Forces change the motion or shape of an object. Every force has a size (measured in newtons) and a direction, so force is a vector quantity.
Contact and non-contact forces
A contact force acts when two objects touch. Examples include friction, air resistance, tension, the normal contact force, and upthrust.
A non-contact force acts without physical contact. Examples include gravitational force, electrostatic force and magnetic force.
| Force | Contact or non-contact | Typical situation |
|---|---|---|
| Friction | Contact | Opposes sliding between two surfaces |
| Air resistance (drag) | Contact | Opposes motion through air |
| Tension | Contact | Pull through a stretched rope, string or spring |
| Normal contact force | Contact | Surface pushes back perpendicular to the contact |
| Upthrust | Contact | Fluid pushes an object upward |
| Weight (gravity) | Non-contact | Earth pulls object toward its centre |
| Electrostatic force | Non-contact | Charged objects attract or repel |
| Magnetic force | Non-contact | Magnets or current-carrying conductors interact |
Direction matters
Forces are vectors. Drawing an arrow in the correct direction and of the correct relative length is essential for free-body diagrams and resultant force calculations.
Worked example
A book rests on a table. Identify the forces acting on the book and state whether each is contact or non-contact.
Weight acts downward (non-contact, due to gravity). The normal contact force acts upward from the table surface (contact). The two forces are equal in size and opposite in direction, so the book is in equilibrium.
Common errors and how to correct them
Calling the normal contact force “reaction” without specifying what reacts to what. The term “reaction” is vague. State “normal contact force from the table on the book” so the direction and origin are clear.
Listing “gravity” as a contact force. Gravity acts without physical contact between the object and the Earth. It is a non-contact force.
How to apply this in an exam
When asked to identify forces, name each force, state its direction, and classify it as contact or non-contact. A labelled free-body diagram earns credit for both identification and direction.
Need help with this concept?
A 0625 specialist can work through the student's current question and help identify whether the difficulty is the concept, the calculation or the exam technique.