Chapter 2. Forces & Newton s Laws

Transcription

1 Chapter 2 Forces & Newton s Laws

2 1st thing you need to know Everything from chapter 1 Speed formula Acceleration formula All their units There is only 1 main formula, but some equations will utilize previous formulas from chapter 1

3 Section 1 Newton s First Law

4 Force - is a push or pull Objects will only speed up, slow down or change direction if something is pushing or pulling it. If you threw a ball forward. Your hand is exerting a force on the ball in a forward direction. Once it leaves your hand the ball moves forwards but curves to the ground because of the Earth s gravity (which is also a force.) Then the ground exerts a force on the ball when it comes in contact with it. Friction slows the ball to a stop.

5 Net Force - is the combination of forces acting on an object. When forces combine IF forces act on an object in the same direction (add them together) If forces act on an object in opposite directions (take the difference)

6 Copy 5 N net force = 5 N right 5 N 5 N net force = 10 N right 15 N 5 N 5 N 5 N net force = 10 N left net force = 0 N

7 Balanced forces - Two or more forces whose effects cancel each other out and do not change the motion of an object Unbalanced forces - two or more forces acting on an object that do not cancel, and cause the object to accelerate.

8 Newton s first law of motion An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted on by an unbalanced force.

9 Basically objects will keep doing what they are doing until Something bothers them

10 examples A ball will just sit there until something moves it. If someone kicks the ball it will keep on going in the same direction the force was applied and same speed (velocity) until something slows it down and stops it

11 Galileo Galilei Sir Isaac Newton Galileo was one of the first to understand force. Newton used Galileo s ideas and was able to better understand, explain, and define how forces act on objects

12 Friction - The force that brings nearly everything to a stop. Force that acts to oppose sliding between two surfaces that are touching. Friction will always slow a moving object. If you are on Earth you must apply more of a force on something than friction is applying on that object

13 3 types of FRICTION Static Sliding Rolling

14 static friction - friction that prevents an object from moving when a force is applied. Moving a couch across a carpet. It keeps an object at rest

15 sliding friction - friction that slows down an object that slides. Eventually a hockey puck will slow down and come to a stop if no one hits it.

16 rolling friction - occurs between the ground and a round object (such as a wheel) Without it wheels on a car would keep turning but wouldn t go anywhere.

17 Section 2 Newton s 2nd Law of Motion

18 Newton s 2nd Law of motion Connects force, mass, & acceleration. An object acted upon by a force will accelerate in the direction of the force.

19 Easy way to think of it: All objects have mass. If an object changes its movement it will be accelerating. This acceleration creates a push or a pull on an object. This push or pull is a force.

20 remember Acceleration is the change in VELOCITY divided by the TIME taken. It s SI UNIT is: m/s 2 The formula is: a = (V f - V i ) t

21 Formula F net = ma Net Force = Mass x Acceleration

22 Example: m = 10 kilograms a = 2 m/s 2 F net =

23 F net = ma F net = (10kg)(2m/s 2 ) F net = 20kg. m/s 2 F net = 20 N

24 The SI Unit for force is the Newton. (N) 1 N (newton) is the same thing as 1kg. m/s 2 (kilograms multiplied by meters divided by seconds squared)

25 BEWARE!!! Even though it is easier to write (N) You need to remember that (N) equals (kg. m/s 2 ) Why is this important If you were given grams, centimeters and minutes you would need to CONVERT these measures into kg, m, & s; other wise it wouldn t be a Newton.

26 GRAVITY It is a FORCE Is the attraction between any 2 objects that have mass. Gravity decreases when: The mass of the objects decrease The distance between the objects increase.

27 Example: You and the person sitting next to you have mass. But you aren t stuck to each other. Your mass isn t great enough to overcome the mass of the Earth. Black holes are the most massive object in the Universe. THEY HAVE THE GREATEST AMOUNT OF GRAVITATIONAL FORCE.

28 The Sun is much more massive that the Earth. we are far away from the Sun We are very close to the Earth. The force of gravity between you and the Earth is 1650 times greater than the force of gravity between you and the Sun.

29 Weight Your weight is the gravitational force between you and the Earth. If you were standing on another planet or moon, your weight would change.

30 The force of gravity causes all objects near the Earth s surface to fall with an acceleration of (9.8m/s 2 ) To Calculate your weight: F=ma F=(your mass)x(9.8m/s 2 )

31 Question In the UNITED STATES we measure weight in pounds. IN European and most other countries they speak of their weight in kilograms. When converting your weight to the metric system should you convert your weight to Kilograms or Newtons? Why?

32 Weight Vs. Mass Weight is a force. Mass is the amount of matter in an object and doesn t depend on its location. Your mass will remain the same everywhere in the universe. Your weight will vary depending on where you are standing in the universe. (this is why when you are in zero gravity they say you are weightless)

33 The 2nd law also explains how to calculate the acceleration of an object if its mass and the forces acting on it are both known. a = Fnet m

34 example Fnet = 10 N m = 2 kg a = a = Fnet m a = 10 N 2kg a = 10kg. m/s 2 2kg a = 5 m/s 2

35 The acceleration does not depend on how fast an object is going. It depends on how much of net force is applied to it and its mass.

36 A bug might be flying 30 mph at a car. The car might be traveling 20 mph, but the car has more mass & more force the car wins.

37 Circular Motion When objects travel in a circle (merry-go-round) they are constantly changing direction, thus they are constantly accelerating. Centripetal force - a force that acts on an object that is moving in a circular path and is directed towards the center around which the object is moving. Means center-seeking Will always travel perpendicular to the motion.

38 Parallel means: Perpendicular means:

39 If you were to swing a ball at the end of a string there would be tension placed on the string. That tension would be a force that is pulling (force) on the string towards the center that would be centripetal force.

40 Center of mass - is the point in an object that moves as if all the object s mass were concentrated at that point.

41 Spin & Slide a ruler across a desk all the mass will appear to be directed towards the center & the object will travel in a straight line.

42 Air resistance Whenever an object passes through air it pushes against that object creating friction that pushes against that object.

43 When an object falls DOWN gravity causes the object to accelerate. It will keep speeding up. BUT air pushes UP on the object. Slowing it down. The faster the object falls the greater the force air will be pushing up on the object. Eventually the force of the air will equal the weight of the object and the object s acceleration will equal zero and will no longer accelerate, therefore the speed will remain constant.

44 That was Terminal Velocity Terminal velocity - the force of gravity is balanced by air resistance. The size of the air resistance force depends on the shape of an object and its speed.

45 Remember: An object can speed up, slow down, or turn in the direction of the net force when unbalanced forces act on it.

46 Section 3 Newton s 3rd Law of Motion

47 3rd Law Forces always act in equal but opposite pairs For every action there is an equal but opposite reaction.

48 When you jump up you push DOWN on the ground BUT you go UP. The ground Pushed UP on you. Did the ground move UP? Yes. But because the ground (Earth) is so massive it is not noticeable.

49 Very massive objects have so much inertia (tendency to stay at rest) that they don t appear to move. This is how rockets are launched into space

50 Action-reaction forces do not cancel out because they are exerted on different objects. When actions-reaction forces are exerted by two objects, the accelerations of the objects depend on the masses of the objects.

51 Weightlessness A falling object is in free fall only if the only force acting on it is gravity. Weightlessness occurs in free fall when the weight of an object seems to be zero. Objects orbiting Earth appear to be weightless because they are in free fall in a curved path around Earth.