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Newton’s Laws. Willow Wood JHS IPC. Facts about FORCE. Unit is the NEWTON(N) Is by definition a push or a pull Can exist during physical contact(Tension, Friction, Applied Force) Can exist with NO physical contact. Newton’s First Law – The Law of Inertia.
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Newton’s Laws Willow Wood JHS IPC
Facts about FORCE • Unit is the NEWTON(N) • Is by definition a push or a pull • Can exist during physical contact(Tension, Friction, Applied Force) • Can exist with NO physical contact.
Newton’s First Law – The Law of Inertia NOTE:MASS and WEIGHT are NOT the same thing. MASS never changes When an object moves to a different planet. What is the weight of an 85.3-kg person on earth? On Mars=3.2 m/s/s)? INERTIA – a quantity of matter, also called Mass. Italian for “LAZY”. Unit for MASS = Kilogram. Weight or Force due to Gravity is how your Mass is effected by gravity.
Newton’s First Law There are TWO conditions here and one constraint. Condition #1– The object CAN move but must be at a CONSTANT SPEED Condition #2– The object is at REST Constraint #3– As long as the forces are BALANCED!!!!! And if all the forces are balanced the SUM of all the forces is ZERO. The bottom line: There is NO ACCELERATION in this case AND the object must be at EQUILIBRIUM ( All the forces cancel out). An object in motion remains in motion in a straight line and at a constant speed ORan object at rest remains at rest, UNLESS acted upon by an EXTERNAL (unbalanced) Force.
Newton’s First Law If the NET FORCE on an object is zero, then the object will have zero acceleration ------that does not mean the object is at rest but means its velocity is constant. Constant zero velocity = at rest. Or constant non-zero velocity = moving at constant velocity. The acceleration is in the same direction as the net force!
Free Body Diagrams FN • Weight(mg)– Always drawn from the center, straight down • Force Normal(FN) – A surface force always drawn perpendicular to a surface. • Tension(FT) – force in ropes and always drawn AWAY from object. • Friction(FF)- Always drawn opposing the motion. T Ff T W1,Fg1 A pictorial representation of forces complete with labels.
Free Body Diagrams Fg Ff FN
N.F.L and Equilibrium Since the Fnet = 0, a system moving at a constant speedor at rest MUST be at EQUILIBRIUM. TIPS for solving problems Draw a FBD Resolve anything into COMPONENTS Write equations of equilibrium Solve for unknowns
Example FN Fa Ff Fg A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50N. Calculate the Force of Friction Calculate the Force Normal
What if it is NOT at Equilibrium? If an object is NOT at rest or moving at a constant speed, that means the FORCES are UNBALANCED. One force(s) in a certain direction over power the others. THE OBJECT WILL THEN ACCELERATE!!!
Newton’s Second Law • Tips: • Draw an FBD • Always write larger force – smaller force. • Solve for any unknowns The acceleration of an object is directly proportional to the NET FORCE and inversely proportional to the mass.
Newton’s Second Law In which direction, is this object accelerating? The X direction! So N.S.L. is worked out using the forces in the “x” direction only FN Fa Ff mg A 10-kg box is being pulled across the table to the right by a rope with an applied force of 50N. Calculate the acceleration of the box if a 12 N frictional force acts upon it.
Newton’s Third Law In every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object.
Newton’s Third Law While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?
Newton’s Third Law Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction
