force. The sensor read the cart movements while crossing the air track. Sensors were installed for distances of 10.0 to 110.0 cm at intervals of 10.0 cm so that we can record 11 times.
Because this experiment uses a sensitive light sensor, the external factor that influences is the ambient light which can influence the reading of the compilation time of the object passing through the path. However, this has been anticipated that the distance is as close as possible between the sensor and the part of the object that is read by the sensor so that the ambient light that enters the sensor will be reduced.

Data Analysis
Time and distance data were then tabulated and graphed. The graphs obtained were then fitted with special exponential functions (Equations 6), which correspond to equation 5. The y-axis is the distance (s) while the x-axis is time (t).
where a = dan b = We used the Origin Lab software for fittings. After graph fitting, we analyzed the graph to find the values of a and b so we can calculate the value of c as the constant of air drag. After we got the value of the air drag coefficient, we plotted the relation graph between velocity and time according to equation (4).

RESULTS AND DISCUSSION
The experimental results show that there is a non-linear relationship between distance and time. If plotted into a graph (Figure 3), then according to equation 6. .
Graphs that are not linear in Figure 3 show that cart motion is not a regular linear motion at a constant velocity. If the velocity is constant, then a linear distance versus time graph will be generated. The pattern shown by the experimental results indicates that there are effects of air drag. These results provide corrections to the study conducted by (Hauko et al., 2018;Hinrichsen & Larnder, 2018;Hinrichsen, 2018;Saphet et al., 2017;Oyelade, 2019) who in his experiment ignored air drag.
Through graph analysis, the values of a and b have been obtained so that the air drag constants can be calculated. The calculation results are shown in Table 1.
If seen for the initial velocity of 12.272 cm/s and 16.286 cm/s, we get the same air drag constant, but there is a difference with the results of the air drag constant for the initial velocity of 24.599 cm/s. This phenomenon occurs because, for higher velocity, the air drag force is no longer proportional to the object velocity but is proportional to the square of object velocity = − 2 (Blanco, 2018;Mohazzabi, 2018;Cross, 2012). These results provide corrections to the study (Merci, 2016) that equation 1 is not always appropriate for motion experiments.
The results of the air drag constants that have been obtained are then used to plot the graph of the velocity versus time (Figure 4).
We set a time scale interval from 0 seconds until 10 seconds. Figure 4 shows that the cart velocity decreases exponentially over time, according to equation 4. This phenomenon indicates that the cart does not move at a constant velocity. The effect of air drag causes a decrease in velocity. These results are consistent with research conducted by Cross (2012), Hackborn (2016), andLi et al. (2016) that the effect of air drag causes the object velocity is not constant (decrease). The existence of air drags when using air track apparatus shows that experiments at school can be more advanced by considering air drag. The results of the air drag constant can be used as a correction factor in subsequent motion experiments.

CONCLUSION AND SUGGESTION
Experiments of linear motion using the air track apparatus turned out not really without friction. The results showed that there was an effect of air drag that affected cart motion. The cart velocity has gradually decreased exponentially. The relation of distance versus time shows a trend that is not linear. Through the approach that the air drag force is proportional to the cart velocity, we got the results that the air resistance constant value is (10.6 ± 0.1) gram/s for the initial velocity of 12.272 cm/s, (10.6 ± 0.2) gram/s for the initial velocity of 16.286 cm/s, and (11.1 ± 0.2) gram/s for the initial speed of 24.599 cm/s. The results of the air drag constants obtained can be additional data, and senior high school laboratories as a factor affecting experiments using linear air track, and this experiment can be an enrichment physics experiment at senior high school laboratory. It would be better if the next experiment on measuring air drag could be a model offered in teaching enrichment of Newtonian mechanics experiments.
The research can be continued to lead to the validation of experimental results by asking students to repeat similar experiments but with different techniques such as mechanical energy or momentum approaches. Research can also lead to the study of students' misconceptions about the concept of air drag.