Forces Introduction Presentation PDF

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

Unit 2: Physics Part 2: Forces – An Introduction What is “Force”  A force is “a push or a pull on an object”  A force has a magnitude (size) and direction  There are many types of forces that can apply in combinations  There can be many different forces acting at once on an object  The symbol for force is a capital ”F”  The 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 motion or “desired” motion. Typically, this force is between 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 a Opposes the motion of an fluid there is a force upwards. object. Equal to the weight of the fluid Depends upon the viscosity displaced. (thickness) of the fluid and the area of the moving object. Spring Force (FSp) Applied Force (FApplied) When a spring is compressed Any external force which isn’t or stretched this force opposes specifically described. the change in length.  Its ok to ignore air resistance unless Free Body Diagram it is mentioned or is a significant force (for example, a skydiver…).  We use a tool called a Free Body Diagram to illustrate the forces acting on an object.  Each force is drawn as an arrow coming FROM the object  The 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 – Newton’s Three Laws Newton’s Three Laws Latin:  Lex. 1. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.  Lex. 2. Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.  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 English translation:  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.  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.  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:  An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.  The acceleration of an object is proportional to the force applied to and inversely proportional to the mass of the object. I.e.  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 Forces motion unless acted upon by a net 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 (both engine) friction and air resistance) Normal Normal Force Force  Equilibrium An object is at equilibrium if all the Which objects are at forces acting on it equilibrium? are balanced. The Car hitting the brakes sum of all forces is Airplane travelling at constant speed zero. Person sitting at rest The object will have no acceleration. Its Child sitting on a ferris wheel as it velocity will not turns  Newton’s Second Law of Motion  The net force on an object is equal to the mass of the object Mass multiplied by the object’s An object’s resistance to acceleration acceleration Measured in kilograms (kg) Don’t confuse with weight  A shorter way of expressing this is Weight = Force using the equation Mass = Mass is the Greek letter S, named “sigma” and in mathematics means “sum of” Practice Problems A truck with a mass of 1500 kg is accelerated at a rate of -5 m/s2. How much force did this require? What do -7500N es the n egative sign ind icate? Practice Problems A 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 A 10-kg cart is pulled with a force of 30N. What is the acceleration of the cart? 3 m/s 2 If 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 r e the e w he n d ta t y a n ’t s u l ar l  For every action h is e s do (force)e n t g s re therei n g : T st u d a ir it ion o e p is sant de equal fi n u r r inand g , s e opposite n tify th b e occ y misid No reaction e(force) sa r e dl f o r c e e a t re p  Any 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 Action / Reaction Forces  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