Physics 161 Lecture 5 Greek: π ρ σ Newton s Laws

Transcription

1 Physics 161 Lecture 5 Greek: π ρ σ Newton s Laws September 18,

2 Announcement 1: exam From the course webpage: Exams are multiple choice and machine graded. There are about fifteen questions in the 80-minute exams. Exams are closed-book. You may bring: one 8.5 x 11" sheet of notes in your own handwriting No typed notes or photocopies. Bring a calculator, backup batteries and several pens and pencils to the exam. No communication devices, cell phones, backpacks, etc. are allowed. If you bring them, you have to leave them at the front of the room during the exam. Students who have any communication devices on them during the exam will receive a zero exam score. 2 Sep. 10, 2015

3 Announcement 1: exam What can be on the exam: Questions like WebAssign Homework Questions like in the Quizzes Questions like in the Syllabus reading sections in the textbook Questions like in the prelab questions Questions like in the lab sessions 3 Sep. 10, 2015

4 Announcement 2: tutors A list of physics graduate students willing to tutor undergraduates is available at Tutoring is for a fee (you negotiate the rate with the tutor). Our own instructors are not so bad either and they are for free. 4 Sep. 10, 2015

5 Lecture 5: learning objectives Review from lecture 4 - Projectile motion. In lecture 5 We will solve problems in two dimensions involving relative velocity. You will introduce Newton s three laws of motion and apply them to study the motion of objects in simple applications. What is the relationship between mass, inertia, and weight. Sep. 1,

6 Complementary angles 6 Sep. 10, 2015

7 Relative Velocity The motion of A may be seen from 2 different frames. (1) From the Earth frame (called E) r AE,v AE. The other (2) from a moving frame (observer in car B) r AB v AB. 7 9/15/2015

8 Problem 3.23 A jet liner moving initially at 300 mi/h due east enters a region where the wind is blowing 100 mi/h in a direction 30.0 north of east. ( J=jet, A=air, E=earth ). a)find the components of the initial velocity of the jet airliner, v JA. b)find the components of the velocity of the air relative to Earth, v AE. c)write down a relation between these vectors and the velocity vector of the jet relative to the Earth, v JE. d)what are the speed and direction of the aircraft relative to the ground? 8 8

9 Newtonian mechanics, Dynamics 1. HOW DOES IT MOVE? KINEMATICS: x, v, a 2. WHY DOES IT MOVE? DYNAMICS: Force F 1. Newton s connection : given the net force, to describe how the object moves in response to that net force. 9

10 Newton s first law Newton s first law: An object moves with constant velocity unless a nonzero net force acts on that object. What is Net Force : The vector sum of all forces acting on an object. Inertia of a body: Tendency of a body to continue in its original motion if forces are absent. A truck has more inertia than you. Mass of a body: Measures a body s resistance to changes in its motion due to a force. A truck has 10more mass than you.

11 Newton s second law Newton s second law: If there is a net force on an object, the object responds by accelerating in the direction of the net force. The acceleration is proportional to the net force and inversely proportional to the object s mass. 11

12 Iclicker question Newton s first law is a special case of Newton s second law. a. TRUE b. FALSE 12

13 Forces Forces are vectors. Newton s second law applies to the net or total force on an object and is a vector equation. The unit of force is the Newton (N), which is equivalent to [mass] x [acceleration] = [kg m/s^2] 13

14 Problem: 4.10 A bullet of mass 5 g leaves the muzzle of a rifle with a speed of 320 m/s. What force (assumed constant) is exerted on the bullet while it is traveling down the 0.82 m barrel of the rifle? 14

15 iclicker question If the net force acting on an object is along the positive x-direction, the object can only move in the positive x-direction. Is this statement true or false? a) True. b) False. 15 Sep. 3, 2015

16 Force of gravity Gravity is a force (more in lec. 10). Weight of an object: The gravitational force acting on an object. The magnitude of an object s weight is given by Weight and mass are not the same! (Even though in daily use, most people don t distinguish them.). 16

17 iclicker question: Newtonian mechanics Which of the following statements are true? a) An object cannot move unless a net force acts on it. b) If the acceleration of an object is zero, there are no forces acting on the object. c) If the velocity of an object is constant, the net force acting on that object is zero. d) If the speed of an object is constant, the net force acting on that object is zero. 17 Sep. 3, 2015

18 Newton s third law Newton s third law: If two objects A and B interact, the force exerted by object A on B is equal and opposite to the force exerted by B on A. 18

19 Normal force Normal force: Contact force exerted by a surface in a direction perpendicular to the surface. A computer standing on the table feels a normal force from the table. 19

20 What is a contact force A contact force is really an electrical force between the atoms of two objects. Newton didn t know about the electrical force, or even atoms. We can ignore the origin of the force and just call this a ``contact force. 20

21 Problem A 75 kg man standing on a scale in an elevator notes that as the elevator rises the scale reads 825 N. What is the acceleration of the elevator? Hint: Mass is 75 kg. Weight is 825 N. F = ma F is the result of gravity and normal force. a is the acceleration of the man and the elevator. 21

22 iclicker question: forces 6 N 3kg 2kg 1kg frictionless What are the accelerations of the 3 kg mass and the 2 kg mass? a)the acceleration of the 2 kg block is larger b)the acceleration of the 3 kg block is larger c) The accelerations are the same 22

23 iclicker question: forces 6 N 3kg 2kg 1kg frictionless What is the force exerted by the 3 kg mass on the 2 kg mass? a)6 N b)3 N c)2 N d)1 N e)0 N 23

24 iclicker question: forces 6 N 3kg 2kg 1kg frictionless What is the force exerted by the 1 kg mass on the 2 kg mass? a)6 N b)3 N c)2 N d)1 N e)0 N 24

25 iclicker question: forces 6 N 3kg 2kg 1kg frictionless What is the net force exerted on the 3 kg mass? a)6 N b)3 N c)2 N d)1 N e)0 N 25

26 iclicker question: rocket cart Which way will the cart move? a) Cart will move forward. b) Cart will move backward. c) Cart will not move. 26 Sep. 10, 2015