This document discusses force and motion, including Newton's three laws of motion. It explains that an object's motion changes when a force acts upon it. Newton's first law states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force. Friction is introduced as a force that opposes motion. The document discusses the two main types of friction - static and sliding friction - and how friction depends on the surfaces in contact and an object's mass. Methods for reducing friction, such as lubrication and rolling motion, are also covered.

1. Force and Motion

2. Motion is the process of an object moving. An object’s motion changes when a force acts upon it.

3. Newton’s Three Laws of Motion

4. Newton’s First Law of Motion--Inertia • an object at rest stays at rest unless acted on by another force • an object in motion stays in motion unless acted on by another force

5. Motion is a relative term. All matter in the universe is moving all the time, but the motion referred to in the first law is a position change in relation to surroundings. We live on the Earth which is rapidly rotating and orbiting the Sun. But when we sit down we say we are at rest.

6. When you are sitting in your seat in an airplane flying through the sky, you are at rest. But, if you get up and walk down the airplanes aisle, you are in motion.

7. In order to understand the first law it is important to understanding balanced and unbalanced forces. If you hold a ball in your hand and keep it still, the ball is at rest. All the time the ball is held there, it is being acted upon by forces. The force of gravity is trying to pull the ball downward, while at the same time your hand is pushing against the ball to hold it up. The forces acting on the ball are balanced.

8. Let the ball go, or move your hand upward, and the forces become unbalanced. The ball then changes from a state of rest to a state of motion.

9. If you are not in motion right now, chances are that you have balanced forces acting on you .

10. Now let’s get back to discussing the first part of Newton 1st Law of Motion. The first part of this law seems pretty obvious—an object stays at rest until a force acts upon it.

11. A ball sitting on the ground is at rest and when it is rolling or flying it is in motion.

12. Furthermore, a resting ball stays resting until a force acts upon it—in this case a moving foot.

13. The second part of this law is less obvious—an object in motion stays in motion until a force acts upon it. This is a difficult concept because in our experience things do slow down and stop, they don’t keep moving in a straight line and at the same speed. The reason, of course, is that there is a force acting on those things. The force is usually friction, which we will study later.

14. This second part of the first law of motion explains why we should wear seat belts. The car and person are both in motion and when the car stops abruptly the person stays in motion flying out of the car.

15. Astronauts who “walk in space” are tethered to the shuttle or space station so they do not float off into space. Otherwise, when they push against the spacecraft, they would start moving away from the ship and continue moving out into space in a straight line until acted on by another force.

16. Activity with car and clay Activity: stack of pattern blocks and penny on index card.

17. Newton’s First Law of Motion combined with the Law of Gravity explains why a planet or moon orbits another (and larger) object. The planet or moon is actually moving in a straight line that would carry it away from the larger object it is orbiting. At the same time, the force of gravity pulls the planet or moon towards the larger object. As a result of the two balanced forces, the planet or moon keeps falling into orbit around the larger object.

18. Newton’s Second Law of Motion An object’s acceleration depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the force acting upon an object is increased, the acceleration of the object also increases. As the mass of an object is increased, the acceleration of the object decreases.

19. Acceleration is either a change in speed (speeding up or slowing down) or a change in direction. Same speed, same direction, this is not acceleration. Speeding up or slowing down, is acceleration. A change in direction is acceleration

20. First: An object’s acceleration is directly proportional to the force. For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater. If you push twice as hard, it will accelerate twice as much.

21. Second: This acceleration is inversely proportional to the mass of the object. For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other. If it gains twice that mass it will accelerate half as much.

22. Sometimes a picture can say more than words. Let’s see. We have a large force and a small mass. The large force is applied to the small mass. The small mass accelerates rapidly.

23. Or, in the other case: We have a small force and a large mass. The small force is applied to the large mass. The large mass accelerates slowly.

24. A speeding bullet and a slow moving train both have tremendous force. The force of the bullet is a result of its incredible acceleration while the force of the train comes from its great mass.

25. A bowling ball has a lot more mass than a soccer ball. If a bowling ball and a soccer ball were both dropped at the same time from the roof of a tall building obviously, because it has more mass, the bowling ball would hit the ground with greater force than the soccer ball. We know that gravity accelerates all objects at the same rate, so both balls would hit the ground at the same time. . Therefore the difference in forces would be caused by the different masses of the balls. Newton stated this relationship in his second law, the force of an object is equal to its mass times its acceleration.

26. Therefore, the differences in force would be caused by the different masses of the two balls. Newton stated this relationship in his second law, the force of an object is equal to its mass times its acceleration.

27. If the mass of an object doubles, you would need to exert twice the force to accelerate it at the same rate. Force 50 N Force 100 N

28. Notice that doubling the force by adding another dog would double the acceleration. Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 m/s2. When you plug in the numbers for force in the illustration above, (100 N) and mass (50 kg), you find that the acceleration is 2 m/s2. Right granted for use for noncommercial use How Stuff Works

29. It is the force of gravity that causes an object to move down a ramp or inclined plane. The more mass an object has the greater the force of gravity pulling on it even in this situation. However, the acceleration of the objects be the same. They will move down the ramp at the same rate regardless of their mass.

30. Experiment to demonstrate Newton’s Second Law of Motion (balls of different masses)

31. Newton’s Third Law of Motion For every action, there is an equal and opposite reaction.

32. The rider steps off the skateboard. In the Third Law, the stepping off the skateboard is called the action. The skateboard responds to that action by traveling some distance in the opposite direction. The skateboard's opposite motion is called a reaction.

33. When you compare the distance traveled by the rider and the skateboard, it appears as if the skateboard has had a much greater reaction than the action of the rider. This is not the case. The reason the skateboard has traveled farther is that it has less mass than the rider—the Second Law of Motion.

34. If two people, both on skateboards, push on one another (action), they move away in the opposite direction as the push (reaction) .

35. When this man on roller skates pushes on the car, the car doesn’t move because it has great mass but he who has little mass rolls backwards.

36. When a gun fires, the bullet moves forward (action) causing the gun to recoil (reaction).

37. When a balloon full of air is sealed, the air pressure on both inside and outside are balanced, same pressure. When the balloon is not tied the air inside the balloon escapes and then the air pressure outside the balloon is greater than inside. As a result of the air moving out of the balloon in one direction, the balloon moves in the opposite direction—action, reaction.

38. In both the balloon and rocket engine shown above, gases rush downward (action) causing the balloon and rocket to go up (reaction).

39. Activity with balloon “rocket”

40. Along with Newton’s Laws of Motion, we now consider Friction. Considered by some to be one of the basic forces, friction is the force that opposes motion when an object’s surface is in contact with other objects. Although we seldom think about the role it plays, friction is crucial to many things we do....often making our lives more difficult and often making it easier.

41. For example, it is friction between the ground and the sole of our shoes that make walking possible and it is lack of friction that makes our feet slip on ice or highly polished surfaces. Without friction, the belts of machines would slip, nails and screws wouldn’t hold, wheels would spin without making things move. At the same time friction wastes energy and causes our machines to break down and to wear out.

42. Friction is the force that opposes motion. To move the blue bar over the orange bar, friction could be a problem. The greater the “load” the more “force” will be needed to overcome “friction.” force

43. The two major types of friction are: Sliding friction: The rubbing together of the surface of a moving body with the material over which it slides. Static friction: the force between two bodies in contact that opposes sliding.

44. Sliding friction-can be easily demonstrated in the classroom. Put both of your hands together and move them back and forth. Push your hands together harder and move them faster. What do you experience? Are your hands warming up? Do you hear the sound of the hands moving against each other? Friction results from the surface of your hands moving in opposite direction over each other. Because your hands are in motion this type of friction is known as sliding friction.

45. Many teachers have dealt with the problem of moving the “big” box of new books when all the carts were already taken. Here it is in graphic form. Sliding friction

46. Sliding friction between the: the broom and the floor the foot and the floor the hand and the hat

47. Now let’s look at static friction—the force between two bodies in contact that tends to oppose sliding. In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting.

48. Football players understand static friction well. When they first hit this blocking sled, it very much resists moving (static friction). Once moving, the sled becomes somewhat easier to push as sliding friction becomes the main force resisting movement.

49. If you have ever pushed a car you have experienced static friction. Initially you have to push really hard to get the car moving. That is static friction. Once you have the car rolling it is easier to keep the vehicle moving. That is sliding friction.

50. This picture shows static friction, just before the block moves. This picture shows sliding friction, while the block is in motion. It takes more force to get the block moving—static fiction than it does to keep it moving—sliding fiction.

51. (Activity: Use scales to determine the force (in newtons) required to move a brick in basket.)

52. The amount of friction encountered, either sliding or static, will depend on two things: 1. How smooth two surfaces are that are touching. 2. The weight of the moving body or the body you are trying to get to move.

53. Even though a surface may look very smooth, friction occurs in part because no surface is perfectly smooth. Rough surfaces have grooves and ridges which catch on one another as the two surfaces slide past each other. When two surfaces try to move past each other these little bumps collide and slow down the motion of the surfaces.

54. The rougher a surface is, the more and bigger bumps it has--more friction. The smoother a surface is, the fewer and smaller bumps—less friction. For example if you slide a wooden block down a ramp it will be slowed by friction. If you sandpaper the block to make it smooth, the block will be smoother and slide faster. If you cover the block in sand paper (making it rougher) the block moves even slower because of the sandpaper’s rough surface.

55. Even surfaces that are apparently smooth can be rough at the microscopic level. Under a microscope, no surface is really "smooth." No matter how smooth the surfaces may look to your eyes, there are many ridges and grooves. The ridges of each surface can get stuck in the grooves of the other.

56. Sandpaper viewed under a microscope

57. Cloth viewed under a microscope

58. Surface of a tile viewed under a microscope

59. Paper viewed under a microscope

60. Friction activity with different surfaces

61. Once more, the amount of friction encountered, either sliding or static, will depend on two things: 1. How smooth two surfaces are that are touching. 2. The weight (or mass) of the moving body or the body you are trying to get to move.

62. The more mass or weight an object has the more friction it has. Therefore it will take more force to get it moving and more force to keep it moving. A dump truck has more mass than a Smart Car.

63. The affect of weight on friction: If it takes 10 newtons of force to slide a block with a weight of 50 newtons, it will take 20 newtons of force to slide a block that weighs 100 newtons:

64. Very interesting!!! Friction does not depend on the amount of surface area in contact between an object and the ground, as demonstrated in Example B.

65. So, is friction good or bad? The answer, of course, is YES. Sometimes friction works against us and sometimes it works for us. It depends on the situation.

66. How does friction works against us? Friction between the moving parts of an engine resists the engine’s motion and turns energy into heat, reducing the the efficiency of the machine and causing it to wear out. Friction also makes it difficult to slide a heavy object, such as a refrigerator or bookcase across the floor.

67. In others situations, friction is helpful. We would be unable to walk if there was no friction between our shoes and the ground. It is that friction that allows us to push off the ground without slipping. On a slick surface, such as ice, shoes slip and slide instead of gripping. This lack of friction, makes walking difficult.

68. Friction allows the tires on our vehicles to grip and roll along the road without skidding.

69. Activity with person walking on board on dowels

70. Friction between nails, screws and beams prevents the nails and screws from sliding out stopping our buildings from collapsing.

71. We want tread on our tires so we can drive our cars and to prevent us from slipping around on wet surfaces.

72. We want tread our footwear so we can gain traction.

73. Often however, we wish to reduce friction. Less friction makes it easier to move things. Reducing the amount of friction in a machine increases the machine’s efficiency. Less friction means less energy lost to heat, less noise and less wear and tear on the machine.

74. People normally use three methods to reduce friction. The first method involves reducing the roughness of the surfaces in contact. For example, sanding materials lessens the amount of friction between the two surfaces when they slide against one another.

75. The second method is to use smooth materials which create less friction.

76. Or by putting a smooth surface under the rougher surface.

77. The third way to reduce friction is often the best way--replace static or sliding friction with rolling friction and/or add a lubricant. Rolling friction: Instead of sliding surfaces together, you can place rollers between them. Lubricant: By adding a thin layer of oil or grease between two objects, you can reduce static or sliding friction and lessen wear on machines.

78. Rolling friction When a cylindrical or spherical body rolls over a surface, the force opposing the motion is called rolling friction. Adding rollers between two surfaces reduces friction.

79. Ball bearings are an example of rolling friction.

80. Friction can be reduced by adding a lubricant such as grease or oil between the two surfaces. Lubricants reduce friction by minimizing the contact between rough surfaces. The lubricant’s particles slide easily against each other resulting in far less friction.

81. Activity with shaving cream as a lubricant and Air Carts using air to separate surfaces and reduce friction.

82. Lubricants decrease the amount of energy lost to heat and damage to machine surfaces.

83. Grease Oil Two common lubricants

84. That’s it for today... MAY THE FORCE BE WITH YOU!