Chapter 5 and 6 Topics: Forces and Friction

1. Chapter 5 and 6 Topics: Forces and Friction

2. A force is a push or pull, or an action that has the ability to change motion. Forces can increase or decrease the speed of a moving object. Forces can also change the direction in which an object is moving. 5.1 The cause of forces

3. 5.1 How are forces created? • Forces are created in many ways. • For example, your muscles create force when you swing a baseball bat.

4. All forces in the universe come from only four basic forces. Electromagnetic forces are important to technology. Gravity is a universal force. Four Elemental Forces

5. Force And Net Force 1 • Force • Produce changes in motion • Produce a change in velocity • Cause an acceleration

6. Force And Net Force 2 • Force • A quantity that is capable of producing motion or a change in motion • Change in velocity • Change in acceleration

7. Force And Net Force 3 • A force may be canceled by one or more other forces • Net effect is zero • If a force acts alone • Change in velocity or acceleration

8. Force And Net Force 4 • Balanced forces • Equal in magnitude • Opposite in direction • Unbalanced forces • Motion occurs

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10. 5.1 Units of force • The pound is a unit of force commonly used in the United States. • For smaller amounts, pounds are divided into ounces (oz.). • There are 16 ounces in 1 pound.

11. 5.1 Newtons • Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons. • The newton (N) is a metric unit of force.

12. 5.1 Unit conversions • The newton (N) is a smaller unit of force than the pound (lb). • If one pound of force equals 4.448 newtons, then a 100 lb person weighs 444.8 newtons.

13. 5.1 The force vector • The direction of a force makes a big difference in what the force does. • That means force is a vector,like velocity or position. • Arrows are often used to show the direction of forces in diagrams.

15. Forces and vectors • Forces and vectors

16. 5.1 Drawing a force vector • The arrow points in the direction of the force.

17. 5.1 How forces act 1 • One way forces act is the result of direct contact. • A contact force is transmitted by matter directly touching other matter such as wind acting to slow a parachute.

18. 5.1 How forces act 2 • The force of gravity between Earth and Moon appears to be what people once called “action-at-a-distance”. • Today we know that the gravitational force is carried from the Earth to the Moon by a force field. Click pic 4 video

19. Classify these forces as contact forces or the result of force fields.

20. 5.1 Contact forces from ropes and springs • Ropes and springs are often used to make and apply forces. • Ropes are used to transfer forces or change their direction. • The pulling force carried by a rope is called tension. • Tension always acts along the direction of the rope.

22. 5.1 Spring forces • The force created by a spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length. • If you stretch a spring twice as much, it makes a force that is twice as strong.

23. 5.1 Gravity • The force of gravity on an object is called weight. • At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

24. 5.1 Weight vs. mass • Weight and mass are not the same. • Mass is a fundamental property of matter measured in kilograms (kg). • Weight is a forcemeasured in newtons(N). • Weight depends on mass and gravity.

25. Weight depends on mass and gravity A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

26. 5.1 Calculating weight

27. Solving Problems • Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars. • Looking for: • …weight of person in newtons on both planets • Given: • …mass = 60 kg; g = 3.7 N/kg on Mars; • …implied g = 9.8 N/kg on Earth • Relationships: • W = m x g • Solution: • 60 kg x 9.8 N/kg = 588 N • 60 kg x 3.7 N/kg = 222 N

28. 5.2 Learning Goals • Define what friction is. • Know what causes friction • Be able to list different types of friction. • From a picture identify the type of friction. • Describe how to reduce friction and uses of friction.

29. 5.2 Friction 1 • Frictionis a force that resists the motion of objects or surfaces. • Many kinds of friction exist. • Video Bill Friction 8 min

30. 5.2 Friction 2 Fill in boxes

31. 5.2 Friction and two surfaces • Friction depends on both of the surfaces in contact. • When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.

32. 5.2 Identifying friction forces 1 • Friction is a force, measured in newtons just like any other force. • Static friction keeps an object at rest from moving.

33. 5.2 Identifying friction forces 2 • Sliding friction is a force that resists the motion of an object moving across a surface. • Friction clip play @ 3:34

34. 5.2 A model for friction • Friction depends on a material’s properties such as roughness, how clean the surfaces are, and other factors. • The greater the force squeezing two surfaces together, the greater the friction force. v

35. 5.2 Reducing the force of friction • Unless a force is constantly applied, friction will slow all motion to a stop eventually. • It is impossible to completely get rid of friction, but it can be reduced.

36. 5.2 Reducing the force of friction • The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat. • Friction can be reduced by placing ball bearings between the shaft and the outer part.

37. 5.2 Using friction 1 • Friction is also important to anyone driving a car. • Grooved tire treads allow space for water to be channeled away from the road-tire contact point, allowing for more friction in wet conditions.

38. 5.2 Using friction 2 • Shoes are designed to increase the friction between their soles and the ground. How do you think these shoes increase friction?

39. 5.2 Friction and energy 1 Friction changes energy of motion into heat energy.

40. 5.2 Friction and energy 2 • Friction is always present in any machine with moving parts. • If the machine is small, or the forces are low, the amount of heat produced by friction may also be small.

41. 5.2 Friction and energy 3 • Each time two moving surfaces touch each other, tiny bits of material are broken off by friction. • Breaking off bits of material uses energy.

42. Chapter 5.3 Learning Goals • Determine the net force acting on an object. • Define equilibrium. • Draw free-body diagrams to represent all forces acting on a body.

43. Newton’s First Law of Motion 1 • What is the natural state of motion? • Aristotle • Objects require a force to stay in motion • Natural state of an object is at rest

44. Newton’s First Law of Motion 2 • Galileo • Objects could naturally remain in motion • Do not come to rest on their own

45. Newton’s First Law of Motion 3 • Newton • First Law of Motion • An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force

46. Newton’s First Law of Motion 4 • Uniform motion in a straight line • Constant velocity • Object at rest • Zero velocity • External forces • Internal forces

47. Newton’s First Law of Motion 5 • Constant velocity • No friction • No gravitational attraction

48. Newton’s First Law of Motion 6 • Inertia • Galileo • Objects tend to remain in • A state of rest • Uniform motion in a straight line

49. Newton’s First Law of Motion 7 • Newton • Mass is a measure of inertia • Greater the mass, the greater the inertia

50. Newton’s First Law of Motion 8 • Newton’s First Law • Law of inertia • Can apply to everyday life