Identifying Forces
Every object in the universe is constantly being pushed or pulled by its surroundings. Before you can apply Newton's laws to predict how something moves, you must first make a complete inventory of every force acting on it. This process — identifying all forces — is the essential first step in force modelling. Miss a single force and your prediction of the object's acceleration will be wrong, no matter how careful your mathematics is afterwards. Think of it like balancing a budget: every penny in and out must be accounted for before the numbers make sense. A force is an interaction between two objects. This seemingly simple statement has a powerful implication: every force you list must have an identifiable source — another object that is doing the pushing or pulling. Gravity pulls you downward because the Earth is pulling on you. The floor pushes you upward because it is in contact with your feet. Air resistance opposes your motion because air molecules are colliding with you. If you cannot name the object causing the force, the force almost certainly does not exist and should not be included in your analysis. Forces in physics are divided into two broad families: contact forces, which require physical touching between objects, and non-contact (field) forces, which act across empty space. Mastering the distinction helps you build a systematic checklist so that nothing is overlooked when you are drawing a free-body diagram or setting up Newton's second law.
- Start with Gravity (Weight)
- Identify All Contact Forces
- Check for Other Field Forces
- Apply Newton's Third Law as a Sanity Check
- Exclude Fictitious Forces and Outputs
Start with Gravity (Weight)
Gravity acts on every object with mass, always pulling vertically downward toward the centre of the Earth (near Earth's surface). Its magnitude is W = mg, where m is the object's mass and g ≈ 9.81 m s⁻² is the gravitational field strength. Gravity is a non-contact force — the Earth does not need to touch the object to pull it. Even a book sitting on a shelf is being pulled down by gravity; the shelf simply provides an upward contact force to balance it. Always list weight first; it is universal and easy to overlook in your diagram.
Identify All Contact Forces
Scan every surface touching your object. Each surface in contact generates at least a normal (perpendicular) force. If there is also sliding or a tendency to slide, friction acts parallel to the surface, opposing relative motion. Additional contact forces include tension (a rope or string pulling the object), compression from a spring (related to Hooke's Law), and the applied force from a person's push or pull. A useful rule: run your finger along every boundary of the object and ask 'is something touching here?' If yes, add the appropriate contact force.
Check for Other Field Forces
Beyond gravity, two other non-contact forces appear regularly at this level: the magnetic force (if the object is a magnet or carries current in a magnetic field) and the electrostatic force (if the object is charged). In most everyday mechanics problems these are absent, but in electricity and magnetism topics they become central. Always ask: 'Is this object charged or magnetic?' before concluding your force inventory.
Apply Newton's Third Law as a Sanity Check
Newton's third law states that forces come in action–reaction pairs: if object A exerts a force on object B, then object B exerts an equal and opposite force on object A. This is useful for checking your list. For every force you have identified, you should be able to describe its reaction counterpart acting on a different object. However, be careful: the reaction force acts on the other object, not on your object, so it does NOT appear in your free-body diagram. Confusing action–reaction pairs is one of the most common errors in force analysis.
Exclude Fictitious Forces and Outputs
Students sometimes include 'the force of motion' or 'the force because it's moving fast'. These do not exist as real forces — velocity is not a force. Similarly, acceleration is the result of the net force, not a force itself. Remove anything from your list that cannot be traced to a specific physical source object. Once you have your final list, each force becomes an arrow in your free-body diagram, drawn from (or to) the object, with length proportional to magnitude and direction matching the physical direction of the push or pull.
The Key Equation
- W — Weight (Newton (N)): The gravitational force acting on the object, directed vertically downward toward Earth's centre.
- m — Mass (kilogram (kg)): The amount of matter in the object; a scalar quantity that does not change with location.
- g — Gravitational field strength (m s⁻²): The acceleration due to gravity near Earth's surface, approximately 9.81 m s⁻². It varies slightly with altitude and latitude.
Worked Example
Problem. A 5.0 kg wooden crate rests on a rough horizontal floor. A person pushes it horizontally with a force of 30 N to the right, but the crate does not move. List all forces acting on the crate and state their directions.
Answer. Four forces act on the crate: weight 49 N downward, normal force 49 N upward, applied force 30 N to the right, and static friction 30 N to the left. The net force is zero, consistent with the crate being stationary.
Origins · 1 minIsaac Newton and how this was discovered
Newton formulated the three laws of motion published in his landmark work Principia Mathematica (1687), providing the mathematical framework for understanding every force acting on an object and how those forces produce acceleration. His systematic treatment of forces — including gravity as a universal attractive force — laid the foundation for all classical force modelling and free-body analysis taught in physics today. Newton reportedly held the post of Warden, and later Master, of the Royal Mint and took the job extremely seriously — personally pursuing counterfeiters and helping to send several to the gallows, treating fraud with the same rigour he applied to natural philosophy.
Related Concepts
- Free-Body Diagrams — Identifying all forces is the prerequisite step before drawing a free-body diagram; each identified force becomes an arrow in the diagram.
- Newton's Second Law — Once all forces are listed, Newton's second law (F_net = ma) uses that force inventory to calculate the object's acceleration.
- Newton's Third Law — Understanding action–reaction pairs helps verify that every identified force has a genuine physical source and prevents double-counting.
- Friction Forces — Friction is one of the most commonly overlooked contact forces; dedicated study of it reinforces systematic force identification skills.