Falling home

Outline

Children love the sensation of free-falling (relying on an adult back-stop). To jump, to control your motion with impulses is a big thing for many children. Accidental bumps, both large and small, happen every day; also more or less painful ways of changing the motion with impulses. Carers talk about these bumps to provide comfort. A bump may be a large or small uh-oh, or a large or small donk, depending on what's happened. The impulse can be a large force for a short time (hitting a hard surface), or a small force for a longer time (landing on a cushion). Here is one way of using the book to introduce some ideas that are important in the ways that physicists think about the world, drawing on jumping, falling and your language for talking about bumps. Explore the adventure with your child, sharing exploratory descriptions, reflecting the wonder of making sense of the world. There are guidelines for some of the story-inventing that you and your child might enjoy in the back cover. Each spread has a suggestion.

Luna, the spacefaring duck, is in free-fall around the Moon, unsupported by the spacecraft which is also in free-fall. Round and round Luna and her spacecraft go, both orbiting the Moon.

A small impulse changes the motion: use the local phrase for getting a bump( an uh-oh, or local equivalent ). A gently changing movement follows, where Luna and the spacecraft are moving away from the Moon, but still being slowed a little by Moon's gravity.

Now Luna and the spacecraft are falling together, in a state of free-fall. There are 'uh-ohs' only when Luna pushes off the insides of her spacecraft.

A large uh-oh, for which Luna must be prepared, by being strapped in, just like a car seat. Followed by a smaller, and reducing impulse, as the parachute reduces the speed to a terminal value. In both cases, the floor of the craft supports Luna, so Luna is not in free-fall.

A big and sudden uh-oh on splashdown, so Luna stays strapped in. Once floating calmly, the floor supports the spacefarer.

Luna tells her friends about her journey, including about experiencing the large and small bumps.

The friends re-play Luna's journey, experiencing the falling and bumps. Then, the friends are planning their next adventure.

You can read multiple panes in the intended order by following the arrows of the pane boundaries:clockwise around the four or six panes, starting top left.

Reading notes

Support with the physics

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 and natural 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.

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.

Not free-fall: 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.

These are for you, so you can have confidence exploring the book. They are not what the book is trying to teach. Think of the book as a scientific toy: something to play with and explore.