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Transcript

Notes: Mechanics The Nature of Force, Motion & Energy I. Force – A push or pull. a) A force is needed to change an object’s state of motion. b) Net force- The sum (addition) of all the forces acting on an object. • Net Force not equal to zero = Unbalanced Force. • Unbalanced Force = acceleration! c) Measured in Newtons (metric) and pounds (English) d) Examples of types of forces. weight drag friction buoyant force thrust lift centripetal support applied normal compression tension many more… II. Mechanical Equilibrium – An object in mechanical equilibrium is stable (no changes in motion). a) Static Equilibrium – object at rest. b) Dynamic Equilibrium – object moving at a constant speed in a straight line. c) An object in equilibrium, net force = zero. S (sigma) means “sum of” III. Vector: An arrow used to illustrate the magnitude (size) and direction of some quantity. a) Force, momentum and velocity are vector quantities. b) Parallelogram Rule - “Tip to Tail” Method Vectors Combining Vectors = Resultant Vector No Change in Motion = 0 Net Forces = Equilibiurm That means if an object…… Mythbusters: •Is at rest, it will stay at rest. Killer Tissue Box Or •If an object is in motion, it will stay in motion at CONSTANT VELOCITY (NO Acceleration). IV. Newton’s First Law of Motion (Law of Inertia): Every object remains at rest or remains in motion at a constant velocity (in a straight line at a constant speed ), unless external unbalanced forces cause the object to change its motion. a) Examples. Golf balls /cups Demo, table cloth trick, seatbelts, Mythbusters – Tissue Box. b) 1st Law ***Main Ideas: 1) Inertia- the resistance for an object to changes in motion. Depends on mass. 2) An object won’t move, stop moving or turn unless something (Net Force > 0, Unbalanced Forces) makes it move, stop moving or turn. V. Mass – The measure of the amount of inertia an object has. a) The more mass an object has, the greater its inertia and the more force is needed to change its state of motion. b) Mass is not Weight!!! • Weight is the force of gravity an object. on c) One kilogram of mass weighs 10 Newtons. VI. Frame of reference: An object or objects assumed to be stationary, used to compare the motion of other objects. Most common – the ground (Earth). Motion is Relative! a) An object is moving if its position relative to a fixed point is changing. b) Examples: Spinning Earth, Solar System, Universe, Virtual reality rides . Motion Formulas 1) 2) 3) Speed = distance/time S = d/t Velocity = distance/time V = d/t Acceleration = change in velocity/time a = (v final – v initial)/ t VII. Speed - distance traveled in an interval of time. a) Example units: meters/second (m/s) kilometers/hour (km/h) miles/hour (mph), etc. b) Average speed – The total distance covered divided by the total time. Speed = distance/time S = d/t c) Instantaneous Speed – The speed at any instant in time. VIII. Velocity - the speed of an object and the direction it is traveling. Velocity is a vector quantity. a) Example units: meters/second east (m/s east) km/hour north b) Calculated by: Velocity = distance/time V = d/t IX. Acceleration - A measure of how fast velocity changes. a) Three ways for velocity to change: speed up slow down turn (change direction) b) Accelerations are caused by unbalanced forces. c) Example unit: meters/second/second (m/s2) d) Calculated by: acceleration = change in velocity/time a = (v final – v initial)/ t Examples of Acceleration X. Falling Objects - The ‘acceleration due to gravity’ (“g”) on earth is 9.8 m/s2. a) All objects fall at this rate when air resistance (friction - drag) is ignored. XI. Projectile Motion – Any object that travels though air or space, acted on only by gravity (and friction). a) Examples- thrown ball, orbiting spacecraft, jumping, shot bullet. b) Projectiles have both a vertical component and horizontal component that are independent of one another. XII. Newton’s Second Law of Motion - The acceleration caused by a force on an object is dependent upon the size of the force and the mass of the object. a) acceleration = Force/mass • a = F/m b) Force = mass x acceleration • F=ma c) 2nd Law ***Main Ideas: • Forces cause accelerations!!! a F = m Examples: Empty shopping cart Regular dart a = F m Full shopping cart Weighted dart Baird XIII. Momentum - Inertia in motion! Refers mainly to how hard something is to stop. a) Ex. Running back b) Calculated by: momentum (p) = mass x velocity XV.Impulse-Momentum – relationship between force, time and momentum. The size of a force and the amount of time that force acts on an object effects the momentum of the object. a) Ex. Athletic movements, airbags b) Calculated by: Force x Time = Mass x velocity Ft = mv c) Law of Conservation of Momentum – Momentum cannot be created or destroyed, it can only be transferred from one object to another. XIV. Newton’s Third Law of Motion a) “Action-Reaction” b) Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. When identifying equal and opposite forces it is important to remember that forces work in pairs. • IMPACT - DEFINE