1. 1. Introduction to Statics
Su-Jin Kim
Gyeongsang National University
Fall 2017

2. OCW
Statics Lecture by Yiheng Wang:

3. OCW
What is statics?
SI Units
Newton's fundamental laws, Law of gravitation
Scalars and Vectors, Vector addition and subtraction
Cartesian vectors

4. Index What is statics? SI Units Newton's fundamental laws
Law of gravitation
Scalars and Vectors
Vector addition and subtraction
Rectangular components
Problem solving in Statics

5. What is statics?
Physics ⊃ Mechanics ⊃ Statics, Dynamics, Fluid mechanics
Mechanics : Physical science that deals with the response of bodies to the action of forces
Statics : Equilibrium of bodies under action of forces Σ F = 0 Σ M = 0
Dynamics : Concerns with the motion of bodies Σ F = ma Σ M = I α
Fluid mechanics: Deals with liquid or gases

6. SI Units [Base Units] Length m Time s Mass kg [Derived Units]
Force N F (N) = m (kg) x a (m/s2)
Velocity m/s
Idealization: Particle, Rigid body, Concentrated force

7. Newton's fundamental laws
Law2. Acceleration motion : F=m a
Law1. Law of inertia : ΣF=0 equilibrium  at rest or continue to move
Law3. Action and reaction : are equal magnitude, opposite direction, collinear
Law2  Dynamics
Law1 & 3  Statics

8. Law of gravitation Law of gravitation : F = G m1m2 / r2
G = 6.673x10-11 m3/kgs2
Earth : m = 5.976x1024 kg, r = 6371 km
Weight : gravitational attraction of the earth : W = mg (g = 9.81 m/s2 ) m1 m2 r F m W

9. Ex) Gravitation Earth-Moon
m1= 5.97x1024 kg, m2=7.35x1022 kg
r = 384x106 m
F = G m1m2 / r2
= 6.673x10-11 m3/kgs2 x 5.97x1024 kg X 7.35x1022 kg / (384x106 m)2
= 1.98 x 1020 N m1 m2 r F

10. Scalars and Vectors
Scalar: length l, mass m, time t, speed v, volume, density, energy (magnitude only)
Vector: displacement r, force F, velocity v, acceleration a, moment M (magnitude + direction)
length r , displacement r
speed v , velocity v

11. Vector addition and subtraction
Magnitude : V = |V|
Parallelogram law : R = A + B
Triangular rule : R = A + B
Subtraction : R = A - B A B R
Parallelogram
Triangle

12. Cartesian(Rectangular) vectors
Cartesian unit vectors: i, j, k
Cartesian components : V = V (Vx, Vy, Vz) = Vx i + Vy j + Vz k
Direction: cos a = Vx / |V|
Addition: add each components i j k V
Vx i
Vz k
Vy j z x y

13. Problem solving in Statics
Free Body Diagram : Isolate object to a rigid body(or particle) + External forces
Equilibrium equation

14. Ex 1-1 Weight of a people? m = 70 kg W ? N
Ans : W = m g = 70 kg x 9.81 m/s2 = 686 N

15. Ex 1-2 Add Vector A and B A 8 A+B 45° 30° 12 B
A = Ax i + Ay j = 8cos45˚ i + 8sin45˚ j
B = Bx i + By j =12cos30˚ i -12sin30˚ j
A+B = (8cos45˚ +12cos30˚) i + (8sin45˚-12sin30˚) j
A+B = i j
Magnitude: |A+B|=16.05
Direction angle: θ=tan-1 (-0.35/16.05) = -1.25˚

16. Ex 1-2
Answer of students

17. Ex 1-3 Vector
|A|=6
|B|=4
60˚
|A+B|=?
a=?