1. Unit 3 - Dynamics How Forces Affect Motion

2. Unit 3 Part 1 – Newton’s Laws of Motion Physics Book Chapter 4 Conceptual Physics Book Chapters 4-6

3. Motion Theories  The old way looked at the what  The new way looked and looks at the why  4 th century BC - Aristotle  Did not experiment  Used only logic and observation  Two states of motion natural and violent  Earth at rest everything else moves in complete circles

4. Motion Theories  1600 Galileo  Used experiments with ramps  Discussed friction came up with the idea of inertia: property of all matter  1670 Newton  Built off of Galileo’s idea

5. 1 st Law of Motion: The Law of Inertia  Newton said, “An object at rest wants to stay at rest unless acted on by a net (unbalanced) force, and an object in motion wants to stay in motion in a straight line at a constant speed unless acted on by an net (unbalanced) force.  Forces do not cause motion they change motion  Inertia is a property of all matter

6. 1 st Law of Motion: The Law of Inertia  Inertia is related to the mass of an object  Mass is the measure of inertia or how easy or hard it is to change an objects position  Mass can also be how much stuff makes up an object.  Units of mass kilograms or slugs  1 slug = 14.59 kg

7. 1 st Law of Motion: The Law of Inertia Which one has more inertia?

8. 1 st Law of Motion: The Law of Inertia http://science360.gov/obj/video/70fadaa8- c3d4-4132-ba1f-c98be5caeb14/science-nfl- football-newtons-first-law-motion

9. Forces and Free Body Diagrams  A force is defined as any push or pull  To display forces free body diagrams are used.  Include only the forces acting on the object.  Force is a vector! Use an arrow to indicate a force.

10. Forces and Free Body Diagrams  Fit in two main categories  Contact forces between 2 objects  Shoot a basketball  Push a shopping cart  Friction  Non-contact or at a distance  GRAVITY  Electricity  Magnetism

11. Forces and Free Body Diagrams  Equilibrium  All forces are equal up = down, lefts = rights  If any one force is not equal then there is a non- zero net force on the object

12. Forces and Free Body Diagrams  Equilibrium involving gravity

13. Proportions

15. Newton’s 2 nd Law: Law of Acceleration

16.  “When a net external force acts on an object of mass m, there is an acceleration that results.”  The acceleration is directly proportional to the force applied and inversely proportional to the mass.

17. Newton’s 2 nd Law: Law of Acceleration  Units of force are Newtons and pounds  A Newton is a kg m/s 2  A pound is a slug ft/s 2

18. Newton’s 2 nd Law: Law of Acceleration https://www.youtube.com/watch?v=iwP4he WDhvw

19. Example Problem  Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other person applies a force of 395 N in the same direction. A third force of 560 N also acts on the car, but in the opposite direction. (This force is due to the friction of the tires on the pavement.) Find the acceleration of the car.

20. Newton’s 3 rd Law of Action/Reaction  Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.  “for every action there is an equal and opposite reaction”  Hammer hits nail and nail stops hammer  Swimming: you push the water backward and the water pushes you forward

21. Newton’s 3 rd Law of Action/Reaction

22. Weight and Normal Force

24.  The normal force is the perpendicular component of the force that a surface exerts on an object with which it is in contact.  Does NOT necessarily point directly upwards!  Equal and opposite of the weight IF no other forces are acting and the object is on a horizontal surface.  Think of “atomic springs” in the surface compressing to produce the normal force.

25. Newton’s 3 rd Law of Action/Reaction https://www.youtube.com/watch?v=91QYouih4bQ

26. Friction  Friction is the parallel component of the force that a surface exerts on an object with which it is in contact.  Two types:  If the object IS NOT moving, then the force is called static friction.  If the object IS moving, then the force is called kinetic friction.

27. Friction

29. True Weight vs. Apparent Weight  The apparent weight is the force that an object exerts on a scale.  If the scale and the object are either at rest or moving at a constant speed (a=0), then True=Apparent.  If the scale and object are accelerating, then apparent weight will be different!

30. True Weight vs. Apparent Weight