2. Mr. P Science – 8 2015/16

3.  Force is defined as a push or a pull. Force is measured in Newtons 1 N = 1 kg x 1 m/s/s

4.  Slow them down  Speed them up  Stop them  Start them  Change their direction  Change their shape

5. Balanced Forces Forces acting on an object that combine to produce a net force equal to zero Think of tug of war with no winner Unbalanced Forces Forces acting on an object that combine to produce a net nonzero force Think of the same tug of war, only this time one team wins Net force = 0 100 N 150 N50 N Net force = 100 N

6. 3 N, right – 6 N, left = 3N, left

7. 4 N, left – 10 N, right =

8. 5 N, right + 10 N, right =

9. a. An applied force is a force that is applied to an object by a person or another object. b. The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. c. The normal force is the support force exerted upon an object that is in contact with another stable object. d. The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. e. The air resistance is a special type of frictional force that acts upon objects as they travel through the air. f. The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. g. The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. h. The protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other. i. Two magnets can exert a magnetic pull or push on each other.

10. Contact Forces Frictional Force Tension Force Normal Force Air Resistance Force Applied Force Spring Force Action-at-a-Distance Forces Gravitational Force Electrical Force Magnetic Force

11.  Gravity is the word we use to describe the pull between masses. Things with more MASS produce a greater Gravity pull on each other. Everything has gravitational force, even you! – Your gravitational force is very small – Earth’s gravitational force is very large

12. 1. The mass of the objects. 2. The distance between objects. As the distance of the object changes so does the gravitational force between them Closer together = stronger Farther apart = weaker

13.  A roller coaster’s ride  A diver’s dive  A skier’s jump  A baseball’s fall

15. Your mass is the amount of material that your body contains. How can you gain mass? I’d have to eat a lot of yummy bread to gain mass!

17. So, having more mass does not necessarily mean taking up more space!

18. Do things that have more mass always weigh more?

19.  When you stand on a bathroom scale, the scale is made of springs. The farther down you push the springs, the higher the number on the scale. The number tells you your weight. On earth it is true that the more mass you have, the more weight you have.

20. Weight? Mass? I am totally confused!

21.  Remember, gravity describes the pull between masses. You have mass, and the earth has mass. There is a pull between you and the earth. We measure this pull with a scale. The more the gravity, the greater the squeeze on the springs of the scale…and the more you weigh.

23. You are floating around. You drift over to a floating bathroom scale and put your feet on it. Your feet do not push down on the scale at all. The scale shows that you weigh 0. You have lost your weight—but did you lose your mass? Oh where, oh where, has my weight gone? Oh where, oh where can it be?

24. OR Earth Moon

25. So, if I ate a lot of yummy tuna fish while standing on the scale, I would gain mass. The pull between the earth and me would become greater—and I would weigh more!

26. During my trip to the moon, my mass does not change, but the moon has much less mass than the earth. So your weight would be about 1/6 of what it is on earth!

27.  Your weight on earth is a measure of the GRAVITY pull between you and the earth. Bodies with more MASS produce a greater GRAVITY pull on each other.

28.  Your MASS is the amount of material in your body.  Your MASS doesn’t depend on where you are.  Your WEIGHT is how much your body pushes down on a scale (your mass plus the pull of gravity).  Your WEIGHT depends on how much MASS you have and where you are.

29.  Motion is the change in position relative to the frame of reference in a certain amount of time.

30.  The background or point that is used for comparison when motion is described – usually a stationary object.  The most common frame of reference is the Earth.  Other examples include …

31.  A way to describe motion › Average speed - Rate of motion calculated by dividing the distance traveled by the amount of time it takes to travel that distance › Constant speed - Speed that does not change › Instantaneous speed - Speed of an object at any given time

32. Speed is calculated by dividing distance by time –

33. A football field is about 100 m long. If it takes a person 20 seconds to run its length, how fast was the football player running? Speed = Distance ÷ Time Speed = 100 m ÷ 20 s Speed = 5 m/s Remember to include the UNITS!!

34.  Speed describes distance and time  Velocity describes distance, time, and direction

35.  A change in velocity – which may be: 1. A change in speed, or 2. A change in direction at a constant speed  Acceleration is caused by unbalanced forces

36. Deceleration is also called negative acceleration - it means an object is slowing down When acceleration is calculated, it may be a negative number

37. acceleration = velocity (final) – velocity (initial) time ora = ∆ v t Δ delta sign means change

38.  In the absence of air resistance, the elephant and the feather hit the ground at the same time.  Why is this so?  All objects (regardless of their mass) experience the same acceleration when in a state of free fall. free fall

39.  When the only force is gravity, the acceleration is the same value for all objects.  On Earth, this acceleration value is 9.8 m/s/s. Aka: the acceleration of gravity  When an object is falling, its velocity increases, but so does the air resistance against it. At a certain point, the air resistance will equal the force of gravity and the object will no longer accelerate. This is called its Terminal Velocity.

40. Friction is the force between two objects in contact that opposes the motion of either object. Frictional force varies depending on the surfaces and pressure. Friction produces a wearing down of the surfaces and heat. Car tires are designed to use friction to increase their grip on the road. – So tell me, why are there so many different types of tires?

41.  Mass in motion  Also has direction  The product of the object’s mass and velocity Momentum = mass x velocity p = mv (for straight line motion)

42.  Larger objects are harder to stop – Greater mass  Speed also effects the ability to stop  The faster an object is moving, the greater its momentum

43. The Law of Conservation of Momentum:  The total momentum of a group of objects that interact remains the same unless outside forces act on the objects.

45.  Moving objects tend to continue moving at a constant speed and in a straight path unless acted upon by an unbalanced force  Objects at rest tend to stay at rest unless acted upon by an unbalanced force  The more mass an object has, the more inertia it has ( inertia – tendency of an object to resist a change in motion) › More massive objects are harder to start moving and stop moving › Smaller objects are easier to start and stop moving

46. Newton’s First Law on Motion describes the idea of inertia  An object at rest or in constant motion is acted upon by balanced forces – an unbalanced force will change the motion  Acceleration of an object at rest or in constant motion is 0 m/s/s (no acceleration)

47.  Describes motion created by unbalanced forces  Mass and acceleration change in opposite ways › The more mass an object has, the more force it takes to accelerate the object, the slower it accelerates › The less mass an object has, the less force it takes to accelerate the object, the faster it accelerates Net force of an object = product of its mass and acceleration f = ma

48.  Describes why forces act in pairs  For every action there is an equal and opposite reaction  Action and reaction forces are equal forces acting in opposite directions on different objects

49. Centripetal forces keep an object or these children on a merry-go-round moving in a circular path. Centripetal forces keep an object or these children on a merry-go-round moving in a circular path. The force is directed toward the center of the circular motion.

50. Uniform circular motion Uniform circular motion is motion along a circular path in which there is no change in speed, only a change in direction. Constant velocity tangent to path. Constant force toward center. Question: Is there an outward force on the ball?

51. The question of an outward force can be resolved by asking what happens when the string breaks! Ball moves tangent to path, NOT outward as might be expected. When central force is removed, ball continues in straight line. Centripetal force is needed to change direction. v

52. You are sitting on the seat next to the outside door. What is the direction of the resultant force on you as you turn? Is it away from center or toward center of the turn?  Car going around a curve. Force ON you is toward the center.

53. There is an outward force, but it does not act ON you. It is the reaction force exerted BY you ON the door. It affects only the door. The centripetal force is exerted BY the door ON you. (Centrally) F’Reaction FcFc

54. We call an object’s tendency to resist a change in its motion its inertia. An object moving in a circle is constantly changing its direction of motion. Although the centripetal force pushes you toward the center of the circular path......it seems as if there also is a force pushing you to the outside. This apparent outward force is called centrifugal force.

55.  Centrifugal force is not a true force exerted on your body.  It is simply your tendency to move in a straight line due to inertia.  This is easy to observe by twirling a small object at the end of a string.  When the string is released, the object flies off in a straight line tangent to the circle.

56. The graph shows an object which is not moving (at rest). The distance stays the same as time goes by because it is not moving.

57. The graph shows that the objects distance increases as time passes. The object is moving and so it has velocity. The straight line shows it is a constant (not changing).

58. Just like the previous graph, this graph shows an object moving with constant velocity

59. The curve in the graph shows that the objects velocity is changing as time passes. This is acceleration.

60. In the first part of the graph the object is moving with constant velocity. In the second part of the graph the object is at rest (not moving). In the third part the object is again moving with constant velocity.

61. The graph shows that the objects velocity does not change as time passes. It shows constant velocity.

62. The graph shows that the objects velocity is increasing as time passes – it is accelerating. The straight line shows that it is constant acceleration.