Raven lays two printed tables on the kitchen table. One lists three forces. The other lists planets.
"Scientists have a word for how a force reaches across a gap," she says. "A field."
Rocket reads the first table. "Electric, magnetic, gravitational. All three can be mapped by what they do to a test object."
"A ball for gravity," Raven says. "A magnet for magnetism. A charged scrap of paper for electric force."
Nova hovers over the planet table. "What do you notice about mass and gravity?" she asks.
Rocket runs a finger down the columns. "The Moon is tiny and its gravity is tiny. Jupiter is huge and its gravity is huge."
"So gravity is a field too," Raven says, "and the biggest one around here belongs to the Sun."
A physics text defines a field as a map of the force around an object, acting on another object at a distance.
The gravitational field around Earth represents the gravitational force a mass would feel at any point. The field is everywhere around the planet.
The field around a charged object extends throughout space. The field around a magnet does the same.
You cannot see a field, but you can map it. Move a test object around and record the force at each spot.
On Wednesday your paper clip was a test object for the magnet's field. The paper scrap was a test object for the tape's field.
The science standards for middle school describe three forces that act at a distance. Here they are as a table.
| Force | Attract, repel or both? | What its size depends on | Test object to map it |
|---|---|---|---|
| Electric | both | the amount of charge and the distance | a charged object |
| Magnetic | both | the strength of the magnets and the distance | a magnet |
| Gravitational | always attracts | the masses and the distance | a ball |
NASA publishes a Planetary Fact Sheet with the mass and surface gravity of every planet. Here are five rows.
Mass is given in units of a trillion trillion kilograms. Gravity is in metres per second squared, how fast a dropped object speeds up.
The standards say a gravitational force exists between any two masses. It is very small unless one object has large mass.
| World | Mass (trillion trillion kg) | Surface gravity (m/s squared) |
|---|---|---|
| Moon | 0.073 | 1.6 |
| Mars | 0.642 | 3.7 |
| Earth | 5.97 | 9.8 |
| Saturn | 568 | 9.0 |
| Jupiter | 1898 | 23.1 |
| Statement | True or false? |
|---|---|
| Gravity between two masses is noticeable only when at least one mass is large. | ? |
| A field can be mapped by moving a test object around and recording the force. | ? |
| Jupiter has less mass than the Moon. | ? |
| Saturn has far more mass than Earth but slightly weaker surface gravity. | ? |
Excellent data reading. Tomorrow you find fields in compasses, dryers, hospitals and every ball you have ever dropped.