Transcript Chapter 1.1
Announcements: - Get lab manual and look at first experiment before your first lab. - Any problems downloading the ppt presentation from web? - Web page for class is: http://www.wfu.edu/~gutholdm/Physics110/phy110.htm - Please bring your i-clicker to class. PHY110 TUTOR SESSIONS (Hours to come soon) MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SUNDAY The tutor: Jillian Bjerke & Maggie Baldwin The tutor sessions in semesters past were very successful and received high marks from many students. All students are encouraged to take advantage of this opportunity. Tutor sessions will last one or two hours. Time/room to be announced. Private tutors are also available: see Judy Swicegood in department office (Olin 100) Chapter 1: The laws of motion, Part I First two chapters: Introduce the “language of physics” Subsequent chapters: Explore objects and underlying physical concepts - Reading assignment for today: Chapter 1.1 - Reading assignment for Monday: Chapter 1.2 - Homework (grader, Jill Bjerke): - (due Thursday, Jan. 21 before class): Exercises: 4, 5, 6, 8, 9, 13, 15 Problem: 1, 2 Chapter 1.1: Demos and Objects Pulling out table cloth Slicing bananas Skating Running around Pushing stuff (students) Concepts Inertia, velocity, acceleration Force, Newton’s 1. & 2. law i-clicker question 1 Your car stopped in front of a stop light, and somebody rear ends you, pushing your car forward a bit. Which part of the car will you head hit? A.The steering wheel B.The head rest. C.The car door D.Not enough information. What we learn from skating (we ignore friction!!): Skating • At rest on a level surface: – If you just wait, you stay stationary – If you’re pushed, you start moving that direction • Moving on a level surface: – If you just wait, you coast steadily in straight line – If you’re pushed, you change direction or speed Katarina Witt Physics Concept • Inertia – A body at rest tends to remain at rest – A body in motion tends to remain in motion Question: So why do cars slow down when they coast? Newton’s first law: An object that is not subject to any outside net force moves at a constant velocity along a straight-line path. Position and velocity are vector quantities - Vector quantities have a magnitude and a direction. - Scalar quantities have just a magnitude (example: temperature) Position (vector): an objects location (example: 5 m east of the origin) Velocity (vector): change in position with time (example: 5 miles/hour to the east) Speed (scalar): distance traveled per amount of time (example: 5 miles/hour (no direction)) distance speed time Force and acceleration are vector quantities A force is a push or a pull If a force (pull or push) is acting on an object of mass m, the velocity of the object is changing the object is accelerating Acceleration: Change in velocity Any change in velocity (slowing down, speeding up, changing direction) is an acceleration. Which direction is the acceleration? 1. A car picking up speed? 2. A race car slowing down? 3. An airplane that is beginning its decent? 4. An elevator that is starting upward from the first to fifth floor? 5. An elevator that is stopping at the fifth floor? 6. An elevator that is moving up at a constant speed? 7. A parked car? 8. A car going up a hill at constant speed? Acceleration points in the direction in which you’d have to pull to obtain the change in velocity Newton’s second law How much does an object of mass m accelerate in response to a given force F (push or pull)? F m a F a m Force and acceleration are vectors. The acceleration is in the same direction as the force Thus far we have learned about four vector quantities: - position (meters, m) - Velocity (meters/second, m/s) - Acceleration (meters/second2, m/s2) - Force (Newtons, N) And one scalar quantity: - Mass (kilograms, kg) Black board example, i-clicker-2 You give two skaters of mass 50 kg and 100 kg a push of 200 N. (a) Which one will accelerate more? (b) What is the acceleration of the heavier one? A.) 0.5 m/s2 B.) 1.0 m/s C.) 2.0 m/s2 D.) 4.0 m/s2 E.) 0 m/s2 Units In mechanics the three basic quantities are: • Length (we will use the unit meter; 1 m) • Mass (we will use the unit kilogram; 1 kg) • Time (we will use the unit second; 1 s) And combinations of these units (e.g. unit of velocity: m/s) • These are units of the SI (Système International) system that is used throughout the world in the Sciences. Conversion of different units: see Appendix B Problem solving: • Always make sure you use the right units. (conversion may be necessary) • Always do an order of magnitude estimation (Ask yourself: “Does the number I’m getting make sense?). Black board example You have bought this nice European car, which only indicates speed in km/h. You take it for a spin and go 140 km/h. Are you still within the speed limit of 65 mi/h?