Part I: Mechanics. Chapter 2 Inertia & Newton s First Law of Motion. Aristotle & Galileo. Lecture 2

Transcription

1 Lecture 2 Part I: Mechanics Chapter 2 Inertia & Newton s First Law of Motion Some material courtesy Prof. A. Garcia, SJSU Aristotle & Galileo Aristotle was great philosopher but not such a good scientist. Aristotle s theory of motion is wrong. Took 2000 years before Galileo got the basic ideas of motion right. 1

2 Newtonian Mechanics Isaac Newton s Laws of Mechanics perfected Galileo s ideas. These laws perfectly explain motion in all normal situations. Newton s laws fail in two circumstances: When the speed of objects exceeds about m/s (13,000,000 mi/h). In this case, we must use Einstein s special theory of relativity (1905 When the objects under study become very small (e.g., electrons, atoms, etc.). In this case we must use quantum mechanics (1926). Also, Newton s Laws are valid only in an inertial reference frame (an environment at rest or in uniform motion) Newton s First Law An object at rest remains at rest and an object in motion remains in uniform motion* unless a net force acts on the object. *Moving in a straight line with constant speed. 2

3 Newton s First Law Object at rest (not moving) Uniform motion then then No force on object Note: Object at rest is just uniform motion at zero speed. Inertia We call the tendency of an object to keep doing what it s doing inertia. The amount of inertia an object has depends on the mass of the object: More mass More inertia SI Unit of mass: kg 3

4 Demo: Rolling, rolling, rolling All moving together WALL Bottom car stops. Other two continue With same speed. Demo: Inertia Balls Upper String Heavy Cannon Balls Which string breaks in each case? Lower String Slow pull Quick yank 4

5 Force A force is a push or a pull. Force is a vector (size & direction). Contact forces: forces that arise from the physical contact between two objects (1 & 2 above). Noncontact forces: forces the two objects exert on one another even though they are not touching (3 above). The SI Unit of force is the Newton (N) (about 1/5 pound). Net Force When several forces act on an object, the forces combine to produce a net force or total force. If the individual forces are all in the same direction, the size of the net force is just the sum of the individual forces. We use arrows to symbolize force vectors; length of arrow proportional to size of force, direction of arrow shows direction of force. 3 Newtons 5 Newtons BRICK same as 8 Newtons 5

6 Forces with Different Directions When individual forces have different directions, the forces may partly or totally cancel each other. In one dimension, we sometimes indicate the direction of a force using + and - signs; + for forces to right (forward); - for forces 5 Newtons BRICK 3 Newtons same as 2 Newtons or Net Force = +5 N + (- 3N) = +2 N Check Yourself? 5N 10N 6

7 Vector Notation When handwritten, use an arrow: When printed, may be in bold print: A r When dealing with just the magnitude of a vector in print, an italic letter may be used: A Sum Notation (Σ) We use the symbol Σ to stand for the sum, or addition of several like items. With this notation, we could write the net force on an object as ΣF or ΣF Equilibrium Rule If an object is at rest, then the net force on it must be zero. Similarly, if the object is in uniform motion, the net force on it must be zero. 3 Newtons 3 Newtons BRICK same as Zero Newtons (No Force) Net Force = ΣF = +3N + (-3N) = 0 When this happens we say that forces balance, or that we have force equilibrium. The book writes this as ΣF = 0. 7

8 Demo: Up+Up=Down+Down 5N+7N=10N+2N Scale reads 5 Newtons Scale reads 7 Newtons STICK (2 Newtons) Stick is at rest. 10 Newton Weight The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass). Force Force: push or pull. Symbol F (also W and N ) Force is a vector it has magnitude and direction. SI unit of force: Newton (N) 8

9 Typical Force Sizes Fundamental Forces Types Strong Nuclear Force Electromagnetic force Weak nuclear force Gravity Characteristics Listed in order of decreasing strength Only gravity and electromagnetic in mechanics 9

10 Practical Forces Some contact forces Normal Force (Support Force) Friction Drag Spring Force Tension force (ropes, etc.) Some non-contact forces Gravitational force (Weight) Electric force What is the fundamental type of each? Support Force Solid surfaces exert a force, called a support force, or normal force, on objects pressed against them. 100 Newton Gold Brick Downward force (weight) balanced by upward force (support). 100 Newton Support force How much is the net force on the brick? 10

11 Normal Force A solid object resists compression with what we call normal force. Normal force always acts outward and perpendicular to surface of compressed object. 11

12 Normal Force A force acting normal (perpendicular) to a surface For example Upward from the floor Upward from a table Away from a wall Down from a ceiling The normal force is the support force exerted upon an object in contact with another stable object. For example, if a book is resting on a surface, then the surface is exerting an upward force on the book to support the weight of the book. Newton s First Law Inertia Key Points of Lecture 2 Nature of Force; SI Unit of force (N) Combining forces to get net force Equilibrium Fundamental and Practical Forces Normal force (support force) Before the next lecture, read Hewitt, 1 st half Chap. 3 Homework Assignment Introduction to WebAssign is due before 11:00 PM on Tuesday, Aug