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Neutron stars consist only of neutrons and have unbelievably high densities. A typical mass and radius for a neutron star might be 2.7 \(\times 10^{28} \mathrm{kg}\) and \(1.2 \times 10^{3} \mathrm{m} .\) (a) Find the density of such a star. (b) If a dime \(\left(V=2.0 \times 10^{-7} \mathrm{m}^{3}\right)\) were made from this material, how much would it weigh (in pounds)?

Short Answer

Expert verified
(a) Density \( \approx 3.73 \times 10^{18} \text{ kg/m}^3 \), (b) Weight of dime \( \approx 1.65 \times 10^{12} \) pounds.

Step by step solution

01

Understand Density Formula

Density is defined as mass divided by volume. The formula used to calculate density, \( \rho \), is \( \rho = \frac{m}{V} \), where \( m \) is the mass and \( V \) is the volume.
02

Calculate the Volume of Neutron Star

Volume for a sphere is given by \( V = \frac{4}{3} \pi r^3 \). Given the radius \( r = 1.2 \times 10^3 \text{ m} \), the volume \( V \) can be calculated as:\[ V = \frac{4}{3} \pi (1.2 \times 10^3)^3 \]
03

Compute the Volume

Calculate the actual numeric value: \[ V \approx \frac{4}{3} \pi (1.2 \times 10^3)^3 = \frac{4}{3} \pi (1.728 \times 10^9) \approx 7.24 \times 10^9 \text{ m}^3 \]
04

Calculate Density of Neutron Star

Now plug the volume \( V \, (7.24 \times 10^9 \text{ m}^3)\) and mass \( m \, (2.7 \times 10^{28} \text{ kg})\) into the density formula:\[ \rho = \frac{2.7 \times 10^{28}}{7.24 \times 10^9} \approx 3.73 \times 10^{18} \text{ kg/m}^3 \]
05

Calculate the Mass of the Dime

Multiply the volume of the dime by the density of the neutron star to find its mass: \[ m = (3.73 \times 10^{18}) \times (2.0 \times 10^{-7}) = 7.46 \times 10^{11} \text{ kg}\].
06

Convert Mass to Weight in Pounds

Use the conversion factor 1 kg = 2.20462 pounds to convert the mass from kilograms to pounds:\[ \text{Weight} = 7.46 \times 10^{11} \times 2.20462 \approx 1.65 \times 10^{12} \text{ pounds} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Density Calculation
Density is a very important physical property that tells us how much mass is packed into a given volume. Whenever you hear about density, think of a box filled with objects. The heavier the box with the same size, the denser it is. The basic formula to calculate density is:\[ \rho = \frac{m}{V} \]where \( \rho \) represents the density, \( m \) is the mass, and \( V \) the volume of the object.
This formula helps find out how tightly the material inside that space is packed. It's used in many areas of science and engineering, like analyzing materials' properties, buoyancy, and stability calculations, and even in understanding how celestial bodies in the universe behave, like our neutron star example.
Once we know the mass and the volume of an object, calculating its density becomes straightforward. For our neutron star, it helps appreciate its extreme nature. Its density is astonishingly high, explaining why neutron stars are such interesting cosmic objects.
Volume of Sphere
The volume of a sphere is crucial for tasks requiring us to find out how much space something takes up. A sphere is a ball-shaped object, and its volume can be determined using the formula:\[ V = \frac{4}{3} \pi r^3 \]Here, \( V \) is the volume, \( \pi \) is approximately 3.14159, and \( r \) is the radius of the sphere.
This formula derives from geometry, using slices of the sphere to calculate the total space within it. Given the radius of the neutron star, which is impressively large, we substitute and solve the formula to get our answer.
By solving this, you can understand why these stars are so dense despite their relatively small size in cosmic terms. The volume gives us the notion of how much space the star takes, while the density tells us how packed the star is within that space.
Mass Conversion
Sometimes, after calculating mass, we need to switch between units. This brings us to mass conversion—translating the mass value from one unit to another without losing actual physical meaning.
In the exercise, converting the mass of the neutron-star-made dime from kilograms to pounds makes it more relatable and understandable for students used to different measurement systems.
The conversion factor between kilograms and pounds is:
  • 1 kg = 2.20462 pounds
You multiply the mass in kilograms by this conversion factor to find the mass in pounds.
This type of conversion is often done in physics and engineering to ensure everyone can use the data seamlessly, regardless of their measurement preferences. It maintains the physical integrity of the quantities we're dealing with, despite using different scales.
Physics Problem Solving
Solving physics problems often involves working methodically, turning complex scenarios into simpler, manageable steps. This is exactly the beauty of physics problem solving.
In our neutron star problem, we broke down tasks, starting from understanding basic formulas, calculating volumes, and densities, to converting units.
Effective problem-solving involves:
  • Clearly understanding the problem or question — what’s given and what needs to be found.
  • Identifying the relevant formulas or theories.
  • Executing calculations step by step, being careful with units and conversions.
  • Re-evaluating results for reasonableness and correctness.
By adhering to these steps, not only do we solve the problem but also enhance our understanding of the concepts involved. This exercise served as a perfect illustration of how cosmic phenomena can be approached systematically, making the vast, intangible universe a bit more understandable through the lens of physics.

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