Forces Introduction Presentation PDF

Unit 2: Physics Part 2: Forces – An Introduction What is “Force” šA force is “a push or a pull on an object” šA force has a magnitude (size) and direction šThere are many types of forces that can apply in combinations šThere can be many different forces acting at once on an object šThe symbo...

Unit 2: Physics Part 2: Forces – An Introduction What is “Force” šA force is “a push or a pull on an object” šA force has a magnitude (size) and direction šThere are many types of forces that can apply in combinations šThere can be many different forces acting at once on an object šThe symbol for force is a capital ”F” šThe unit for force is the Newton (N) What are some different forces acting on you right now?  Some Common Forces Gravitational Force (FG) Friction (FF) Acts on everything that has mass. Resistive force the acts against the direction of Typically, this force is between motion or “desired” motion. the Earth and another object. Amount of friction depends on the two surfaces. Points towards the center of Earth Normal Force (FN) A perpendicular force that acts Tension (FT) in response to a contact force. Force from a rope/cable/etc. This force will typically be a surface pushing up on an object They can only pull – not push.  Other Forces Bouyancy (FB) Water/Air Resistance (FDrag) When an object is submerges in Opposes the motion of an object. a fluid there is a force upwards. Depends upon the viscosity Equal to the weight of the fluid displaced. (thickness) of the fluid and the area of the moving object. Spring Force (FSp) Applied Force (FApplied) When a spring is compressed or Any external force which isn’t stretched this force opposes the specifically described. change in length.  Its ok to ignore air resistance unless it Free Body Diagram is mentioned or is a significant force (for example, a skydiver…). šWe use a tool called a Free Body Diagram to illustrate the forces acting on an object. šEach force is drawn as an arrow coming FROM the object šThe length of the arrow corresponds to the strength of the force.  Newton’s Three Laws Isaac Newton (1643-1727) Philosophiæ Naturalis Principia Mathematica First published 5 July 1687 (digitised copy at Sydney Uni – https://digital.library.sydney.edu.au/nodes/view/7454) Newton’s Three Laws Newton’s Three Laws Latin: š Lex. 1. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare. š Lex. 2. Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur. š Lex. 3. Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi. Newton’s Three Laws Translating… Lex. 1. Corpus š Law omne Every body perseverare perseveres in itsinstate statuof suo quiescendi rest, vel or of uniform movendi motion in uniformiter a right line,inunless directum, nisi quatenus it is compelled a viribusthat to change impressis state cogiturimpressed by forces statum illum mutare. thereon. Lex. 2. Mutationem š Law The alteration motus proportionalem of motion esse vi motrici is ever proportional to the impressae, motive etimpressed; force fieri secundum and lineam is maderectam qua vis illa in the direction of the imprimitur. right line in which that force is impressed. Lex. 3. Actioni š Law To everycontrariam semper action there et æqualem is always opposed esse an equal reactionem: reaction; sivemutual or the corporum duorum actions actiones of two bodies in se mutuo upon each other semper are esseequal, always æquales and et in partes directed tocontrarias dirigi. contrary parts. Newton’s Three Laws English translation: š Law 1. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon. š Law 2. The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. š Law 3. To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. Newton’s Three Laws Modern English: š An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force. š The acceleration of an object is proportional to the force applied to and inversely proportional to the mass of the object. I.e. Σ𝐹 = 𝑚𝑎 š When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.  Newton’s First Law of Motion An object in motion will remain in Balanced motion unless acted upon by a net Forces unbalanced external force Unbalanced Forces  Newton’s First Law of Motion How does this slo-mo video demonstrate the First Law of Motion?  Balanced or Unbalanced? Describe the motion of this car with the forces shown in red Weight Thrust Force Drag Force (from engine) (both friction and air resistance) Normal Normal Force Force  Equilibrium An object is at equilibrium if all the Which objects are at equilibrium? forces acting on it are balanced. The sum of Car hitting the brakes all forces is zero. Airplane travelling at constant speed The object will have Person sitting at rest no acceleration. Its velocity will not Child sitting on a ferris wheel as it turns change.  Newton’s Second Law of Motion šThe net force on an object is equal to the mass of the object multiplied Mass by the object’s acceleration An object’s resistance to acceleration šA shorter way of expressing this is Measured in kilograms (kg) using the equation Don’t confuse with weight Weight = Force Σ𝐹 = 𝑚𝑎 Mass = Mass 𝚺 is the Greek letter S, named “sigma” and in mathematics means “sum of” Practice Problems šA truck with a mass of 1500 kg is accelerated at a rate of -5 m/s2. How much force did this require? W hat d o -7500N e s th e n egative sign indic a te ? Practice Problems šA batter hits a ball with 500 N and the ball is accelerated at 2500 m/s2. What is the mass of the ball? 0.20 kg Practice Problems šA 10-kg cart is pulled with a force of 30N. What is the acceleration of the cart? 3 m/s 2 šIf the cart started from rest, what is the cart’s velocity after 5 seconds? (remember motion concepts…) 15 m/s  Newton’s Third Law of Motion e r e the t e wh a nd s ta rl y šFor every action o sn’t e(force) re ula gthere is h is d e nt s i n g : T u d i r anesequal e fi n n itioand g , s o st y the pareaction opposite b t d ccurrin identif o N (force) s a re o edly mis force repeat šAny time an object A applies a force to object B – object B will apply an equal and opposite force to object A Better definition, but not as jingly to remember L Action / Reaction Forces š Any time you identify one object applying a force to another (action), you should also be able to identify the equal and opposite reaction Action Reaction Windshield applying a force to a bug Bug applying a force on the windshield Student pushes down on Earth Earth pushes up on student Bat hits ball Ball hits bat Rocket pushes gasses out the bottom Gasses push up on the rocket