Fields and Field Strength

Overview

This lesson introduces gravitational fields as a way to describe force at a distance. You will define gravitational field strength, use field lines to represent gravitational fields, and connect the direction and spacing of field lines to the force on a test mass.

What You Need to Know

• Use gravitational fields to model forces acting at a distance around a mass.
• Use gravitational field strength to compare the force per unit mass at different points.
• The field direction is the direction of the force on a positive test mass equivalent for gravity: toward the attracting mass.
• Field lines show the direction of the gravitational field.
• Closer field lines represent a stronger field.
• Near Earth’s surface, the field can often be treated as uniform over small height changes.

How to Work Through It

1. Start by comparing contact forces with gravitational force at a distance.
2. Define field strength and practise using g = F / m in simple examples.
3. Sketch field-line diagrams for uniform fields and radial fields.
4. Interpret diagrams by describing direction and relative strength.

Check Your Understanding

• What does gravitational field strength measure?
• Why do gravitational field lines point toward a mass?
• How can you tell from a field-line diagram where the field is stronger?
• What is the difference between gravitational force and gravitational field strength?

Common Mistakes

• Treating field strength as a force rather than force per unit mass.
• Drawing field lines away from a mass instead of toward it.
• Saying field lines are physical objects rather than a representation of the field.
• Ignoring the test mass when explaining the meaning of field strength.

Next Steps

• Keep the definition of field strength secure before using Newton’s law of gravitation.
• Be ready to connect force between masses to field strength around one mass.