I've been thinking about the Sum of Forces equation and the fact that gravity isn't really a force. So what is the proper way to think about this equation when dealing with objects in a gravitational field? My thinking goes like this: The sum of forces equation for flat (Lorentzian) spacetime i...

If a jet engine is bolted to the equator near ground level and run with the exhaust pointing west, does the earth speed up, albeit imperceptibly? Or does the Earth's atmosphere absorb the energy of the exhaust, and transfer it back to the ground, canceling any effect?

In the explanation for rainbow formation, it is said that the spherical droplet refracts and then Totally Internally reflects the incident sunlight. But examining the angles suggest ( see fig below ) that the light is incident on the water-air interface at an angle less than the critical angle. ...