a push or a pull -we could name hundreds…

1. a push or a pull -we could name hundreds… Stretch, squeeze,press,twist,crumple,bend,punch, etc etc

2. Net Force • The cause of an acceleration or the change in an object’s velocity. • Net Forces can cause objects to… • Start moving • Stop moving • Change their direction

3. For years physicists have been grouping forces. Eventually they would like to have all forces in one unified theory GUT’s (Grand Unification Theory) TOE’s (Theory Of Everything)

4. For now– 4 groups of forces • Gravitational : exists between all things with mass • Electromagnetic : forces giving materials strength, bending squeezing, shattering. (object interaction) • Strongnuclear: holds particles of atomic nucleus together **strongest of four forces! • WeakNuclear : involved in decay of atomic nuclei *

5. Forces • a force is a push or a pull (bend stretch squeeze, accelerate) • a force is a vector quantity (magnitude & direction) • and there are 4 basic forces • gravitational • electromagnetic • strong nuclear • weak nuclear • Aristotle, Copernicus & Galileo all explained force and motion. • Isaac Newton (1642 – 1727) explained what we study today • Einstein extended our knowledge of gravity and forces – You need to take a higher level of Physics to learn about it.

6. Forces can act through contact or at a distance • Contact Forces – forces resulting from physical contact between 2 objects. • Field Forces – do not involve physical contact. An example could be gravity, the earth exerts a force even though its not necessarily contacting the object. Also magnets and electricity have forces without touching.

7. Unit of force is named after Isaac Newton Newton Symbol = N N= Kg m/s2

8. Pounds and Newtons measure Force • 1 pound = 4.448 Newtons • 1 Newton = 0.225 pounds • Remember that a Newton is a small thing. If you weigh 150lbs, you also weigh 667N

9. Free Body Diagram; FBD • Free Body Diagrams isolate an object and the forces acting on it. • We draw a simple picture of the object and then draw arrows (vectors) to represent the forces on the object. Be sure to label them. • Force diagrams are similar but they have multiple objects.

10. The Net Force causes acceleration • There are many forces that act on objects…. • Weight • Normal • Friction • Applied • Tension • Centripetal

11. On page 127 in the Text book • Lets look at the picture of this red car getting towed and do a sample FBD together.

12. Do you see 4 forces acting on the car? Weight 14700N, Ground 13690N, Truck hitch 5800N, Friction 775N Draw a FBD for this scene.

13. Holt worksheet FBD

14. Chapter Packet p. 160 I & J • Let’s do some of these together, in pairs, and on our own.

15. Section Review p. 128 • Answer questions 1-6. • Work in groups of 4. Everyone completes the assignment on their own paper, turn in the best one of four with ALL four names on it.

16. 1. List three examples of each of the following: a. a force causing an object to start moving b. a force causing an object to stop moving c. a force causing an object to change direction 2. Give two examples of field forces described in this section and two examples of contact forces you observe in everyday life. Explain how you know that these are forces. 3. Draw a free-body diagram of a football being kicked. Assume that the only forces acting on the ball are the force of gravity and the force exerted by the kicker. 4. Physics in Action Draw a force diagram of a crash-test dummy in 4, 5, and 6 can be done together at once. a car at the moment of collision. For this problem, assume that the forces acting on the car are 19 600 N downward, 17 800 N forward, and 25 000 N backward. The forces acting on the dummy are 585 N downward, 175 N backward, and 585 N upward. 5. Physics in Action Use the information given above to draw a freebody diagram showing only the forces acting on the car in item 4. Label all forces. 6. Physics in Action Use the information given above to draw a freebody diagram showing only the forces acting on the dummy in item 4. Label all forces.

17. Weightp.141 in text • Is the magnitude of the force of gravity acting on an object. • Weight depends on location – if you change the value of g, your weight will change. (I weigh 220lbs on Earth : I would weigh 37lbs on the moon: 80lbs on Mars).

18. Mass and Weight we must know the difference Mass Weight - is measured on - is measured on a balance scale a spring scale - is the amount of - is the force of gravity matter in a body pulling on the mass SCALAR - VECTOR - is the same - Depends on where wherever you go you are (planets, levels) - Has the units of… - Has the units of… Kg metric Newton Metric slug imperial Pound imperial - Is a fundamental unit - derived unit N= kg* m/s2

19. however, mass and weight are related Force = mass • acceleration Weight(force) = mass • gravity N= kg • m/s2 W= mg g= W/m m= W/g • Look at units • Mass = Weight/ Gravity • Mass = N/ m/s2 • M= kg m/s2 / m/s2 • M= Kg

20. The Normal Forcep.141 in text • A force exerted by one object on another in a direction perpendicular to the surface of contact. • Many times the normal force will be the contact force an object has with the ground. • If the object is on a slope(ramp), the normal will be perpendicular with the slope.

21. Normal Force Draw a FBD for this scene, the object is the t.v. t.v. table

22. Normal Force on a ramp Normal Force can be calculated. It’s the perpendicular component of the weight. The angle with the weight vector and perpendicular component is the same as the ramp angle.

24. p. 142 text Friction • any force opposing motion • occurs between any two surfaces that are touching • No surface is perfectly smooth • 2 Kinds of Friction • Static: occurs when no motion is present between two surfaces • Kinetic: (dynamic) occurs when objects on surfaces slide past one another (sliding) • STATIC is the greater of the two

25. Coefficient of friction  • is a unit less number used to describe how much friction is present for certain material • lesser the friction, lower the number • greater the friction, greater the number

26. hot car tire • car tire • sand paper • tile floor • soapy water • ice • Teflon

27. µ is the symbol for the coefficient of friction (mu or mew) • Normal force • this is an equal and opposite force to the weight of an object that sits on a level floor. On an incline the normal force will be different than the weight. Fn W

28. Friction, µ, and normal force are related this way µ = Ff Fn Ff= µ Fn Ff= µ W Ff= µ m g Fn= Ff µ

29. Practice Problems 4C p.145 • 1 0.23 • 2 a. 1.5 b. 1.3 • 3 a. 870N , 670N b. 110N , 84N c. 1000N , 500N d. 5N , 2N

30. Fnormal • many forces act on a moving object Fapplied Ffriction Fweight

31. Balanced and Unbalanced Forces • Sometimes all the forces on an object will cancel out. We say that object is in equilibrium or a state of balance • Sometimes all the forces will not cancel out and we say there is a Net Force acting on the object. • The Net Force is the Force that causes acceleration.

32. Consider the Forces on this box, are they balanced or not?

33. **over head bucket problem • sometimes Fnet = Zero • when fnet = 0 -- No acceleration will occur • Remember, even without acceleration, you can still have velocity • When Fnet= 0 for a moving object, that object is moving at CONSTANT VELOCITY

34. for the case of constant velocity friction force = applied force Ffr = Fapp this occurs when you are on cruise control at highway speed, going in a straight path. -- no acceleration but you still have velocity

36. Free Fall • all objects fall at the same accelerating rate, with out air resistance • This is ONLY true when air resistance is not present • Galileo showed this is true, but he couldn’t explain why

37. NEWTON”S 2nd LAW • Here is why its true: A= F/m F/ m = F/m

38. lets compare a 1 kg ball to a 10 kg ball Weight = 10N = 100N 10 N/ 1 KG = 10 m/s2 100N / 10 Kg= 10 m/s2 You cannot just look at mass or weight alone, you must consider them together A big mass has a big force A small mass has a small force

39. Falling With Air • air resistance is a frictional force, air resistance causes objects to slow, even to where no acceleration occurs. When Fnet = 0 No Acceleration When air resistance = weight of object Fnet= 0 N

42. http://www.youtube.com/watch?v=jybIWhW7BNM

43. terminal velocity occurs when • Acceleration = 0 • Fnet = 0 • Weight - resistance = 0 A = Fnet/ mass = Weight – Resistance/ Mass = 0 / Mass = 0 m/s2 Falling ants Sky Divers Flying squirrels

45. Terminal Velocity • As an object moves through the air, it collides w/ air molecules that exert a force on it. (drag, Ffr, air resistance) The force depends on the size and shape of the object, the density of air, and the speed of motion. • The faster you go, the more air resistance you get, at some point drag force will equal the force of gravity ** overhead plane model**

46. THE END Studyfor thetest!!