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# Forces

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1. Forces

2. Forces • A FORCE is a push or a pull exerted on an object • Forces can cause objects to speed up, slow down, or change direction. • Force is measured in Newtons

3. Force • Forces come in pairs • A force is a vector • Forces have a magnitude and a direction Magnitude: 5N Direction: north (up) 5N, north (up)

4. Types of Forces • There are two types of forces • Contact forces • Forces at a distance

5. Contact Forces • Contact forces- types of forces that result when the two interacting objects are perceived to be physically contacting each other • Examples of contact forces: • Applied force • Friction force • Tension force • 4. Normal force • 5. Air resistance

6. Applied Force • Identified as Fa • Applied to an object by a person or another object • Acts in the horizontal direction, parallel to the surface

7. Friction Force • Identified as Ff • Acts opposite sliding motion between surfaces • Acts in the horizontal direction, parallel to the surface and opposite the direction of sliding

8. Tension Force • Identified as FT • Pull exerted by a rope, string, or cable attached to a body and pulled tight • Acts away from the object and parallel to the string, rope or cable

9. Normal Force • Identified as FN • Exerted by a surface on an object (presses two surfaces together) • Acts in an upward, vertical direction, perpendicular to the surface • Sometimes referred to as the support force

10. Air Resistance Force • Identified as Fair • Type of frictional force that acts upon objects as they travel through the air • Acts opposite motion

11. Long Range Forces • Long Range Forces- forces that occur between objects that are not touching. Examples of long range forces: • Gravitational force • Electrical force • Magnetic force

12. Gravitational Force • All objects on Earth experience a force of gravity that is directed downward towards the center of the earth. • Identified as FW • Referred to as the force of weight

13. Balanced and Unbalanced Forces • Forces occur in pairs and they can be either balanced or unbalanced

14. Balanced Forces • Balanced forces do not cause change in motion, they are in equilibrium • They are equal in size and opposite in direction

15. Unbalanced Forces • An unbalanced force always causes a change in motion • Forces are not in equilibrium • When unbalanced forces act in opposite directions you can find the net force • The net force is the sum of all the forces acting on an object

16. Net Force • A net force shows magnitude and direction • The magnitude is the difference between forces acting on an object • The resulting motion of an object is in the direction of the largest force

17. Unbalanced Forces 3 N, right – 6 N, left = 3N, left

18. Unbalanced Forces 4 N, left – 10 N, right = 6N, right

19. Force Body Diagrams • Used to show all the forces acting on an object (can be 1,2,3, or 4 forces) • Can help you understand the motion of an object • Force vectors are represented by arrows • The length of the arrow represents the magnitude (size) of the force

20. How to Draw a Force Body Diagram • Step 1: Draw a sketch of the situation • Step 2: Identify the horizontal forces • Step 3: Identify the vertical forces • Step 4: Draw a box, place a dot in the middle • Step 5: Draw and label the horizontal forces • Step 6: Draw and label the vertical forces • Step 7: Calculate the sum of all the forces

21. Example • A book is at rest on a tabletop. Diagram the forces acting on the book.

22. Review: Net Force • The sum of all forces acting on an object • When net force is greater than zero, the object accelerates in the direction of the largest force • If the net force is zero the object is either • At rest • Moving with constant velocity

23. Force Body Diagram Interactive

24. Physics of Football http://www.nsf.gov/news/special_reports/football/lawofmotion.jsp What is Inertia? What does Newton’s First Law of Motion state?

25. Newton’s First Law • An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. • Newton’s first law is sometimes called the Law of Inertia

26. Inertia • Inertia is a property of matter that tends to resist a change of motion • If an object is at rest, it wants to stay at rest • If an object is in motion, the object wants to stay in motion

27. Newton’s 2nd Law • Newton’s 2nd Law helps is identify how an object’s motion changes when a net force is applied to the object • When forces are unbalanced, the object accelerates • Acceleration is dependent on two variables • Net force • The mass of an object

28. Newton’s 2nd Law • http://www.nbclearn.com/nfl • Causes a change in motion • A change in motion means a change in velocity • A change in velocity is called an acceleration • Newton’s 2nd law describes motion when unbalanced forces act on an object • If forces are unbalanced, the net force is greater than zero.

29. Mass Mass is the amount of matter that makes up an object Mass is a measure of an object’s inertia The more mass an object has, the greater its inertia The unit for measuring mass is the kilogram

30. Weight Weight is the force that results from the pull of gravity on an object’s mass The SI unit for weight is Newtons Can be determined by using the equation

31. Example Mass = 59.7 kg Force on moon: 95.5 N On Earth, a scale shows that you weigh 585 N. What is your mass? What would the scale read on the moon, where gravity is 1.67 m/s2?

32. Example 2: On Planet X a 50 kg barbell can be lifted by exerting a force of 180 N. What is the acceleration of gravity on Planet X?

33. Example 3: A 71.1-kg man travels from an area where the gravity is 9.78 m/s2 to where it is 9.80 m/s2. • What is his change in mass? • What is his change in weight?

34. Second Law Simulation • Forces Simulation • Independent variable: • Dependent variable: • Control Variable:

35. Newton’s 2nd Law Summary • The acceleration of an object depends directly on the net force acting on the object, and inverselyon the mass of the object. • As the force acting on an object is increased, the acceleration of the object is increased. • As the mass of an object is increased, the acceleration of the object is decreased • The acceleration is always in the direction of the net force

36. Example 1 A net force of 24 N is applied to an object whose mass is 14 kg. What is the object’s acceleration?

37. Example 2 • Thomas, the 72 kg star forward on Buford High School’s basketball team, collides with Killer, a guard for the other team, and is brought to a stop with an acceleration of –20 m/s2. What force does Killer exert on Thomas?

38. Question • If you push a skateboard with your hand does the skateboard keep moving at a constant speed after it leaves your hand? • What force is acting on the skateboard that is causing it to slow down? • What would happen if I stood on the skateboard and had someone push, would that affect the distance the skateboard would be able to travel? • http://www.nbclearn.com/portal/site/learn/science-of-the-olympic-winter-games

39. Predict • Will the toy walk faster on the lab table or the sandpaper? • Try it . . . Do several trials to make sure you are accurate. • Was your hypothesis correct? Explain why or why not.

40. A force that opposes motion Types of Friction Friction Notes Amount of friction depends on: KINETIC FRICTION STATIC FRICTION 1) Force (weight) pushing surfaces together Friction b/w MOVING surfaces Applying a force & NO movement 2)Roughness of the surface Rougher the surface, greater the friction SLIDING ROLLING Examples Examples Increasing force/weight, increases the friction

41. Harmful friction • Causes wear and tear • Example: car brakes • Helpful friction • Allows us to walk on certain surfaces • Example: different types of sport shoes

42. Changing Friction • To reduce friction • Use a lubricant (oil, wax, grease) • Use rolling friction • Make the surfaces smoother • Decrease the weight • To increase friction • Make the surfaces roughter • Increase the weight

43. Mu (Coefficient of Friction) • The friction between surfaces is a direct result of the surface type and the force pressing surfaces together, the normal force • On a flat surface, when vertical forces are balanced, the normal force is equal to the force of weight

44. Mu • The coefficient of friction refers to the “stickiness” of two surfaces • The closer to 1, the greater the friction • The closer to 0, the lower the friction

45. Mu • Is symbolized with μ • Mu is calculated using the ratio between the frictional force and the normal force

46. Example • A hockey puck has a coefficient of kinetic friction of μk = .10. If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck?

47. Example • A block has a weight of 5 N. An applied horizontal force of 1.5N is required to cause the block to slide with constant speed on a smooth concrete floor. • What does “constant speed” tell you about the shoe? • Draw a force diagram for the block. • What is the coefficient of sliding friction? • What is the mass of the block? • If the coefficient of friction were 0.10, what would be the acceleration of the block?