# Einstein's General Theory of Relativity | Lecture 1 | Summary and Q&A

7.0M views
January 13, 2009
by
Stanford
Einstein's General Theory of Relativity | Lecture 1

## Summary

This video explores the concept of gravity and Newtonian gravity in particular. It explains the basic equations and principles that describe gravity, including Newton's laws of motion and the force of gravity between two objects. The video also discusses the concept of the equivalence principle, which states that the motion of an object is independent of its mass and that falling in a gravitational field is indistinguishable from being in free space.

### Q: What is gravity?

Gravity is a force that is different from electrical and magnetic forces and is connected to the geometric properties of space and time. It is described by the general theory of relativity.

### Q: How is gravity different from other forces?

Gravity is unusual because it is connected to the geometric properties of space and time. It also has the special property that the strength of the gravitational force is proportional to the mass of an object, unlike other forces which are usually proportional to other properties like electric charge.

### Q: What is Newtonian gravity?

Newtonian gravity is the oldest and simplest mathematical form of gravity. It is described by Newton's equations, which state that the force acting on an object is equal to the mass of the object times its acceleration. This force is attractive and proportional to the product of the masses and inversely proportional to the square of the distance between them.

### Q: What is an inertial frame of reference?

An inertial frame of reference is a frame in which an object will move with uniform motion and no acceleration if there are no forces acting on it. In an inertial frame of reference, Newton's first law (the law of inertia) holds true.

### Q: How is gravity described in an inertial frame of reference?

In an inertial frame of reference, the force on an object is equal to the mass of the object times its acceleration. This vector equation can be written in component form, where each component of the force is equal to the mass of the object times the corresponding component of acceleration.

### Q: What is the relationship between force, mass, and acceleration in Newton's equations?

Newton's equations state that the force on an object is equal to the mass of the object times its acceleration. This equation is a vector equation, where each component of the force is equal to the mass of the object times the corresponding component of acceleration. The mass is a conserved quantity that does not change.

### Q: How does gravity in the flat space approximation differ from gravity in real space?

In the flat space approximation, gravity points in the same direction everywhere and the gravitational force is independent of height. In real space, due to the curved shape of the Earth, the gravitational force varies with height. This leads to differences in the acceleration experienced and the presence of tidal forces.

### Q: Why is gravity considered a weak force?

Gravity is considered a weak force compared to other forces, such as the electromagnetic force. This is because Newton's constant of gravitation (G) is very small. While gravity can have significant effects on large objects due to their mass, it is usually negligible when compared to other forces acting on smaller objects.

### Q: How does gravity affect objects of different masses?

The motion of an object in a gravitational field does not depend on its mass. This means that objects of different masses will fall at the same rate in a gravitational field. This property, known as the equivalence principle, is a fundamental characteristic of gravity.

### Q: How did Newton derive the equation for the gravitational force?

Newton derived the equation for the gravitational force based on observations and mathematical analysis. Specifically, he used Kepler's laws of planetary motion and the information they provided about the relationship between the period of motion, the radius of orbit, and the acceleration towards the center of the orbit. He found that an inverse square law of force would yield the correct predictions for planetary motion.

### Q: What are tidal forces?

Tidal forces are the differential gravitational forces experienced by an extended object due to the variation in gravitational acceleration across its size or shape. These forces can cause stretching or compression of the object, leading to distortions and changes in its shape.

## Takeaways

Gravity is a unique force that is connected to the geometric properties of space and time. In its simplest form, described by Newtonian gravity, the force of gravity is proportional to the product of the masses of two objects and inversely proportional to the square of the distance between them. In an inertial frame of reference, the motion of an object can be described using Newton's equations, where the force is equal to the mass times the acceleration. The equivalence principle states that the motion of an object is independent of its mass in a gravitational field. Tidal forces can cause distortions and changes in the shape of objects in a gravitational field. Gravity is considered a weak force compared to other forces, such as the electromagnetic force.