Transcript 333 UNIT 2 - mrdsample
333 PHYSICS - UNIT 2 FORCES Explanations as to why things occur versus just observation. Copernicus & the moving Earth What idea did Copernicus propose that revolutionize science? What view was held prior to Copernicus proposal? In the 1500’s, Galileo reasoned… In the year 1642, Galileo died and Issac Newton was born. He was a genius who essentially wrote the book of physics in 18months. He furthered Galileo’s idea and generated Newton’s Laws of Motion. NEWTON’S LAWS OF MOTION Law of INERTIA OR 1ST LAW: INERTIA Mass • Mass is • Mass is measured Mass is not weight. You can change your weight without changing your mass. How? LOCATION MASS WEIGHT EARTH 50kg 500 Nwtns MOON ? 83 Nwtns JUPITER ? 1250 Nwtns Question: A top view of a circular pie tin with a quarter section cut out. Which path would a marble follow after exiting the circular track? Explain the purpose of seatbelts using the concept of inertia. An argument against a moving Earth (Copernicus) was the following: Consider a bird sitting at rest at the top of a tall tree. If the earth is really moving around the sun at 30km/s and also rotating at 1000mph, then how does a bird dive down to catch a worm? Why doesn’t classroom wall hit us if we jump up in air? Why do all objects fall at the same rate? (g=-10m/s2) Aristotle felt heavy things fell faster than lighter ones. However, without air resistance, a light object falls with the same acceleration as a heavy object… Explain using weight and inertia. 3rd LAW or Action/Reaction Walking or Jumping Swimming Use 3rd law to describe the motion of: A Rocket taking off A helicopter hovering over the ground A school bus collides with a bug and kills it. QUESTION: Which body receives the greater force of impact, bug or bus? WHY? Who pulls harder on the rope? Who wins the tug of war? NET FORCE (FNET ) Net = Net Force = NET FORCE Newton’s 2nd Law Fnet ma If net force then acc If net force then acc If mass then acc If mass then acc Fnet ma When FNET = 0, acceleration is 0….,why? When FNET = 0, velocity is constant…,why? Net Force causes Acceleration Mass Resists Acceleration TYPES OF FORCES Normal force = N Friction force = f Tension force = T Weight force = W 1 kg = 2.2lbs Force diagrams FBD’s with 2nd law (constant acc) Example 1: A 7.5kg box sits on a table. It is then pulled from rest to the right by rope with a constant force of 50N. There is a resistive force of 13N on the box. a)Draw a force diagram on the box. b) Write the 2nd law force equation with specific forces. c) Determine the acceleration of the box. d)Determine the speed reached by the box after 3.5sec of being pulled. Example 2: An empty elevator of mass 2700 kg is pulled upward by a cable. The elevator accelerates up at 1.3m/s2. a) Draw a force diagram for the elevator. b) Write the 2nd law force equation with specific forces. c) Determine weight of elevator. d) Determine the tension in the cable. e) Determine the tension in the cable if it was moving with a constant velocity. Draw the forces acting on the skydiver. Fnet ma When a skydiver reaches terminal speed, they are moving _________________ What is the value of the acceleration of the skydiver when they reach terminal speed? _________________ What is true about the forces acting on the skydiver when they reach terminal speed?___________ Which direction does skydiver move (relative to ground) when they open up the parachute? What does the parachute change about the skydiver? What happens to FORCE of air resistance due to the parachute? What happens to the skydiver’s weight? What is the direction of the NET FORCE on the skydiver right after opening parachute? What happens to speed of skydiver after parachute has been open for awhile? Forces on skydiver? FRICTION FORCE Two (2) types of friction: 1)Static: 2)Sliding: Static Friction is present when man pushes on box, until box moves. Also present on his feet. Sliding Friction between player and dirt STATIC FRICTION BEHAVIOR Normal force Friction force Coefficient of friction Friction Example: The coefficient of sliding friction is 0.40 for a box being pulled to the right across a table. The mass of the box is 2kg. a) Draw a force diagram for the box. b) Determine the weight of the box. c) If the normal force equals the weight force on the box, determine the sliding friction between the box and table surface. d) If the box is being pulled by a force of 12N, determine the net force on the box. e) Determine the acceleration of the box. Equilibrium • EQUILIBRIUM IS A CONDITION WHERE FORCES CANCEL OUT IN A PARTICULAR DIRECTION. • FNET = 0 • BALANCED…ZERO ACCELERATION Assuming no air resistance, EXPLAIN the following picture.