1. Test #3 Notes Forces and the Laws of Motion Circular Motion and Gravitation Chapters 4 and 7

2. Kinematics The study of HOW particles move Do they move fast, slow, change directions, etc.

3. Dynamics The study of WHY objects move Do they move b/c of friction, gravity, etc.

4. Force A push or a pull All forces are VECTORS Unit for force is NEWTONS 1 Newton = kg x m/s 2

5. 4 categories of forces Gravitational force: attractive force that exists b/w all objects Electromagnetic force: force b/w charged particles (proton – electron) Give materials their strength, ability to bend, stretch, squeeze Nuclear force: holds particles together in the nucleus of an atoms Weak force: responsible for radioactive decay

6. Weight Weight is a force measured in newtons! 1 pound is 4.448 Newtons 1 newton is 0.225 pounds

7. Free body diagrams/Force diagrams Example: See page 123

8. Free body diagrams-Practice Do practice A on pg. 124

9. Newton’s Laws An object at rest remains at rest, and an object in motion remains in motion unless the object experiences an external force Often referred to as the Law of Inertia Inertia: an object’s resistance to changes in motion When the external force on an object is zero, its acceleration is zero

10. Inertia The inertia of an object is proportional to the object’s mass Greater mass, less acceleration Smaller mass, more acceleration

11. Net Force The vector sum of all forces acting on an object Objects remain at constant velocity as long as the vector sum of the forces is equal to zero

12. Newton’s 2 nd Law Force is proportional to mass and acceleration The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass

13. Formula Force = mass x acceleration ΣF = ma Σ sigma : the sum of the quantities that come after it.

14. Example Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2kg book toward Roberto. If the net force acting on the book is 1.6N to the right, what is the book’s acceleration?

15. Newton’s 3 rd Law Every action has an equal (in magnitude) but opposite (in direction) reaction-force. Action-Reaction forces: = in size, opposite in direction

16. Everyday forces Weight A measure of the gravitational force exerted on an object Can change based on location W = F g (force of gravity) To calculate W: W = mg (g acceleration due to gravity)

17. Normal force A force that acts on a surface in a direction perpendicular to the surface F N – normal force ___________

18. Friction A force that opposes the motion of 2 touching objects Acts parallel to the surfaces in contact

19. Types of friction Static friction: opposes the initiation of sliding motion b/w 2 surfaces that are in contact and at rest Also called starting friction F s As long as the object isn’t moving, F S = F A (force applied) Applied force and friction act in opposite directions

20. Kinetic friction Force that opposes the movement of 2 surfaces that are in contact and are sliding over each other Also called sliding friction F k Remember if the object is at rest all forces are equal in size

21. Coefficient of friction μ - mu The force of friction depends on the composition of the surfaces in contact You will encounter more friction sliding across concrete than if you are sliding across ice μ – coefficient of friction The ratio of F K to F N

22. Formula F f = μF N Example: a 24kg crate at rest on a horizontal floor requires a 75N force to set it in motion. Find μ b/w the crate and the floor. F N = weight which equals mg F f is given = 75N

23. Example A smooth wooden block is placed on a smooth wooden tabletop. You find that you must exert a force of 14N to keep the 40N block moving at a constant velocity. What is the coefficient of kinetic friction b/w the block and the table? Draw a picture Givens: F A = W = a = Solving for μ

24. Continued If a 20N brick is placed on the block, what force will be required to keep the block and brick moving at constant velocity? Given: W = Constant velocity

25. Forces on an Accelerating Object Other forces besides friction act on objects. A stone with a mass of 10kg is lying on the ground. The weight of the stone, W is ____N in a downward direction. The ground exerts an equal and opposite force, 98n upward. The net force is.... F NET = F GROUND + W

26. Example Givens:Formula: F GROUND = __F NET = F GROUND + W Mass = ____ Weight = mg Plug in and solve F NET = 98N + -98N = ______

27. Example A spring scale hangs from the ceiling of an elevator. It supports a package that weighs 25N. What upward force does the scale exert when the elevator is not moving? What force must the scale exert when the elevator and object accelerate upward at 1.5m/s 2 ? Draw a picture.