Earth pulls the Moon across 384,000 km of empty space. A magnet tugs a paper clip through a tabletop. Charged balloons push apart without touching. How can a force act across a gap? Physicists describe it with a field: the space around a mass, a charge or a magnet is changed, and any other object placed there feels a force. You met the first one already: gravity.
Key idea
A field is the region around an object where another object feels a force. Gravitational fields act on masses, electric fields on charges, magnetic fields on magnets and moving charges. Field lines show the direction of the force, and they are crowded where the field is strong.
Why it works
The field lets us split one question into two: "what does this object do to the space around it?" and "what force does that space put on whatever is placed there?" Near Earth's surface the gravitational field is 9.8 N/kg: each kilogram placed there feels 9.8 N. That is the same $g$ as in weight: $W = m\,g$ is just "mass times field."
Worked example: Field strength
On Mars the gravitational field is about 3.7 N/kg. What force does it put on a 20 kg rover?
- Force = mass × field: $20\,\mathrm{kg} \times 3.7\,\mathrm{N/kg} = 74\,\mathrm{N}$.
Answer: 74 N
Common mistake
Thinking field lines are real strings, or the paths objects must follow. They are a map: at each point, a line shows which way the force would push, and how close the lines are shows how strong it is. Objects don't slide along them; a ball tossed near Earth curves even though Earth's field lines point straight down.