1. Day 30, Wednesday, 7 October, 2015 Everything you ever wanted to know about forces * * but were afraid to ask

2. The unit of Force - the newton kg m/s 2 = N The force required to accelerate 1 kilogram by one meter per second per second

3. What is Mass? Mass is the source of gravity and it is also affected by gravity. Weight is a force. Weight is the force one gets when mass is multiplied by gravity. Mass is NOT weight. Mass is a fundamental quantity measured in kilograms. Weight is the force one gets when mass is multiplied by the acceleration of gravity. Weight is measured in units of newtons. Mass and weight easily confused because they are related, but they are not the same. Weight changes with location in space. Mass never changes.

4. The Difference between People and Gravity Gravity is everywhere and invisible, people exist in one place and are visible. Sure, but lets get serious, really! People apply one human power of force and get vastly different accelerations from different masses. Gravity applies one acceleration and has vastly different forces for different masses.

5. Newton’s 3 Laws of Motion 1. Inertia: An object at rest will remain at rest. An object moving will move in a straight line with constant speed, if and only if the net force acting on it = 0. 2. F = MA originally written as A = F/M 3. F a on b = -F b on a (for every action there is an = and opposite reaction)

6. Newton’s First Law: Inertia Inertia: An object at rest will remain at rest. An object moving will move in a straight line with constant speed, if and only if the net force acting on it = 0.

7. The Law of Inertia in action Click on the box

8. Newton’s Second Law: Acceleration A = F/M or F = MA

9. Newton originally wrote it as a = f/m. It has been taught for a few hundred years as f = ma. Lately the original form is coming back in favor. The current preferred form is a = f/m. A Note on Notation

10. Demo with toy pistols

11. Newton’s Third Law: Action and Reaction F a on b = -F b on a (for every action there is an equal and opposite reaction)

12. Demo with fruit

13. Strive for clarity Aristotle: 3 volumes Newton: f = ma

14. Free Body Diagrams They are bodies free of rotation, hence the name, “Free Body Diagrams”

15. How to draw and interpret the diagrams The forces are always shown with the tails at the body (the particle, the point). If the force vectors have equal and opposite components in each dimension then there is no “net” force and no acceleration. If the components are not equal and opposite then there is a “net” force and an acceleration.

16. Some Common Forces 1. Friction 2. Normal 3. Restoring force of a spring 4. Tension 5. Thrust 6. Gravity (weight)

17. Friction 1. Static F f s =  s F N 2. Kinetic F f k =  k F N Opposite direction from velocity  s is the Coefficient of Friction, Static  k is the Coefficient of Friction, Kinetic

18. The Normal Force Always perpendicular to a surface (“Normal” is the mathematical term for perpendicular)

19. Restoring Force of a Spring Always opposite direction of displacement Always has a negative sign Hooke’s LawF = - k x

20. Tension Pull of a string or other material Always points away from the body

21. Thrust Manifestation of Newton’s 3 rd law (law of action and reaction) Does not need a medium to push against

22. Gravity (weight) Always acts towards the center of mass f = ma  f = mg

23. Let’s solve problems Chapter 4, practice A, # 1 Chapter 4, practice A, # 2 Chapter 4, practice B, # 1 Chapter 4, practice B, # 2 Chapter 4, practice B, # 3

24. Credits Free Body Diagrams http://www.goiit.com/posts/list/mechanics- find-the-tension-in-the-string-24966.htm