Problem 64 - College Physics

Question

A hot-air balloon has a volume of 2200 \(\mathrm{m}^{3} .\) The balloon fabric (the envelope) weighs 900 \(\mathrm{N} .\) The basket with gear and full propane tanks weighs 1700 \(\mathrm{N}\) . If the balloon can barely lift an additional 3200 \(\mathrm{N}\) of passengers, breakfast, and champagne when the outside air density is \(1.23 \mathrm{kg} / \mathrm{m}^{3},\) what is the average density of the heated gases in the envelope?

Step-by-Step Solution

Answer

The average density of the heated gases in the balloon is approximately 0.9602 kg/m³.

Step 1: Understand the Problem

We need to find the average density of the heated gases inside the balloon when it is barely lifting its total load. This means it should just be in equilibrium with the weights it is lifting plus its own weight.

Step 2: Calculate Total Load

The total weight that the balloon must support (total load) is the sum of the weight of the balloon fabric, the basket with gear and tanks, and the weight of passengers, breakfast, and champagne. \[ \text{Total Load} = 900 \text{ N} + 1700 \text{ N} + 3200 \text{ N} = 5800 \text{ N} \]

Step 3: Buoyant Force Calculation

The buoyant force on the balloon is given by Archimedes' principle and is equal to the weight of the air displaced by the balloon: \[ F_b = \rho_{\text{air}} \times V \times g \] where \( \rho_{\text{air}} = 1.23 \text{ kg/m}^3 \), \( V = 2200 \text{ m}^3 \), and \( g = 9.8 \text{ m/s}^2 \). Calculate the buoyant force:\[ F_b = 1.23 \times 2200 \times 9.8 = 26532.6 \text{ N} \]

Step 4: Determine Additional Lift Provided by Balloon

We set the buoyant force equal to the total load to find the additional lift provided by the balloon with heated gas:\[ F_b = W = 5800 \text{ N} \]

Step 5: Solve for Density of Heated Gas

The average density of the heated gas inside, \( \rho_{\text{gas}} \), can be found from:\[ F_b = (\rho_{\text{air}} - \rho_{\text{gas}}) \times V \times g = W \]Rearrange for \( \rho_{\text{gas}} \):\[ \rho_{\text{gas}} = \rho_{\text{air}} - \frac{W}{V \times g} \]Substitute the known values:\[ \rho_{\text{gas}} = 1.23 - \frac{5800}{2200 \times 9.8} \]\[ \rho_{\text{gas}} = 1.23 - 0.2698 \approx 0.9602 \text{ kg/m}^3 \]

Step 6: Conclusion

The average density of the heated gases in the envelope is approximately \( 0.9602 \text{ kg/m}^3 \).

Key Concepts

Archimedes' Principle

Archimedes' principle is fundamental to understanding how objects float or sink in fluids. It states that any object submerged in a fluid is buoyed up by a force equal to the weight of the fluid that the object displaces. In simpler terms, if something is submerged in water and takes up a space that water would otherwise occupy, the object faces an upward or buoyant force. This principle is crucial for things like ships, submarines, and our case, hot-air balloons.In the context of our hot-air balloon exercise, Archimedes' principle helps us realize that the balloon ascends because the air inside has a lower density than the outside air, allowing the balloon to displace a heavier weight of air and thereby creating a buoyant force sufficient enough to lift its total load. This force is computed using the formula: \[ F_b = \rho_{\text{air}} \times V \times g \]Here, \( F_b \) (the buoyant force) directly relates to the outside air density \( \rho_{\text{air}} \), the balloon's volume \( V \), and gravitational acceleration \( g \). Understanding this basic principle is the key to knowing how and why the hot-air balloon can lift loads.

Density Calculation

Density is a measure of how much mass is contained in a given volume. It's an essential factor for many scientific calculations, especially in understanding how and why things float. When you compare the densities of two substances, the one with lower density will tend to float on or in the higher density substance. The formula for density is: \[ \rho = \frac{m}{V} \] where \( \rho \) is density, \( m \) is mass, and \( V \) is volume. In our exercise, the task is to find the density of the heated gas inside the balloon. This involves knowing the atmospheric air density and using it to compute the difference between it and the density of the hot air.For the hot-air balloon problem, the density of heated gases \( \rho_{\text{gas}} \) is calculated based on Archimedes' principle where the buoyant force equals the weight of the displaced cold air and must match the total weight the balloon needs to lift, allowing for equilibrium. The rearranged formula used is: \[ \rho_{\text{gas}} = \rho_{\text{air}} - \frac{W}{V \times g} \]Understanding density calculations thus helps us determine that the gases inside the balloon are specifically less dense than the air outside, causing the desired lift.

Hot-Air Balloon Dynamics

Hot-air balloons work because of the interplay between heated air inside and the cooler surrounding air. When you heat air, it expands, becoming less dense and allowing the balloon to rise because of the buoyant force we discussed. To optimize this dynamic, the balloon must have a carefully balanced weight distribution, including the weight of its passengers and any cargo. Balloon dynamics are directly linked to the overall ability of the heated air to lift the balloon; this is in alignment with Archimedes' principle. In our exercise, the goal was to adjust the density of the air within the balloon by heating it. As long as this internal air density remains less than the outside air density, the balloon can remain buoyant and lift off. Key aspects of hot-air balloon dynamics include:

• Controlling the temperature of the air inside to manipulate buoyancy.
• Balancing the overall weight, hence knowing the payload capacity is vital.
• Understanding how reduced air density enables elevation.

The interplay of these factors not only determines how well a balloon can rise, but also how well it can control its altitude and navigate effectively. By mastering these dynamics, pilots can safely enjoy the soaring adventures of hot-air ballooning.