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Why This Science Matters | Explore This Topic

Why This Science Matters

Supercomputer simulations allow us to test our theories of how galaxies, galaxy clusters, and superclusters form. Cosmologists have developed a general framework for understanding how such structures form from gravitational amplification of small matter density fluctuations present in the early universe. However, the precise parameters are not yet pinned down. These parameters correspond to rather fundamental properties of the universe as a whole, such as the mean density of matter in various forms, the amplitude of the matter fluctuations, and the value of the cosmological constant. By comparing the detailed predictions of the simulations with astronomical observations, we can home in on the correct set of parameters.

Even if we know the cosmological parameters exactly, there still remains many uncertainties about the physical processes governing the formation of galaxies. That is because of the complex interplay of gravity, gas dynamics, thermodynamics, star formation and feedback in forming galaxies. Supercomputer simulations solve the relevant physics equations all together. Visualizations and quantitative analysis allow us to understand the galaxy formation process in an analogous way to how atmospheric scientists use supercomputers to understand tornado formation or predict the weather.

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The following questions will further challenge your knowledge of gravity in the Universe. To reveal an answer, place the cursor over "REVEAL THE ANSWER". To study the answer further, click on the answer to open it in a window.

1. The gravitational field follows an inverse square law. Each mass in an infinite line of identical masses is separated from its neighbors by a constant distance. Draw to scale and state the relative size of the gravity vector on each mass due to its interaction with the mass at the origin.

2. Newton's Third Law states that if Body A puts a force on Body B, then Body B exerts an equal and opposite force on Body A. Transfer all of the force vectors from the various masses in Question 1 to the origin to demonstrate that the mass at the origin feels no net gravitational force.

3. In questions 1 and 2, we assumed that there were an infinite number of masses, therefore, every mass feels no net gravitational force. State two reasons to support this conclusion.

4. Suppose the line of infinite masses was not infinite, but instead had only nine masses in a line.

(a) Show that the middle mass still feels no net gravitational force.

(b) Find the relative size and direction of the gravitational force on the other masses.

Answer A:

Answer B:

5. An infinite line of identical masses separated by a constant distance is in equilibrium. This means that all of the masses feel no net gravitational force. Stable equilibrium means that if we disturb one of the objects a little bit, it will return to its original location. Unstable equilibrium means that if we disturb one of the objects and let it go, it will have a tendency to move even further away from its original location.

Suppose we move the mass at the origin a little bit to the right.

Is this stable or unstable? Briefly describe the subsequent behavior of the original line of masses?

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