Falling home: the physics 

Two big ideas

There are just two big physics ideas in the story: “impulse changes motion” and “unforced natural motion”. But they are both essential ideas, and took a long time to be accepted. That's because everyday experience suggests other descriptions. Chairs and boxes move only as long as you push them, but this is a consequence of local conditions, and a change of point of view reaps rewards.

As with taking a different viewpoint in other areas of life, starting early is best.

Impulse changes motion


Things don't just start moving by themselves. An impulse gets something going.

Motion doesn't just peter out by itself. Impulses bring things to a halt.

So, what is an impulse?

A “donk” or “uh-oh” is an example of an impulse, as are soft or hard landings. A large force for a short time gives a hard landing, or a painful “donk”. Reduce the force and increase the time to get the same impulse, but a softer landing, or a less painful “donk”.

An impulse changes the motion of something whenever a force acts on that thing for a time.

Impulses can accumulate to keep on changing motion, as time ticks on.

One particular accumulation gives an orbit.

Impulses from particles

The rockets throw a stream of particles in one direction, exerting an impulse on the spacecraft. Later the spacecraft collides with the particles of the atmosphere, and these collisions result on an impulse that slows the spacecraft down. Later still the parachute opens so that more particles collide: this increases the impulse, further slowing the spacecraft.

Luna is supported by her chair or the walls of the spacecraft. Otherwise, she's in free-fall: Luna experiences weightlessness when she is unsupported. 


Free-fall is natural motion

Newton had the idea that “natural motion“ happened when there were no forces; Einstein updated “natural motion“ to things being in free-fall or unsupported. They had completely different views of gravity.


Support in two ways


On the Earth, Luna, the spacecraft or her friends are supported in one of two ways, depending on their environment. If one of the friends or the spacecraft is making a hole in the water, then there is a buoyancy force.

If something is squeezing a solid underneath it (the chair or floor in the spacecraft, the ground), then expect a compression force.