1. Introduction to Dynamics Dynamics is the study of why objects move.

2. Dynamics What causes objects to move?

3. Force Force: is a push or pull. Force can cause objects to accelerate and change direction.

4. Types of Forces Contact Force: A force that arises from the physical contact between two objects. Field Forces: Forces that can exist between objects in the absence of physical contact.

5. Quantoons, ed. NSTA press (2006) p. 11

6. Quantoons, ed. NSTA press (2006) p. 67

7. SI Units of Force Unit of force is the Newton – N A Newton is defined as the amount of force required to accelerate a 1 kg mass, 1 m/s2. 1 N = 1 kgm/s2

8. Forces are Vector Quantities The effect of a force depends on it magnitude and direction, thus it is a vector quantity and is represented graphically with arrows Downward 50 N Right 20 N

9. Free-Body Diagrams Table pushing up on the cow Gravity Physical model which represents the forces acting on a system

10. Free-body Diagram Frope on ball F Gravity

11. Newton’s First Law( Law of Inertia) “An object at rest remains at rest, and an object in motion will continue with a constant velocity unless the object experiences a net external force”

12. Newton’s First Law Newton’s first Law is really saying: When the net external forces on an object is zero, its acceleration is zero.

13. Newton’s First Law Newton’s first Law is really saying: When the net external forces on an object is zero, its acceleration is zero.

14. Equilibrium When the net external forces on an object are zero, the object is said to be in equilibrium. Its acceleration is zero and its velocity is constant.

15. Quantoons, ed. NSTA press (2006) p. 67

16. Class Activity For the following draw a free-body diagram, label all forces, and indicate the direction of the acceleration, velocity, and net force. Draw vectors of appropriate lengths and directions. • A flowerpot falling freely from a window • A sky diver falling toward earth with a constant velocity (The air exerts an upward force of the person). • A cable pulling a crate at a constant speed across a horizontal surface. (The surface provides a force that resists the crate’s motion

17. Sample Problem 1 A crate is pulled to the right with a force of 82 N. There is a frictional force to the left of 115 N. The crate has an upward force of 565 N and a downward force of 236 N acting on it. • Find net force in x-direction • Find net force in y-direction • Find magnitude and direction of net external force on crate

18. Sample Problem 1 565 N 82 N 115 N 236 N In X direction ∑Fx = 82 N – 115 N = - 33 N In Y direction ∑Fy = 565 N – 236 N = 329 N

19. Sample Problem 1 565 N 82 N 115 N -33 N 236 N 329 N In X direction ∑Fx = 82 N – 115 N = - 33 N In Y direction ∑Fy = 565 N – 236 N = 329 N

20. Sample Problem 1 -33 N A2 + B2 = C2 (-33 N)2 + (329 N)2 = C2 109330 N2 = C2 C = 331 N Tanθ = -33/ 329 Θ = -5.70 96 0 counterclockwise from the positive X-axis 329 N