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Relativistic Mechanics

by Ron Kurtus (updated 9 January 2023)

Relativistic Mechanics—also known as Relativity—concerns the study of the motion of objects and the forces involved. It is divided into two areas: Special Theory of Relativity and the General Theory of Relativity.

The objects involved are of normal to extremely large in size and are traveling close to the speed of light. At such high speeds, physical laws change, according to the Theory of Relativity.

Albert Einstein was a leader in developing the relativity theories.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion


The main idea of relativity is that motion is always relative to some frame of reference.

For example, when you are walking down the sidewalk, you are moving relative to the stationary ground, which is a frame of reference. However, when you are not walking and see a car drive by, the car is moving relative to you. Likewise, the person sitting in the car is also moving relative to you but is stationary with respect to the frame of reference of the car.

A major premise in the study of relativity is that there is no fixed frame of reference about which everything moves. Everything in space is moving relative to other objects, and there is no fixed reference in space.

This brings up questions about whether the laws of physics vary and the what the speed of light is with respect to moving objects. This is studied in the Special Theory of Relativity. There are also questions about what happens when object accelerate and what causes the graviational force, as studied in the General Theory of Relativity.

Special Theory of Relativity

The Special Theory of Relativity is concerned with frames of reference that are not experiencing any acceleration. They are called inerial reference frames.

First postulate

The first postulate of the theory states that the laws of physics have the same form in all inertial reference frames. In other words, the result of an experiement done in a bus moving at a constant velocity is the same as done in a stationary vehicle.

Second postulate

The second postulate states that light propagates through empty space at a constant speed independent of the velocity of the source or observer.

Results of the second postulate are that time dilates at high speeds and lengths contract when the velocity of an object approaches the speed of light. Another factor is that as an object approaches the speed of light, its mass approaches infinite mass.

General Theory of Relativity

The General Theory of Relativity concerns what would happen when objects or reference frames are accelerated.

One concept is that all frames of reference undergoing the same acceleration are equivalent to each other.

Acceleration and gravitation

The theory says that local observations in an accelerating frame of reference cannot be distinguished from observations made in a classic or Newtonian gravitational field. One result is that observers can not determine by experiment whether they are accelerating or are in a gravitational field.

Curvature of space

The General Theory of Relativity states that determined that massive objects cause a distortion in space and time (space-time). Thus gravitation is the effect of this curved space-time.

This is a different concept than defiining gravitation as a force acting at a distance.


Albert Einstein was a leader in developing Relativistic Mechanics or Relativity, which concerns the study of the motion of objects and the forces involved.

The Special Theory of Relativity concerns laws of physics when the frame of reference is not accelerating. It also states that the speed of light is independent of the speed of the source or observer.

The General Theory of Relativity states that gravitatation cannot be distinguished from acceleration and that large masses curve space to cause the gravitational force.

Use your imagination to solve problems

Resources and references

Ron Kurtus' Credentials


Relativistic mechanics - Wikipedia

Mechanics outline - HyperPhysics

Einstein's Theory of Special Relativity - Space.com

Einstein's Theory of General Relativity - Space.com

Physics Resources


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