Dynamics (Forces)

DYNAMICS (FORCES) Types of Forces Balanced Forces Forces can be classified into two main types: If the forces acting on the object are balanced, the resultant force is zero. Definition Contact Forces Non-contact forces Friction Gravitational force (Weight) A force is a push or pull that one exerts on another Normal reaction Electric force Newton’s First Law of Motion states that every object will body. It can produce, slow down, speed up, stop or Tension Magnetic force continue in its state of rest or uniform motion in a straight change direction of motion. line unless a resultant force acts on it. Unbalanced Forces Action-Reaction Forces Vector Addition Newton’s Third Law of Motion states that for every Newton’s Second Law of Motion states that the resultant action force acting on a body, there is an equal and For parallel vectors: Same direction (add), opposite direction force acting on an object is equal to the product of mass opposite reaction force acting on the opposite body. (Subtract) and the acceleration of the object; the force acts in the Non-Parallel vectors same direction as the acceleration of the object. Characteristics action-reaction forces: Method 1: Parallelogram method F : resultant force (N) 1) The forces are of the same nature. �! � = �� m : mass (kg) 1. Choose an appropriate scale. -2 2) They are equal in magnitude and opposite in a : acceleration (ms ) direction. 2. Complete parallelogram by 3) Action and reaction act on different bodies. �! If the object is moving at constant speed, the acceleration adding dotted lines, parallel to is zero. The resultant force is also zero. original forces. Friction 3. Resultant force is the diagonal Free-Body Diagrams (FBD) �! of the parallelogram. Friction is a contact force that opposes or tends to • Identify and isolate the body. oppose motion between surfaces in contact.

Method 2: Tip-to-tail method • Draw all the physical forces acting on the body clearly. Length of arrow represents the magnitude of the force. Effects of Friction Take note of the relative magnitude of forces. Positive effects Negative effects � 1. Choose an appropriate scale. ! • Ensure correct point of application of force. Helps us to walk without Reduces efficiency of slipping machines nd • Label all forces drawn. �! 2. Place 2 force to the tip of the Help us to grip things Causes wear and tear 1st force. �! Normal Reaction Moving vehicles can slow Causes unwanted down when needed. heating st 3. Join the tail of the 1 force to nd the tip of the 2 force to obtain Friction can be reduced by using the resultant force. Friction � When using the tip-to-tail method, if the ! 1. Wheels resultant force is zero, the vector diagram 2. Ball bearings � would form a closed triangle. ! Weight 3. Lubricants and polished surfaces 4. Air cushion (hovercrafts, maglev etc) The forces are said to be in equilibrium. �!

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