# Gravity | Summary and Q&A

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August 14, 2014
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MIT OpenCourseWare
Gravity

## TL;DR

Gravity is a fundamental force that shapes planets, determines orbits, and governs the behavior of the universe, with three different expressions: gravity near Earth's surface, Newton's universal law of gravity, and Einstein's general relativity.

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### Q: How does gravity near Earth's surface affect our daily lives?

Gravity near Earth's surface provides a constant acceleration that keeps us on the ground, affects elevator operation, and influences the action of pile drivers used in construction.

### Q: What are the applications of Newton's law of universal gravitation?

Newton's law explains why celestial bodies tend to be spherical, predicts the elliptical orbits of planets around the sun, and allows for the discovery of new planets through the analysis of disturbances in existing orbits.

### Q: How does Einstein's general relativity explain the existence of black holes?

Einstein's field equations describe the gravitational effects of massive objects, such as black holes, which bend space and time to the extent that even light cannot escape them.

### Q: What does the cosmological constant in Einstein's field equations represent?

The cosmological constant, denoted as lambda, is used to describe the expansion rate of the universe, which appears to be accelerating. It can signify a collapsing, steady, slowly expanding, or rapidly expanding universe.

## Summary & Key Takeaways

• Gravity predicts the formation of planets, explains why they are spherical, and why planets have elliptical orbits around the sun.

• Newton's law of universal gravitation explains why all objects are pulled towards each other's centers, leading to spherical celestial bodies and elliptical planetary orbits.

• Einstein's field equations, part of general relativity, describe the effects of matter and energy on gravity and have applications in understanding black holes, gravitational lensing, and the nature of the expanding universe.