# How does the temperature of a squash ball affect its rebound height when dropped from rest, from a fixed height onto a fixed surface?

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## Introduction

Recently, I played my first game of squash. Before one starts a competitive game, one must ‘warm up’ the squash ball. Before this act, the ball barely bounces. After multiple impacts, the ball begins to get warm. At this point the ball bounces two to three times as much as it did previously, implying collisions go from inelastic to more elastic. The increase in bounce height can be mathematically represented through the coefficient of restitution (COR). This is a ratio between the velocity after a collision and the velocity before a collision. When COR = 1, the collision is perfectly elastic: all kinetic energy is conserved. When COR = 0, the collision is perfectly inelastic: no kinetic energy is conserved (no bounce). The COR for a ball varies with a number of factors including the elasticity of the ball, the pressure inside the ball, and the hardness of the surface on which the ball is dropped.

In Topic 3 of the IB, students are taught about gas concepts and the relationship between pressure, volume, and temperature. In Topic 2, students are taught about forces and energy. This investigation aims to combine knowledge from Topic 2 and 3 to comprehensively answer the research question.

## Background information

If an object is dropped from rest and there is negligible air resistance, the loss in gravitational potential energy is equal to the gain in kinetic energy.

$$mgh_1 = \frac{1}{ 2} mu^2$$

Where m is the mass of the object (kg),

g is the acceleration due to gravity on earth (ms -2),

u is the velocity of the object upon impact (ms -1)

and h1 is the initial drop height of the ball (m).

$$∴ u = \sqrt{2gh_1}$$            (Equation 1)

By the same principle, the loss in kinetic energy after the collision is equal to the gain in gravitational potential energy, therefore the velocity after the collision is given by

$$v = \sqrt{2gh}_2$$            (Equation 2)

Where v is the velocity of the object after collision (ms-1),

and h2 is the maximum height reached by the ball (m)

The coefficient of restitution is a ratio that describes the velocity of an object before an impact and after an impact and is defined by the following equation

$$e=\frac{v}{u}$$            (Equation 3)

Equations 1 and 2 can be substituted into Equation 3

$$e=\frac{\sqrt{2gh_2}}{\sqrt{2gh_1}}$$

$$∴ e =\sqrt\frac{h_2}{h_1}$$            (Equation 4)

Although the gas in the ball cannot be assumed to be ideal, there is a positive relationship between temperature and pressure. As temperature increases, gas molecules have greater kinetic energy and thus move faster. When molecules collide with the walls of the container, there is a greater change in momentum. By Newton’s Second Law, a greater change in momentum applies a greater force onto the surface. Thus the force exerted on the inner walls of the ball increases. When the ball collides with the ground, the force due to pressure acts to restore the ball to its original shape, thus it exerts a force on the walls of the ball and therefore the ground. Newton’s third law of motion states that when object A exerts a force on object B, object B will exert a force equal in magnitude and opposite in direction onto A. Thus the ground exerts a force equal in magnitude and opposite in direction onto the ball causing a resultant upward force. Newton’s Second Law of motion states that the change in momentum of an object is directly proportional to the resultant force acting on the object, thus a greater force causes a greater change in momentum, giving the ball a greater initial post-collision velocity. Referring to Equation 1, the maximum height will be greater. Thus the COR will approach 1.

Temperature also affects the elastic properties of the squash ball. As temperature increases, the ball experiences greater deformation when an equal force is applied. Hooke’s Law (F = kx) states that force is the product of k, the spring constant, and x, the length of deformity. If x increases, k must decrease as force stays constant. Elastic potential energy is given by the equation

$$E_p=\frac{1}{2}kx^2$$

Since x is proportional to $$\frac{1}{k}$$, this can be substituted in for x.

$$Ep∝ k(\frac{1}{k})^2$$

$$∴ E_p ∝\frac{1}{k}$$

Since a greater amount of energy is stored in the ball, less is lost to the surrounding in the form of heat. This energy become kinetic energy as the ball expands to its original shape. This increases its rebound height, increasing COR.

Thus theory predicts a graph of COR versus temperature to have a decreasing positive gradient and asymptotic towards COR = 1.

### Preliminary trials

A range of preliminary trials were conducted to investigate control variables, test the range of temperatures that will be used, and to develop a method.

Two squash balls at different temperatures (5°C and 60°C) were dropped from different heights (1.00m and 1.70m). This was recorded with the slow-motion feature on a smartphone camera. The footage was analysed using LoggerPro to determine the maximum height reached by the squash ball after the first bounce. 60°C was chosen as a maximum temperature as this 15°C above a squash ball after a usual game of squash (Sportscentaur). 5°C was chosen as it was the lowest temperature the ball could get to with the means available (ice). Even if the ball was initially at 0°C, some heat energy was gained in exposure to air before the ball could be dropped. At 5 ± 0.5 °C and an initial drop height of 1.00 ± 0.02 m, the rebound height was 0.143 ± 0.005 m.

Temperature (°C) ± 0.5Initial drop height (m) ± 0.02Rebound height (m) ± 0.005
51.000.143
51.700.250
601.000.477
601.700.823

Figure 1 - Table On Temperature (°C) ± 0.5

The accuracy of the rebound height, ± 0.005 m, is based on the uncertainty of the ruler used for measurements. But the ball's position against the ruler was observed by the camera, hence the accuracy.

Upon further testing, it was found that the squash ball can be brought up to a temperature of 70°C safely. Thus a range of temperatures from 5°C to 65°C was chosen.

The error in measuring bounce height was 0.005m. For the lowest achieved bounce height (highlighted in blue), the percentage error is $$\frac{0.005}{ 0.143} × 100 = 3. 5\%$$ . Although this is reasonably low, a drop from a larger distance would give a larger range of data. The drop height was limited by safety. A drop height above 1.85m became unsafe due to it’s height in comparison to the person dropping the ball, especially considering the ball may be at 60°C. Thus a height of 1.80m was chosen.

## Variables

Independent variable - The temperature of the squash ball in °C ± 0.5, changed using a water bath 5, 15, 25, 35, 45, 55, and 65.

Dependent variable - The maximum height the squash ball reaches after one bounce in metres ± 0.005 This was measured using video analysis software. A slow-motion recording of the experiment was done. Within the frame was a one-meter ruler in order for the video analysis software to calibrate distances. A frame-by-frame analysis of the recording was done to determine the point of change in direction of the ball. The displacement between this point and the floor was found and recorded.

Control VariableValueMethod/Justification
The initial height of the ball1.80m ± 0.02

A retort stand was set up on top of a desk. A retort clamp was set at a height of 1.80m above the ground. The retort clamp was opened to a width slightly smaller than the width of the squash ball. The ball was then placed on the arm of the clamp, and the width was increased to allow the ball to fall through. This ensured that the ball was dropped from the same point in each trial and was dropped from rest at each trial. The same retort stand and clamp were used for all trials. Different drop heights would have changed the velocity on impact due to acceleration due to gravity.

The initial velocity of the ball

0 ms-1

A retort clamp was used to position the ball 1.80 metres above the ground. The clamp was opened to about 3.5 cm. This was the minimum width at which the squash ball could rest upon the surface of the clamp. The clamp was then opened to allow the ball to drop through.

Because the change in velocity is constant depending on the height, an initial velocity would have affected the final velocity. Although terminal velocity would have an effect on the change in velocity, at low velocities and surface areas, this factor is negligible.

The elasticity of the ballOne red dot

Squash balls come in various standards, these are identified by coloured dots on the balls. The standard varies in quality, durability, and elasticity. 10 single red dot balls were selected for the investigation. This ensured there were no systematic errors in data due to a difference in the ball.

Elasticity and consistency of ground---The balls were dropped onto the same flat surface for each trial. This ensured there were no errors with balls bouncing to the side or differences in the change in momentum. An elastic surface would have increased force exerted onto the ball, increasing its kinetic energy and therefore maximum height. . Since this would affect the dependent variable, it was kept constant.

Figure 2 - Table On Control Variable

### Safety

Exposure to 65°C water for two seconds or more can cause third-degree burns (CPSC). When dealing with this temperature, caution was taken to avoid contact with the water and the ball. This was done by using tongs to remove squash balls from the water bath. Furthermore, eye protection was worn in case of splashing. Temperatures above 50°C can still cause severe burns if in contact with skin, thus caution was taken with all temperatures above 50°C. Another safety hazard was the retort stand. Due to its heavy base, knocking it over would likely lead to damage of property or harm to an individual. This was especially important considering it was at the edge of a table. The experiment was done with minimal interference from peers to ensure no accidents occured.

### Environmental and ethical issues

Water baths were turned off when not in use and were set to the minimum temperature possible in order to reduce energy consumption. All squash balls used in the process were reused. There were no other environmental issues.

Due to the small scale of the experiment, there were no ethical issues.

NameQuantityUncertainty
Retort stand1---
Retort clamp1---
Squash ball10---
Ruler (1.000m)1± 0.001m
Smartphone slow-motion camera1---
Water bath1± 0.1°C
Digital infrared thermometer1± 0.1°C

Figure 3 - Table On Apparatus

### Method

• Apparatus was set up as shown in Figure 4.
• 5 squash balls were placed in a water bath set to 65°C.
• The slow-motion camera was turned on and started recording.
• When the balls reached 65°C (checked with a digital infrared thermometer), they were taken out of the water bath one at a time with tongs and placed on the retort clamp as quickly as possible. The ball retort clamp was quickly loosened until the ball fell.
• Step 4 was repeated four more times to obtain a more accurate mean value of h2
• Steps 2-5 were repeated at temperatures of 55°C, 45°C, 35°C, 25°C, 15°C, and 5°C. With temperatures below 25°C, squash balls were placed in an ice bath with their temperatures being monitored.

### Sample calculation

Consider T = 5.0 ± 0.5 °C

h2 mean

$$\frac{h _{2(1)} +h_{ 2(2)} +h_{ 2(3)} +h_{ 2(4)} +h_{ 2(5) }}{5}$$

$$=\frac{ 0.175 + 0.190 + 0.182 + 0.173 + 0.191)}{5}$$

= 0.182m

h2 mean uncertainty

= max|h (1, 2, 3, 4, 5) -h mean |

= 0.191 - 0.182

= 0.009m

COR

$$COR = \sqrt\frac{h_{2}}{h_1}$$

Where h1 = 1.80 ± 0.01 m

Consider h2 =0.182 ± 0.009.

$$COR = \sqrt\frac{0.182}{1.80} =0.318$$

COR Uncertainty

Consider h2 =0.182 ± 0.009

$$=\frac {\% unc\ h_{2}}{2} + \frac{\%unc\ h_1}{2}$$

$$\%unc\ h_2 = 100×\frac{0.009}{0.182}$$

= 4.95%

$$\%unc\ h_1 = 100× \frac{0.01}{1.80}$$

= 0.556%

$$\%unc \ COR =\frac{4.95}{2}\frac{0.556}{2}$$

%unc COR = 2.48+0.28

%unc COR = 2.76%

$$unc\ COR = \frac{2.76}{ 100} × 0.318$$

unc COR = 0.009

The graph shown in Fig 2 has an increasing positive gradient between 5 and 25°C. Between the temperatures of 35 and 65°C, there is a positive relationship with a decreasing gradient.

To investigate the mathematical relationship between COR and T, the assumption was made that

T ∝ CORn

∴ T = k × CORn

log(T) = log(k × CORn)

log(T) = nlog(COR)+ log(k)

Thus, assuming there is a mathematical relationship between T and COR, a graph of log(T) versus log(COR) will be a straight line graph with gradient n, where n determines the relationship between the two variables.

log(T)

log(CORmean)

0.699 (± 0.046)-0.497 (± 0.011)
1.176 (± 0.028)-0.475 (± 0.030)
1.398 (± 0.017)-0.345 (± 0.005)
1.544 (± 0.012)-0.232 (± 0.009)
1.653 (± 0.010)-0.179 (± 0.007)
1.740 (± 0.008)-0.158 (± 0.007)
1.813 (± 0.007)-0.142 (± 0.008)

Figure 8 - Table On

### Sample calculations

log(T) uncertainty

Consider T = 5.0 ± 0.5 °C

log(t) unc =

max[log(T) − log(T 0. 5), log(T + 0. 5) − log(T)]

= max[0. 6990. 653, 0. 7400. 699]

= 0. 046

log(COR) uncertainty

Consider COR = 0.318 ± 0.009

log(COR) unc =

max[log(COR) − log(COR − 0.009), log(COR + 0.009) − log(COR)]

= max[− 0. 498 −− 0. 510, − 0. 485 −− 0. 498]

= 0. 013

## Conclusion

• Fig 3 plots COR versus T. Between T = 5°C and T = 25°C, COR increases at an increasing rate. Between T = 35°C and T = 65°C, COR increases at a decreasing rate.
• Theory predicts a graph of COR versus T to have a decreasing positive gradient and asymptotic towards COR = 1 (pg 2). Experimentally, a graph with a positive gradient, decreasing gradient was obtained when T > 25°C. The graph appears to be asymptotic, however, a greater range of data is needed to confirm this.
• All points follow the predicted theory other than the point at (5.0, 0.182). Although this may be an anomaly, since five trials were taken, it is unlikely. Thus the point suggests that when T < 15.0°C, the ball loses its elasticity at a decreasing rate.
PointImpact
Use of a digital infrared thermometer

The use of an infrared thermometer allowed the temperature of the squash ball to be measured directly (a liquid thermometer would not be able to measure the temperature of the ball, thus the temperature of the water would have had to be measured. This may differ from the temperature of the ball depending on how long the ball was in the water.). In addition, the digital thermometer allowed the temperature to be measured within one second ensuring the ball did not cool while measuring its temperature. Furthermore, the uncertainty on the thermometer was 0.1°C. This was changed to 0.5°C due to cooling in the time spent between taking the ball out of the water bath and placing it in the retort clamp.

Number of trialsFive trials were conducted at each temperature. This greatly reduced random uncertainty as anomalies could be identified more easily. Furthermore, it allowed for a greater amount of certainty in data points.
Surface of the groundBecause the experiment took about one hour, it could be completed in one go. This made it extremely easy to control the surface the ball was bouncing on. Since the retort clamp was not moved, the surface below the initial position of the ball was kept constant.

Figure 10 -Table On Strengths

PointImprovement

There was a relatively small range of data, 5.0°C to 65°C. Since the relationship between COR and T is non-linear, the investigation only gave insight into the effect of T on the COR from 5 - 65°C.

A greater range of data could be used. A commercial freezer has a temperature of -18°C thus a lower limit range could be a temperature of -15°C (FDA). Water boils at 100°C, a squash ball in a bath of boiling water can also reach 100°C, thus the maximum range of temperatures will be -15°C to 100°C.

The internal temperature of the ball could not be measured. Since the thermometer used could only measure the surface temperature of the ball, the temperature of the gas inside the ball could not be measured. Thus the temperature measured may not represent the temperature of the ball.

Leaving the ball in the water bath for at least 10 minutes would ensure that heat is transferred throughout the ball.

Although logger pro is extremely precise, there may be some systematic error in the measurement. This may be due to human error as the calibration length had to be drawn by hand allowing for human error. Furthermore, there may have been systematc errors in the measurement due to distortion due to the angle of recording. This would affect the calculated COR, thus affecting the investigation.

A lightgate velocity sensor could be used to measure the initial velocity before the bounce and the velocity after the bounce allowing the direct calculation of COR. The lightgate has an uncertainty of 0.0001ms-1 . The minimum rebound height was 0.182m. This gives an initial velocity of 1.89ms-1 . Thus the largest percentage uncertainty would be $$100 × \frac{0.0001}{1.89}=0.005\%$$.

Figure 11 -Table On Limitations

## Extension

Although temperature affects the pressure inside the ball, this investigation cannot investigate the direct relationship between pressure and COR mostly because the gas cannot be assumed to be ideal but also because temperature also affects the elasticity of the ball. One way to measure the effect of pressure on the COR is through the use of a different type of ball. A basketball can be used as its pressure can easily be adjusted using an air pump. The method could stay relatively the same, using a slow-motion camera to measure the rebound height. The usual pressure of a basketball is around 51,000 to 58,000 pascals, thus a range of pressures from 50,000 to 60,000 pascals can be used (NBA).

Pressure can be increased in a few ways: decreasing the volume, increasing the number of moles of gas, or increasing the temperature. This method would increase the number of moles of gas using a pump. Due to a greater number of gas molecules, there would be more collisions with the inner wall, thus a greater force would be applied.

The pressure inside the ball can be measured using a digital pressure gauge. This has an accuracy of 0.01 kPa, for a minimum pressure, 50kPa, the percentage uncertainty would be $$\frac{0.01}{ 5} × 100 = 0. 02\%$$, therefore extremely small.

### Works cited

"Are You Storing Food Safely?" U.S. Food and Drug Administration, 9 Feb. 2021, www.fda.gov/consumers/consumer-updates/are-you-storing-food-safely. Accessed 22 June 2022.

CPSC. CPSC.gov, www.cpsc.gov/s3fs-public/5098-Tap-Water-Scalds.pdf. Accessed 22 June 2022.

NBA. The Of icial Site of the NBA for the Latest NBA Scores, Stats & News. | NBA.com, www.nba.com/resources/static/team/v2/thunder/explorers-1819-y2a4-activity.pdf. Accessed 22 June 2022.

Williams, Martin. "How To Warm Up A Squash Ball." Sports Centaur, 30 July 2018, sportscentaur.com/how-to-warm-up-a-squash-ball. Accessed 22 June 2022.