Here is the finished map of Earth's field, with three of the mapped points marked.
Each arrow records the direction of the force the field would exert at that point. Joined up, the arrows form the drawn lines.
The mapmaker's record at those three points makes a table: the point, the force direction there, and the arrow that direction puts on the page.
Three points, one rule. At every point the force direction is toward Earth's center, and on the page that is a different arrow at each point.
Lines that show, at each point, the direction of the force the field would exert are called field lines.
The map is read the same way it was made: the arrow at any point on a line shows the direction of the force the field would exert at that point.
One warning. The lines are a drawing tool. They show the force's direction at each point. They are not tracks, and an object placed in the field does not have to travel along them.
Check what you know
At a point on a field line, what does the arrow show?
The arrow at the marked point aims at Earth's center. Soren says the force on a rock held there would act toward Earth's center. Is Soren correct?
Type the name for the lines that show, at each point, the direction of the force a field would exert.
What do field lines give you at every point along a line?
A field drawing around the Moon marks point P on one line. Which question does the arrow at P answer?
A student maps a field by holding a small mass at many points and marking the force direction at each one. Joined up, what do the marks form?