1. FORCES

2. Chapter Five: Forces  5.1 Forces  5.2 Friction  5.3 Forces and Equilibrium

3. Chapter 5.1 Learning Goals  Define force as a vector and describe how it is measured.  Explain how forces are created.  Compare and contrast types of forces.

4.  Key Question:  What is force and how is it measured?  What are the ways forces cause change? Forces Change Things Work and Forces in Living Things

5. The cause of forces  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.

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

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

8. 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.

9. Pounds  When you measure weight in pounds on a postal scale, you are measuring the force of gravity acting on an object.

10. 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. 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. Forces in Living Things Plant responses How does a seed respond to its environment? It responds to gravity, light, temperature, and water. What pushes or pulls are involved in the seed growing? Gravity pulls on the root while it pushes through the soil, and the leaves push upward to reach the sunlight..

14. Forces in Living Things Plant responses  Plants respond to a stimulus (internal forces)by growing either away or toward the stimulus.  Plant motion affected by internal forces is called tropism. A change in the growth of a plant due to light is called phototropism.

15.  Plants also grow in response to gravity (gravitropism).  If a plant is turned upside down, it will grow away from the pull of gravity and turn upward.

16.  What forces affect the motion of plants? Plant motion is affected by internal forces called tropisms.  What is Turgor pressure pressure exerted by water inside the cell on the cell wall  What are some examples of force? Push, pull, gravity, magnetism, turgor pressure)

17.  Part 2- Plaster Seeds  What affect did the seeds have on the plaster? Over time, the plaster cracked.  Why do you think this occurred? The seeds broke the plaster  What was going on in the seed? The seed soaked up water which caused the cells of the seed to swell. This swelling created pressure inside the seed, which broke the plaster surrounding the seed. As the seedling grows, the number of cells increases. This growth continued to increase the pressure cracking the plaster. This force continues as the seedling emerges.) 

18. Part 3- Potato Strips  1.Describe the differences in the two potato strips. The strip soaked in salt water was limp and bent rather than broke when force was applied. The potato soaked in water was crisp and snapped when force was applied) 

19.  What force caused these changes? The salt in the water surrounding the potato pulled the water inside the cells of the potato to move out of the cell. This caused the potato to become limp. When the potato was soaked in fresh water, water entered the cells of the potato causing the cells to swell. The cell walls exert pressure, called turgor pressure, as the cells take up water. This pressure caused the potato to be crisp and break easily.

20.  I was in the grocery store yesterday, and as I was walking by the lettuce and carrots, the sprayers came on. Has this ever happened to you? Why do they need to spray water on their vegetables? The water in vegetables evaporates over time causing the produce to wilt. Spraying water on them increases the turgor pressure keeping the vegetables crisp.

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

22. 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.

24. 5.1 Spring forces  Springs are used to make or control forces.  The force from a spring always acts to return the spring to its resting shape. Which of these springs is designed to be stretched? Which is designed to be compressed?

25. 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.

26. 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.

27. 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 force measured in newtons (N).  Weight depends on mass and gravity.

28. 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.

30. 5.1 Calculating weight  The weight equation can be rearranged into three forms to calculate weight, mass, or the strength of gravity.

32.  Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars. 1.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 2.Relationships:  W = m x g 3.Solution:  60 kg x 9.8 N/kg = 588 N  60 kg x 3.7 N/kg = 222 N Sig. fig. = 600 N Solving Problems Sig. fig. = 200 N

33. Investigation 5C  Key Question: How does gravity affect the motion of falling objects? Gravity and Falling Objects

34. Chapter Five: Forces  5.1 Forces  5.2 Friction  5.3 Forces and Equilibrium

35. Chapter 5.2 Learning Goals  Define friction.  Identify causes of friction.  Distinguish among various types of friction.

36. Investigation 5B  Key Question: How does friction affect motion? Friction

37. 5.2 Friction  Friction is a force that resists the motion of objects or surfaces.  Many kinds of friction exist.

38. 5.2 Friction

39. 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.

40. 5.2 Identifying friction forces  Friction is a force, measured in newtons just like any other force.  Static friction keeps an object at rest from moving.

41.  Sliding friction is a force that resists the motion of an object moving across a surface. 5.2 Identifying friction forces

42. 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.

43. 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.

44. 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.

45. 5.2 Using friction  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.

46. 5.2 Using friction  Shoes are designed to increase the friction between their soles and the ground. How do you think these shoes increase friction?

47. 5.2 Friction and energy  Friction changes energy of motion into heat energy.

48. 5.2 Friction and energy  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.

49. 5.2 Friction and energy  Each time two moving surfaces touch each other, tiny bits of material are broken off by friction.  Breaking off bits of material uses energy.

50. Chapter Five: Forces  5.1 Forces  5.2 Friction  5.3 Forces and Equilibrium

51. 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.

52. 5.3 Forces and Equilibrium  The sum of all the forces on an object is called the net force.  The word net means total but also means the direction of the forces has been taken into account. In what direction will this plane go?

53. 5.3 Adding forces  To figure out if or how an object will move, we look at ALL of the forces acting on it.  Four forces act on a plane: 1.weight 2.drag (air friction) 3.the thrust of the engines, and 4.the lift force caused by the flow of air over the wings.

56. 5.3 Equilibrium When several forces act on the same object: 1.The net force is zero, or 2.The net force is NOT zero.

58. 5.3 Normal forces  When the forces are balanced, the net force is zero.  When the net force on an object is zero, we say the object is in equilibrium.

59. 5.3 Equilibrium and normal forces  A normal force is created whenever an object is in contact with a surface.  The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight. The normal force is sometimes called the support force.

60. 5.3 The free body diagram  How do you keep track of many forces with different directions?  Draw a free-body diagram that contains the objects, like a book on a table.

62. 5.3 Solving equilibrium problems  For an object to be in equilibrium, all the forces acting on the object must add up to zero. Is this object in equilibrium?

63. Solving Problems Two chains are used to support a small boat weighing 1,500 newtons. One chain has a tension of 600 newtons. What is the force exerted by the other chain?

64. 1.Looking for:  …tension on chain 2 2.Given  …weight boat = 1,200N; tension 1 = 600 N  Implied: weight and tension are forces 3.Relationships:  Net force on boat = zero Solving Problems

65. 4.Solution:  Draw free body diagram Solving Problems  Upward force of chains = weight of boat  600 N + tension 2 = 1,200 N  tension 2 = 900 N

66. Parabolic Flights  NASA has been conducting parabolic flights since the 1950s to train astronauts. Scientists and college students have also gone on parabolic flights to perform a wide variety of chemistry, biology, and physics experiments.