1. Chapter 4 4-2 Newton’s first law

2. Newton’s first law  An object at rest remains at rest, and a object in motion continue in motion with constant velocity unless the object experiences a net external force.

3. Law of Inertia  What is Inertia?  Is the tendency of an object not to accelerates.  Newton’s first law is often referred to us as the law of inertia. Because it states that in the absence of a net force, a body will preserve its state of motion.

4. Newton’s first law  In other words, Newton’s first law states that when the net external force on an object is zero, the object’s acceleration is zero.

5. Net force  What is a net force  Answer: Is a single force whose external effects on a rigid body are the same as the effects of several actual forces acting on a body.  In other words a net force is the vector sum of all forces acting on an object

6. Determine Net Force  Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. The free-body diagram below shows the forces acting on the book. Find the net force acting on the book.

7. Example#1 1. Define the problem, and identify the variables. Given: F gravity-on-book = F g = 22 N F friction = F f = 11 N F table-on-book = F t = 18 N  Unknown:  Fnet = ?

8. continue  2. Select a coordinate system, and apply it to the free-body diagram.  Choose the x-axis parallel to and the y- axis perpendicular to the incline of the table, as shown in (a). This coordinate system is the most convenient because only one force needs to be resolved into x and y components.

9. continue  Find the x and y components of all vectors.  Draw a sketch, as shown in (b), to help find the components of the vector Fg. The angle  is equal to 180  – 90  – 35  = 55 .

10. continue

11.  4. Find the net force in both the x and y directions.  Diagram (d) shows another free-body diagram of the book, now with forces acting only along the x- and y-axes.

12. continue  For the x direction:   Fx = Fg,x – Ff  Fx = 13 N – 11 N  Fx = 2 N For the y direction:  F y = F t – F g,y  F y = 18 N – 18 N  F y = 0 N

13. continue Find the net force. Add the net forces in the x and y directions together as vectors to find the total net force. In this case, F net = 2 N in the +x direction, as shown in (e). Thus, the book accelerates down the incline.

14. Equilibrium  Objects that are either at rest or moving with constant velocity are said to be in equilibrium. What is equilibrium? Equilibrium is the state in which the net force on an object is zero.

15. Equilibrium  Newton’s first law describes objects in equilibrium. Tip: To determine whether a body is in equilibrium, find the net force. If the net force is zero, the body is in equilibrium. If there is a net force, a second force equal and opposite to this net force will put the body in equilibrium.