WORLD FOR CIVIL

Basic Vector Review

1. Definitions:

Scalar: Any quantity possessing magnitude (size) only, such as mass, volume, temperature
Vector: Any quantity possessing both magnitude and direction, such as force, velocity, momentum

2. Vector Addition:
Vector addition may be done several ways including, Graphical Method, Trigonometric Method, and Component Method. We will be reviewing only the Component Method, as that is the method which will be used in the course. Other methods are detailed in your textbook.

3. Vector Addition - Component Method: (2-dimensional)
The component method will follow the procedure shown below:

1. Choose an origin, sketch a coordinate system, and draw the vectors to be added (or summed).

2. Break (resolve) each vector into it's "x" and "y" components, using the following relationships:
- A_x = A cosine Ø, and, A_y = A sine Ø, where A is the vector, and Ø is the vector's angle.

3. Sum all the x-components and all the y-components obtaining a net resultant R_x, and R_y vectors.
- R_x = A_x + B_x + C_x + . . ., &, R_y = A_y + B_y + C_y + . . .

4. Recombine Rx and Ry to obtain the final resultant vector (magnitude and direction) using

See Example below

Example - Vector Addition

Three ropes are tied to a small metal ring. At the end of each rope three students are pulling, each trying to move the ring in their direction. If we look down from above the students, the forces and directions they are applying the forces are as follows: (See diagram to the right)

Find the net (resultant) force (magnitude and direction) on the ring due to the three applied forces.

Choose origin, sketch coordinate system and vectors (done above)

Resolve vectors into x & y components (See Diagram)
A_x = 30 lb cos 37^o = + 24.0 lbs ; A_y = 30 lb sin 37^o = + 18.1 lb
B_x = 50 lb cos135^o = - 35.4 lbs ; B_y = 50 lb sin135^o = + 35.4 lb
C_x = 80 lb cos240^o = - 40.0 lbs ; C_y = 80 lb sin240^o = - 69.3 lb

Sum x & y components to find resultant R_x and R_y forces.
R_x = 24.0 lbs - 35.4 lbs - 40.0 lbs = -51.4 lbs
R_y =18.1 lbs + 35.4 lbs - 69.3 lbs = -15.8 lbs