# 9.4 Forces and Kinetic Energy | Summary and Q&A

3.2K views
November 8, 2021
by
MIT OpenCourseWare
9.4 Forces and Kinetic Energy

## TL;DR

This content discusses the relationship between forces and special relativity, highlighting the differences in kinetic energy calculations and the transformation of forces under Lorentz transformations.

## Install to Summarize YouTube Videos and Get Transcripts

### Q: What is the formula for calculating kinetic energy in special relativity?

The formula for calculating kinetic energy in special relativity is k = (m0c^2)(γ - 1), where m0 is the rest mass, c is the speed of light, and γ is the Lorentz factor.

### Q: How does the calculation of kinetic energy differ for velocities close to the speed of light?

At higher velocities, the classical calculation of kinetic energy is inadequate, and the relativistic calculation shows a significant increase in the kinetic energy due to the Lorentz factor γ.

### Q: What are the two components of the force in special relativity?

In special relativity, the force acting on a particle has two components. One component is parallel to the acceleration (mγa), while the other component (m0u∆γ) is not parallel to the acceleration.

### Q: How does the force transform under Lorentz transformations in special relativity?

The force in special relativity does not remain parallel to the acceleration under Lorentz transformations. It has two components, one parallel to the acceleration and the other not parallel to it, making it counterintuitive.

## Summary & Key Takeaways

• The content explores the concept of forces in special relativity, starting from Newton's second law and its application in calculating kinetic energy.

• The calculation of kinetic energy involves integrating the force over the path of the particle, which leads to a formula involving the mass, velocity, and the speed of light.

• For small velocities, the classical calculation of kinetic energy matches the relativistic calculation, but at higher velocities, there is a significant difference.