4.5.1.2 - Contact and non-contact forces

Forces as interactions

A force is a push or pull that acts on an object. There must always be another object involved, because the force is produced by an interaction between two objects.

When you describe a force, try to name both objects:

• "the hand pushes the box"
• "the table pushes the book upwards"
• "the Earth pulls the ball downwards"

This sentence pattern matters because it tells you which object the force acts on and which object produces the force. A force description such as "there is a push" is usually too vague for an exam answer.

Forces are measured in newtons, N, but this lesson does not require force calculations. The key skill is to identify the interaction correctly.

Contact and non-contact forces

All forces between objects are either contact forces or non-contact forces.

Friction is a contact force because it acts where surfaces touch. Air resistance is also a contact force: the air particles are touching and pushing on the moving object, even though the air is hard to see.

Other important contact force examples are:

• tension, such as a rope pulling a sledge
• normal contact force, such as a table pushing up on a book resting on it

Non-contact forces do not need the objects to touch. The required examples are:

• gravitational force, such as the Earth pulling an object downwards
• electrostatic force, such as a charged balloon attracting small pieces of paper
• magnetic force, such as one magnet attracting or repelling another magnet

Do not decide only from whether the object is moving. A stationary book on a desk still has a contact force from the desk. A magnet can pull on another magnet even when both magnets are stationary and separated by a gap.

Force as a vector

Force is a vector quantity. This means a force has both a magnitude and a direction.

The magnitude is the size of the force, measured in newtons, N. The direction tells you which way the force acts, for example upwards, downwards, left, right, towards the centre of the Earth, or away from another charge.

A force vector is represented by an arrow:

• the arrow points in the direction of the force
• a longer arrow represents a larger force if the same scale is being used
• the arrow should be drawn on, or from, the object that the force acts on

A written answer should include direction when direction matters. "A force of 10 N" is incomplete if the question asks for a vector description; "a force of 10 N to the left" gives both magnitude and direction.

Paired-object interactions

When two objects interact, each object experiences a force due to the other object. The useful exam habit is to write the two force descriptions as a pair:

1. force of object A on object B
2. force of object B on object A

The two descriptions swap the object names. Each force acts on a different object, so do not treat the pair as two forces cancelling on one object.