力：定義と種類（Oレベル物理）

Force: Definition and Types (O Level Physics) Skip to main content lesson o-level students core What Is A Force? Show/Hide Sub-topics (Forces & Turning Effect Of Forces | O Level Physics) What Is A Force? Inertia Gravitational Field Strength & Weight Free Body Diagrams (FBD) Balanced Force - Newton's First & Third Law Of Motion Unbalanced Force - Newton's Second Law Of Motion Friction Terminal Velocity (Air Resistance) Moment Of A Force Equilibrium, Rotational Equilibrium & Translational Equilibrium Centre Of Gravity Stability & States Of Equilibrium How To Add Forces? Three Forces in Equilibrium (Graphical Method) 1. Definition A. Force A force is a push or pull that can change an object’s motion (speed or direction) or change its shape . A force is an interaction that can change motion (speed or direction) or deform an object. Unit and type SI unit: newton (N) Force is a vector (it has magnitude and direction). 2. Key Ideas A force is drawn as an arrow : longer arrow = larger force; arrow direction = force direction. Forces can be classified as: contact forces (need contact): normal reaction, friction, tension, air resistance non-contact forces (act at a distance): gravitational, electrostatic, magnetic The resultant force is the vector sum of all forces on the object. If the resultant force is zero , forces are balanced : object is at rest, or object moves with constant velocity (straight line, constant speed) If the resultant force is not zero , forces are unbalanced and the object accelerates. 3. Detailed Explanations What Can A Force Do? A force can: make a stationary body move change the speed of a body change the direction of motion of a body change the size or shape of the body A. Contact vs non-contact forces (6091) This syllabus skill is to identify forces and classify them: Contact forces (need contact): normal reaction, friction, tension, air resistance Non-contact forces (act at a distance): gravitational, electrostatic, magnetic B. Common forces in O Level questions Here are the forces you see most often and how to draw them: Weight , (non-contact): gravitational force on the object, acts downwards (see Gravitational Field Strength & Weight ). Normal reaction , (sometimes written for reaction) (contact): acts perpendicular to the surface (e.g. table pushes up on a book). Friction , (contact): acts parallel to the surface, opposes motion (or attempted motion) (see Friction ). Tension , (contact): a pulling force in a string/rope, acts along the string, away from the object. Air resistance / drag (contact): acts opposite to the direction of motion through air. C. Resultant force (balanced vs unbalanced) Forces add as vectors. The resultant force is the single force that has the same effect as all the forces together. If resultant force , forces are balanced → no acceleration. If resultant force , forces are unbalanced → the object accelerates in the direction of the resultant force. You’ll use these ideas in: Free Body Diagrams (FBD) How To Add Forces Balanced Force (Newton’s First Law) Unbalanced Force (Newton’s Second Law) A Level extensions (optional) For deeper theory (not needed for 6091): Fundamental forces: Fundamental Forces Drag models ( and ): Drag Force 4. Common Mistakes “Object at rest means no forces.” False. It can have forces that balance (resultant ). Treating force as a scalar (forgetting direction). Calling any contact force “contact force” (be specific: normal reaction, friction, tension, etc.). Drawing friction in the wrong direction (it always opposes relative motion or attempted motion). Mixing up action–reaction pairs with “balanced forces” (action–reaction forces act on different objects). 5. Exam Tips When asked to “state the forces”, name them clearly (e.g. weight , normal reaction , friction , tension , air resistance ). For diagram questions, draw forces as arrows starting from the object and label each force. If the question says “constant speed” or “at rest”, write: resultant force is zero . Always include units when giving a force value: N . 6. Worked Examples Example 1 Book on a table (identify + classify) A book is at rest on a horizontal table. State the forces acting on the book and classify each as contact or non-contact. Show Answer Forces on the book: Weight down (non-contact: gravitational) Normal reaction up (contact: table on book) Because the book is at rest, forces are balanced: . Example 2 Pulling a box at constant speed (direction of friction) A box is pulled to the right across a rough floor at constant speed. State the direction of the friction force on the box. Show Answer Friction acts opposite to the direction of motion (or attempted motion), so friction on the box is to the left . Example 3 Hanging mass (tension vs weight) A mass hangs at rest from a light string. State the forces on the mass and the relationship between them. Show Answer Forces on the mass: Weight down Tension up At rest, forces are balanced, so: . Example 4 Pushing a box that doesn’t move (balanced contact forces) A box is pushed to the right on a rough floor, but it does not move. State the horizontal forces on the box and their relationship. Show Answer Horizontal forces on the box: the push to the right static friction to the left (contact force from the floor) Because the box is at rest, the forces are balanced, so the friction equals the push (in magnitude). Example 5 Highest point of a throw (rest does not mean “no forces”) A ball is thrown vertically upwards. At the highest point, its speed is zero (ignore air resistance). State the force(s) acting on the ball at that instant, and the direction of its acceleration. Show Answer The only force is the weight acting downward (non-contact gravitational force). So the acceleration is also downward. 7. Mind Stretchers A. “Balanced forces” vs “action–reaction” A book rests on a table. Many students say: “weight and normal reaction are an action–reaction pair.” Is that correct? Explain briefly. Show Answer Not correct. Weight and normal reaction act on the same object (the book), so they are a pair of forces that can be balanced. An action–reaction pair acts on different objects . The reaction to the book’s weight is the gravitational pull of the book on the Earth. The reaction to the normal reaction is the force of the book on the table. See: Balanced Force – Newton’s Third Law . B. Does “constant velocity” mean “no forces”? A car moves at constant velocity along a straight, level road. Does this mean there are no forces on the car? Explain. Show Answer No. It means the resultant force is zero . For example, the driving force can balance air resistance and friction, and the normal reaction can balance the weight. 8. Practice Practice Time! Ready to test your understanding? Go to Dynamics Quiz Next lesson: Free Body Diagrams (FBD) Topic hub: Forces & Turning Effect Of Forces Next Step: Practice This Topic Forces & Turning Effect Of Forces: take one targeted next step, then escalate difficulty only after the first route stabilizes. Best next step: Dynamics Quiz Fast concept check before deeper follow-up. Harder follow-up: Paper 2 Structured Practice — Forces Longer-response follow-up with mark-scheme scoring. Repair with simulation: Forces & Motion Explorer Repair step: visualize the concept, then retake a timed route. Forces Hub Quick recap of the full topic before timed practice. Moments & Stability Lab Repair step: visualize the concept, then retake a timed route. Categories O Level Forces & Turning Effect Of Forces Tags O Level Physics Forces