Describe, with examples, how objects can interact a at a distance without…

The idea

Non-contact interactions arise through fields. A gravitational field surrounds any mass, so the Earth pulls a falling apple and the Moon without touching either. An electrostatic field surrounds a charged object, so a charged balloon attracts small paper pieces across a gap. A magnetic field surrounds a magnet or current-carrying wire, so two magnets repel or attract before they touch. Contact interactions need surfaces to press together: the normal contact force acts perpendicular to a surface and supports a book on a table, while friction acts along a surface and opposes sliding, as when a box is dragged across a floor. Every interaction produces a pair of forces, one on each object, equal in size and opposite in direction, and each force can be drawn as an arrow whose length shows magnitude and whose direction shows the line of action.

What earns the marks

• Identifies a non-contact interaction and links it to the correct field, such as a falling object to the gravitational field.
• Identifies a contact interaction and names the normal contact force or friction acting between the surfaces.
• States that an interaction produces a pair of forces acting on the two objects, equal in size and opposite in direction.
• Describes a force as a vector with both magnitude and direction, represented by an arrow.
• Distinguishes a field interaction from a contact interaction by whether the objects touch.

Common mistakes and how to put them right

• Treats friction as a non-contact force.

  Put it right: Recognise that friction needs surfaces in contact and acts along them.

• Draws only one force for an interaction.

  Put it right: Show the paired force acting on the other object.

• Gives a force's size but not its direction.

  Put it right: Represent each force as a vector arrow.

• Links a magnetic attraction to the gravitational field.

  Put it right: Match each example to its own field.

Exam tips

• Name the field explicitly whenever you describe a non-contact example.
• For each contact example, state which surface pushes or resists and in which direction.
• When drawing forces, use arrow length for magnitude and arrow direction for line of action.