1. 2 Outline • ideas about motion • Newton’s 1st Law & equilibrium • diagramming forces • Homework: • RQ: 1, 7, 10, 11, 12, 14, 16. • Ex: 1, 3, 9, 15, 22, 27, 31.

2. Force • “push” or a “pull” in a direction • SI unit is newton (N) • US customary unit is pound (lb) • 1 lb = 4.45 N

3. Net Force • is sum of all forces acting on an object • The net force can be zero (and often is) even when several forces act. • Sign Convention for Forces: • + denotes upward or rightward direction • - denotes downward or leftward direction • If forces act in the same or opposite direction, the net force is the algebraic sum of force-values.

4. Ex. Net Force for Up & Down Applied Forces • Fill in the missing information • F1 F2 Net Force • -2 +5 +3 • -10 0 • -10 +5 • -10 +10 • -30 0 -10 -5 0 +30

5. Newton’s 1st Law of Motion • When the net force on an object is zero, the object will either • a) continue in a state of rest, or • b) maintain a constant speed & direction.

6. Mechanical Equilibrium • State in which the net force acting on an object is zero • This also means the object is not accelerating

7. Example of Mechanical Equilibrium and Newton’s 1st Law. What happens to the speed of the block?

8. Common Forces Encountered • Outward forces (compressed spring or any object under compression) • Inward forces (stretched spring or any object under tension) • Downward force of Weight • Reaction forces (object on top of spring) • Frictional forces (oppose motion)

9. What is the unknown tension? Assume the man weighs 800 N and the platform has negligible weight. Net Force = +170 + ???? + (-800) = 0

10. Inertia (Mass) • the property of objects to resist changes in motion. • measurement of is called “mass” • Scientific unit is “kilogram” (kg) • US customary unit is “slug”

11. What is important here, the size or the mass of the books? Would a stiff but light box of the same size protect her as much? Why would a very light foam pad of the same size as the books protect her from the hammer blow?

12. When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity.) A B C

13. 2 summary • Galilean and Newtonian ideas about force and motion prevailed over Aristotle’s ideas. • A net force of zero leaves the motion of an object unchanged (called equilibrium even when moving) • Pages 3-5 in practicing physics.

14. 0 Motion Diagrams • Are velocity-position diagrams • Length determines speed (zero velocity is a “dot”) • Arrow determines direction • force required to change velocity • Example: slowing, reversing direction

15. 2 agenda • lecture • practicing physics: • p3 static equilibrium • p4 equilibrium rule • p5 vectors and equilibrium • lab: vector forces with spring scales