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9TH CLASSES PHYSICS DAILY PLAN SUBJECT: Force and the laws of motion GOALS: DURATION: IN PRACTICE: If object A exerts a force F on object B, then object B exerts a force –F on object A. These two forces are equal in magnitude but opposite in direction. Ex 8: PRESENTATION: DYNAMICS Dynamics is about the relation between force and motion. Force: A force is a pull or push on an object. A force can cause a change in the shape or velocity of the object, start it to move or stop the motion. Net Force (or Resultant Force): is the vector sum of all forces acting on that object. Fnet=F1+F2+F3+… (vector sum) F F’ F = - F’ Unit of force: is Newton (N) or Dyne (dyn) Newton’s laws of motion: N First Law of Motion: a) An object at rest will remain at rest if there is zero resultant force acting on it; and; b) If the net force on a moving object is zero, the object continues to move with constant velocity (=on a straight path) v This object stops. When an object is at rest on a graund, the forces acting on the object are the normal force, N, and the force of gravity, W. Here ; W = N = mg W N N F F v pump air This object does not stop. The reason is: friction force stops the first object. But there is no net force to stop the second object. Therefore: Fnet 0 v const If an object is moving, no force is needed to keep it moving. The object will move forever without any force on it. This is because of inertia. W W N = W+ F N=W-F Second Law of Motion: Newton’s second law answers the question of what happens to an object that has a nonzero resultant force acting on it. Newton’s second law states that; if a net force acts on a body, the body will accelerate. The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. [ a m F m 2a 2F aF Third law of motion: Units: SI: 1 Newton = 1 kg m/s2 m F a/2 2m and F net ma 3F a/3 Inertia: Inertia is the tendency of an object at rest to remain at rest; and of an object in motion to remain in motion with its original velocity. Mass: Mass is measure of inertia. (unit : kg) Big mass big inertia a 3a m F 1 a m 3m F ] F a m (Fnet and a are in the same direction) Therefore 1 N is the force which gives a 1-kg mass an acceleration of 1 m/s2. Also we have the cgs unit: 1 dyne = 1 gr cm/s2. So: 1N=100000 dyn Forces of Friction When a body is in motion on a rough surface, or through a viscous medium such as air or water, there is resistance to motion because of the interaction of the body with its surroundings. We call such resistance a force of friction. Ex 10: A 2 kg object accelerates from rest to 5 m/s in 0.4 s. What is the net force on the object? Ex 11: A force of 20 N acts on a 4 kg object. What distance does the object travel in 3 s? (v0=0) Mass and Weight Mass is the amount of matter. Mass is scalar. (unit: kg) Weight is the gravitational force pulling the object down. Weight is vector. (Unit: Newton) Mass Static and kinetic friction: When there is friction, it is easier to keep an object moving than to start it to move. Ffrictio n starts to move maximum static friction Weight kinetic friction Fapplie d The Earth pulls each 1-kg object with a 9.8 N force. This is called gravitational constant (g) 2 kg 1 kg 3 kg Kinetic friction depends on two factors: 1) The force pushing the objects together (N) 2) Material of the object (k) k: coefficient of friction g=9.8 N/kg v g 10 N/kg 10 N Ff = k N 20 N 30 N Ff W=mg k N Ex 37: Inclined Plane Forces on the object: v0=0 F=25 N 5 y N k=0.8 [Sol: kN gives 40 N but friction can not be 40 N (or it will start to move back). Ff= 25 N.] mg Sin x Therefore: Formula Ff=kN gives us the friction if the object is moving. If object does not have any intention to move then Ff=Fext. a mg Cos mg Ex 38: What is the acceleration of the object in figure? v0=0 Ex 32: g m a) Find the accelerationn of the object. b) In which direction is it moving? 37° Find the tension in therope and the acceleration. Ex 35: What is the final velocity of the object after sliding for 5m on a smooth inclined plane, if it is released from rest? Lmax=? F=15 N 2 k=0.5 HOMEWORK: MULTIMEDIA: DEMONSTRATION: EXPERIMENT: v0 = 8 m/s TEACHER: 30° DIRECTOR: