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Transcript
Physics 1 Dynamics Newton’s second law of motion N II If you push a vehicle with a force, the velocity of the vehicle is increasing: the vehicle accelerates. If you double the net force, the acceleration will be double. If you double the mass of the vehicle, the acceleration will be half of the first one if You use the same force. The basic law of dynamics (Newton II). (Dynamiikan peruslaki) The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass: a=F/m where m=the mass of the body F = the net force= sum of the forces acting the body The force F can be counted from equation F=ma Units of force: [F]=[m]/[a] = Physics 1 Examples 1. Net force Count the net force affecting to the object. In what kind of state of motion is the object. F1 F3 F1= 8N, F2=16N, F3=10N F2 Physics 1 Newton’s second law of motion (cont) Newton’s second law is giving a more precise definition to force as an action capable of accelerating an object. Force is needed to chance the direction of motion and to change the velocity of the body. Force is also needed to chance the shape of a body. For instance friction is slowing down the velocity. Friction is a braking force to the motion. Example1 Estimate the force capable of giving to a 1000kg car an acceleration of ½g. Example2 A car (m=1000kg) is accelerating from rest to the speed of 100km/h in 10,5 s. Count the average force that’s making the acceleration. Physics 1 Weight Weight is the force of the gravity acting on a body. The basic law of dynamics states that weight G of a body is counted as G=mg (compare with F=ma) where m = the mass of the body g= 9,81 m/s2 Ex2. If your mass is 57 kg so what is your weight a) on Earth b) on Moon? Mass m Mass is the quantity of matter. More precisely mass is a measure of the inertia of a body. The more mass a body has, the harder is to change its state of motion, for instance to accelerate or to slow down or to turn. A truck has more inertia than a bicycle. It therefore has much more mass. Notice Be careful when speaking of mass and weight. Mass is the quantity of matter. Weight is force of gravity acting on a body . Physics 1 Newton’s First Law of Motion N I =the law of inertia (in Finnish “hitauden laki” “jatkavuuden laki”). If the net force on an object is zero, an object originally at rest remains at rest, and an object in motion remains in motion in a straight line with constant velocity. Net force= sum of all forces acting on the object Examples Pulling a papersheet, A puck on ice, a steel ball on the floor, a passenger in a car, a car turning in a curve Physics 1 Forces resisting motion Friction and resistance on medium Static friction Sliding friction Rolling resistance Resistance of medium: liquids, air prevents moving resists sliding Physics 1 Forces resisting motion Vortex Rolling resistance Lift Air resistance Thrust Force due to the gravity Physics 1 Forces resisting motion Bearing Flat tyre