PENJUMLAHAN GAYA TUJUAN PEMBELAJARAN Mahasiswa dapat menentukan besar

PENJUMLAHAN GAYA TUJUAN PEMBELAJARAN: • Mahasiswa dapat menentukan besar dan arah resultan dari beberapa gaya dengan metode analitis. • Mahasiswa dapat menentukan besar dan arah resultan dari beberapa gaya dengan metode grafis. 1

Review PRINSIP DASAR Dalam Mekanika 1. Hukum PARALELOGRAM dalam penjumlahan gaya 2. Prinsip TRANSMISIBILITAS 3. Hukum NEWTON 1 4. Hukum NEWTON 2 5. Hukum NEWTON 3 6. Hukum GRAFITASI NEWTON 2

Sistem Satuan • Four fundamental physical quantities. Length, Time, Mass, Force. • We will work with two unit systems in static’s: SI & US Customary. Bagaimana konversi dari SI ke US atau sebaliknya ? 3

GAYA Apakah gaya itu ? 4

Menyusun atau menjumlahkan gaya dimaksudkan untuk menentukan resultante (R), dengan kata lain dua buah gaya atau lebih dapat digabung menjadi satu gaya pengganti yang disebut resultante (R). 5

Dapat dilakukan dengan 2 cara Cara lukisan Cara hitungan 6

APPLICATION OF VECTOR ADDITION There are four concurrent cable forces acting on the bracket. How do you determine the resultant force acting on the bracket ? 7

Addition of Vectors • Trapezoid rule for vector addition • Triangle rule for vector addition • Law of cosines, C B C • Law of sines, B • Vector addition is commutative, • Vector subtraction 8

Sample Problem SOLUTION: • Trigonometric solution - use the triangle rule for vector addition in conjunction with the law of cosines and law of sines to find the resultant. The two forces act on a bolt at A. Determine their resultant. 9

Sample Problem (Lanjutan) • Trigonometric solution - Apply the triangle rule. From the Law of Cosines, From the Law of Sines, 10

ADDITION OF SEVERAL VECTORS • Step 1 is to resolve each force into its components • Step 2 is to add all the x components together and add all the y components together. These two totals become the resultant vector. • Step 3 is to find the magnitude and angle of the resultant vector. 11

Example of this process, 12

You can also represent a 2 -D vector with a magnitude and angle. 13

EXAMPLE Given: Three concurrent forces acting on a bracket. Find: The magnitude and angle of the resultant force. Plan: a) Resolve the forces in their x-y components. b) Add the respective components to get the resultant vector. c) Find magnitude and angle from the resultant components. 14

EXAMPLE (continued) F 1 = { 15 sin 40° i + 15 cos 40° j } k. N = { 9. 642 i + 11. 49 j } k. N F 2 = { -(12/13)26 i + (5/13)26 j } k. N = { -24 i + 10 j } k. N F 3 = { 36 cos 30° i – 36 sin 30° j } k. N = { 31. 18 i – 18 j } k. N 15

EXAMPLE (continued) Summing up all the i and j components respectively, we get, FR = { (9. 642 – 24 + 31. 18) i + (11. 49 + 10 – 18) j } k. N = { 16. 82 i + 3. 49 j } k. N y FR FR = ((16. 82)2 + (3. 49)2)1/2 = 17. 2 k. N = tan-1(3. 49/16. 82) = 11. 7° x 16

Sample Problem SOLUTION: • Resolve each force into rectangular components. • Determine the components of the resultant by adding the corresponding force components. Four forces act on bolt A as shown. Determine the resultant of the force on the bolt. • Calculate the magnitude and direction of the resultant. 17

Sample Problem (cont’) SOLUTION: • Resolve each force into rectangular components. force mag x - comp y - comp r + 129. 9 + 75. 0 F 1 150 r - 27. 4 + 75. 2 F 2 80 r - 110. 0 F 3 110 0 r + 96. 6 - 25. 9 F 4 100 • Determine the components of the resultant by adding the corresponding force components. • Calculate the magnitude and direction. 18

READING QUIZ 1. The subject of mechanics deals with what happens to a body when ______ is / are applied to it. A) magnetic field B) heat D) neutrons E) lasers C) forces 2. ________ still remains the basis of most of today’s engineering sciences. A) Newtonian Mechanics B) Relativistic Mechanics C) Euclidean Mechanics C) Greek Mechanics 19

READING QUIZ 3. Which one of the following is a scalar quantity? A) Force B) Position C) Mass D) Velocity 4. For vector addition you have to use ______ law. A) Newton’s Second B) the arithmetic C) Pascal’s D) the parallelogram 20

CONCEPT QUIZ 5. Can you resolve a 2 -D vector along two directions, which are not at 90° to each other? A) Yes, but not uniquely. B) No. C) Yes, uniquely. 6. Can you resolve a 2 -D vector along three directions (say at 0, 60, and 120°)? A) Yes, but not uniquely. B) No. C) Yes, uniquely. 21

ATTENTION QUIZ 7. Resolve F along x and y axes and write it in vector form. F = { ______ } N y A) 80 cos (30°) i - 80 sin (30°) j x B) 80 sin (30°) i + 80 cos (30°) j C) 80 sin (30°) i - 80 cos (30°) j 30° F = 80 N D) 80 cos (30°) i + 80 sin (30°) j 8. Determine the magnitude of the resultant (F 1 + F 2) force in N when F 1 = { 10 i + 20 j } N and F 2 = { 20 i + 20 j } N. A) 30 N B) 40 N D) 60 N E) 70 N C) 50 N 22

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