Physics is the Science of Measurement Length Weight Time We begin with the measurement of length: its magnitude and its direction. Distance: A Scalar Quantity Distance is the length of the actual path taken by an object. distance = 20 m A B A scalar quantity: Contains magnitude only and consists of a number and a unit. Displacement—A Vector Quantity • Displacement is the straight-line separation of two points in a specified direction. D = 12 m, 20o A q B A vector quantity: Contains magnitude AND direction, a number, unit & angle. (12 m, 300) Distance and Displacement • Displacement is the change of position based on the starting point. Consider a car that travels 4 m, E then 6 m, W. D Net displacement: 4 m,E x = -2 x = +4 6 m,W D = 2 m, W What is the distance traveled? 10 m !! Identifying Direction A common way of identifying direction is by reference to East, North, West, and South. (Locate points below.) Length = 40 m N 40 m, 50o N of E W 60o 60o 50o 60o E 40 m, 60o N of W 40 m, 60o W of S S 40 m, 60o S of E Identifying Direction Write the angles shown below by using references to east, south, west, north. N W 45o E 50o S N W E S 0 S of 50 Click to Esee the Answers 450 W. of . .N Rectangular Coordinates y (-2, +3) (+3, +2) + (-1, -3) + x Reference is made to x and y axes, with + and - numbers to indicate position in space. Right, up = (+,+) - Left, down = (-,-) (+4, -3) (x,y) = (?, ?) Trigonometry Review • Application of Trigonometry to Vectors Trigonometry R y q x y sin q R x cos q R y tan q x y = R sin q x = R cos q R2 = x2 + y2 Example 1: Find the height of a building if it casts a shadow 90 m long and the indicated angle is 30o. The height h is opposite 300 and the known adjacent side is 90 m. opp h tan 30 adj 90 m 0 h 300 h = (90 m) tan 30o 90 m h = 57.7 m Finding Components of Vectors A component is the effect of a vector along other directions. The x and y components of the vector (R,q) are illustrated below. x = R cos q R q x y y = R sin q Example 2: A person walks 400 m in a direction of 30o N of E. How far is the displacement east and how far north? N N R q x 400 m y 30o E y=? x=? The x-component (E) is ADJ: x = R cos q The y-component (N) is OPP: y = R sin q E Example 2 (Cont.): A 400-m walk in a direction of 30o N of E. How far is the displacement east and how far north? N Note: x is the side 400 m 30o y=? x=? E x = (400 m) cos 30o = +346 m, E adjacent to angle 300 ADJ = HYP x Cos 300 x = R cos q The x-component is: Rx = +346 m Example 2 (Cont.): A 400-m walk in a direction of 30o N of E. How far is the displacement east and how far north? N Note: y is the side 400 m 30o y=? x=? E opposite to angle 300 OPP = HYP x Sin 300 y = R sin q y = (400 m) sin 30o The y-component is: = + 200 m, N Ry = +200 m Example 2 (Cont.): A 400-m walk in a direction of 30o N of E. How far is the displacement east and how far north? N 400 m 30o Rx = Ry = +200 m E The x- and ycomponents are each + in the first quadrant +346 m Solution: The person is displaced 346 m east and 200 m north of the original position. Resultant of Perpendicular Vectors Finding resultant of two perpendicular vectors is like changing from rectangular to polar coord. R q x R x y 2 y 2 y tan q x R is always positive; q is from + x axis Example 3: A woman walks 30 m, W; then 40 m, N. Find her total displacement. +40 m 40 0 tan ; = 59.1 30 R q = 59.1o N of W -30 m R (30) (40) 2 2 R = 50 m (R,q) = (50 m, 126.9o) Component Method 1. Start at origin. Draw each vector to scale with tip of 1st to tail of 2nd, tip of 2nd to tail 3rd, and so on for others. 2. Draw resultant from origin to tip of last vector, noting the quadrant of the resultant. 3. Write each vector in x,y components. 4. Add vectors algebraically to get resultant in x,y components. Then convert to the total vector (R,q). Example 4. A boat moves 2.0 km east then 4.0 km north, then 3.0 km west, and finally 2.0 km south. Find resultant displacement. 1. Start at origin. Draw each vector to scale with tip of 1st to tail of 2nd, tip of 2nd to tail 3rd, and so on for others. D 2 km, S N 3 km, W C B 4 km, N A 2 km, E E 2. Draw resultant from origin to tip of last vector, noting the quadrant of the resultant. Note: The scale is approximate, but it is still clear that the resultant is in the fourth quadrant. Example 4 (Cont.) Find resultant displacement. 3. Write each vector in i,j notation: A = +2 x B= +4y C = -3 x D= -2y R = -1 x + 2 y 1 km, west and 2 km north of origin. D 2 km, S N 3 km, W C B 4 km, N A 2 km, E E 4. Add vectors A,B,C,D algebraically to get resultant in x,y components. 5. Convert to resultant vector See next page. Example 4 (Cont.) Find resultant displacement. Resultant Sum is: R = -1 x + 2 y D 2 km, S N 3 km, W C Now, We Find R, q R (1) (2) 5 2 2 B 4 km, N A 2 km, E R = 2.24 km 2 km tan 1 km = 63.40 N of W R Rx = -1 km Ry= +2 km E Conclusion of Chapter 3B - Vectors