Y3 Y4 ACSI IP Physics Chapter 3

CORE NOTES CHAPTER 3: FORCES By: Oscar Chiang Page 1 - Part I: Forces • Types of Forces Contact Forces Forces exists when the two objects are in contact. Weight Tension Normal reaction force Friction / Air resistance Vertically downwards from centre of gravity Against weight of object Perpendicular to surface Agaisnt motion of object / body Non-contact Forces Forces that exist but do not require object to be in contact. e.g. gravitational force, electric force, magnetic force. • Effects of Forces Forces can move a stationary object. Forces can change the direction in which an object is moving. Forces can change the speed of a moving object. Forces can stop a moving object. - Part II: Vector Diagrams When there can be more than one vector acting on an object with differing magnitudes and direction, vector diagrams can be used to find the resultant vector. (Note: Vector diagrams can be used for other vectors such as velocity) A vector quantity is represented by an arrow. The length of the arrow is proportional to the magnitude of the vector. The direction of the arrow indicates the direction of the vector. Condition Line Object at equilibrium (FR = 0) Draw “connecting” line with single head as there is no start and end point – lines keep going in circles. Moving object (FR ≠ 0) Draw “connecting” line with double head as there is a start point and end point. • Addition of parallel vectors Resultant vector is indicated by a double-headed arrow. • Addition of non-parallel vectors 1. Tip-to-tail method (recommended) Move the tail of an arrow to the tip of the other arrow. Method: A character needs a clear straightforward path to walk. Rearrange arrows to do so! << Character does not have a straightforward path to walk. Character has a straightforward path to walk. Checklist 1. Scale 2. Magnitude 3. Arrowheads 4. Angle

By: Oscar Chiang Page 2 2. Parallelogram method Drawing duplicate lines to form a parallelogram. - Part III: Newton’s First Law of Motion Newton’s First Law of Motion (also known as law of inertia) states that every body continues in its state of rest or uniform motion in a straight line unless an external force acts on it. When forces are balanced à 𝐹!=0𝑁 à 𝑎 = 0𝑚𝑠"# (remain at rest or remain in constant velocity) - Part IV: Newton’s Second Law of Motion Newton’s Second Law of Motion states that when a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force. The product of the force and the acceleration of the object gives the resultant force. When force is constant, acceleration is inversely proportional to mass. 𝒂∝ 𝟏𝒎 When mass is constant, acceleration is directly proportional to force. 𝒂∝ 𝒇 Note! 𝐹!=exerted force−drag force, where drag force is a force opposing the object. When there is a resultant (unbalanced) force, an object produces acceleration. 𝐹!≠0𝑁 à 𝑎 ≠0𝑚𝑠"# (start moving from rest or its velocity changes) - Part IV: Newton’s Third Law of Motion Newton’s Third Law of Motion states that if Body A exerts a force 𝑭𝑨𝑩 on Body B, then Body B will exert an equal and opposite force FBA on Body A. FAB and FBA will always occur as a pair. Each pair is made up of an action and reaction. • Condition for action-reaction pair 1. Equal in magnitude 2. Act in opposite directions 3. Act on mutually opposite bodies Examples (action-reaction pairs) Gravitational force that earth acts on object and gravitational force that object acts on earth. Friction that floor acts on object and friction that object acts of floor. Contact force that stool acts on floor and contact force that floor acts on stool. - Part V: Friction Friction is the contact force that opposes or tends to oppose motion between surfaces in contact. << 𝐹=𝑚∙𝑎 Formula Checklist 1. Scale 2. Magnitude 3. Arrowheads 4. Angle