General Physics

1. General Physics 賴 光 昶 第一醫學大樓六樓 自然科學共同實驗室

2. Textbook: • Principle of Physics, by Halliday, Resnick and Jearl Walker • E-learning: • www.wileyplus.com

3. Office time: • Whenever I am in the office • Tue. 16:00—18:00 • Thu. 10:00—12:00 • Grading: 作業 30% • 期中考 20% x 2 = 40% • 期末考 30% • Exercise class: Thu. 18:00—20:00

4. Introduction • Measurement and Uncertainty; Significant Figures • Units, Standards, and the SI System • Converting Units • Order of Magnitude: Rapid Estimating • Dimensions and Dimensional Analysis

5. Measurement and Uncertainty; Significant Figures Estimated uncertainty is written with a ± sign; for example: 8.8 ± 0.1 cm. Percent uncertainty is the ratio of the uncertainty to the measured value, multiplied by 100:

6. Measurement and Uncertainty; Significant Figures The number of significant figures is the number of reliably known digits in a number. It is usually possible to tell the number of significant figures by the way the number is written: 23.21 cm has four significant figures. 0.062 cm has two significant figures (the initial zeroes don’t count). 80 km is ambiguous—it could have one or two significant figures. If it has three, it should be written 80.0 km.

7. Measurement and Uncertainty; Significant Figures Scientific notation is commonly used in physics; it allows the number of significant figures to be clearly shown. For example, we cannot tell how many significant figures the number 36,900 has. However, if we write 3.69 x 104, we know it has three; if we write 3.690 x 104, it has four. Much of physics involves approximations; these can affect the precision of a measurement also.

8. Units, Standards, and the SI System

9. Units, Standards, and the SI System We will be working in the SI system, in which the basic units are kilograms, meters, and seconds. Quantities not in the table are derived quantities, expressed in terms of the base units. Other systems: cgs; units are centimeters, grams, and seconds. British engineering system has force instead of mass as one of its basic quantities, which are feet, pounds, and seconds.

10. Dimensions and Dimensional Analysis Dimensions of a quantity are the base units that make it up; they are generally written using square brackets. Example: Speed = distance/time Dimensions of speed: [L/T] Quantities that are being added or subtracted must have the same dimensions. In addition, a quantity calculated as the solution to a problem should have the correct dimensions.

11. Dimensions and Dimensional Analysis Dimensional analysis is the checking of dimensions of all quantities in an equation to ensure that those which are added, subtracted, or equated have the same dimensions. Example: Is this the correct equation for velocity? Check the dimensions: Wrong!

12. Vectors and Scalars • Addition of Vectors—Graphical Methods • Subtraction of Vectors, and Multiplication of a Vector by a Scalar • Adding Vectors by Components • Unit Vectors • Vector Kinematics • Projectile Motion • Solving Problems Involving Projectile Motion • Relative Velocity

13. Vectors and Scalars A vector has magnitude as well as direction. Some vector quantities: displacement, velocity, force, momentum A scalar has only a magnitude. Some scalar quantities: mass, time, temperature

14. Addition of Vectors—Graphical Methods For vectors in one dimension, simple addition and subtraction are all that is needed. You do need to be careful about the signs, as the figure indicates.

15. Addition of Vectors—Graphical Methods If the motion is in two dimensions, the situation is somewhat more complicated. Here, the actual travel paths are at right angles to one another; we can find the displacement by using the Pythagorean Theorem.

16. Addition of Vectors—Graphical Methods Even if the vectors are not at right angles, they can be added graphically by using the tail-to-tip method.

17. Subtraction of Vectors, and Multiplication of a Vector by a Scalar In order to subtract vectors, we define the negative of a vector, which has the same magnitude but points in the opposite direction. Then we add the negative vector.

18. Adding Vectors by Components Any vector can be expressed as the sum of two other vectors, which are called its components. Usually the other vectors are chosen so that they are perpendicular to each other.

19. Adding Vectors by Components If the components are perpendicular, they can be found using trigonometric functions.

20. Adding Vectors by Components The components are effectively one-dimensional, so they can be added arithmetically.

21. Adding Vectors by Components • Adding vectors: • Draw a diagram; add the vectors graphically. • Choosex and y axes. • Resolve each vector into x and ycomponents. • Calculate each component using sines and cosines. • Add the components in each direction. • To find the length and direction of the vector, use: . and

22. Adding Vectors by Components Mail carrier’s displacement. A rural mail carrier leaves the post office and drives 22.0 km in a northerly direction. She then drives in a direction 60.0° south of east for 47.0 km. What is her displacement from the post office?

23. Adding Vectors by Components Three short trips. An airplane trip involves three legs, with two stopovers. The first leg is due east for 620 km; the second leg is southeast (45°) for 440 km; and the third leg is at 53° south of west, for 550 km, as shown. What is the plane’s total displacement?

24. Unit Vectors Unit vectors have magnitude 1. Using unit vectors, any vector can be written in terms of its components:

25. Vector Kinematics In two or three dimensions, the displacement is a vector: