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Forces 1 Dynamics FORCEMAN

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What causes things to move? Forces What is a force? –A push or a pull that one body exerts on another. 2

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Balanced 3 No change in motion

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Unbalanced If the forces acting on a body are "unbalanced" this means that there is a "net" or "resultant" force. The body will do one of the following: 1.Speed Up 2.Slow Down 3.Change the Direction of its Motion In other words The Body will Accelerate 5

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Newtons SI unit of force is the Newton (N). Defined as the amount of force that when acting on a 1 kg object produces an acceleration of 1m/s 2. Therefore, 1N = 1 kg m s 2 7

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Newton’s First Law 8

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An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force. What do you mean by “Net Force”? Net means: Final Sum, so net external force is the sum of all the forces acting on the object. 10

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Newton’s First Law Inertia is the tendency of an object to resist any change in its motion. Newton’s First Law is also called the Law of Inertia. 11

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Why does the ladder continue to move? Inertia! 12

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Why does the person continue to move? Inertia! Clip 13

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Newton’s Second Law The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass. F=ma Clip 14

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Force = mass x acceleration Newton's 2nd Law of Motion: Mass is in kg Acceleration is in m/s 2 Force is in kg x m/ s 2 (or N) F = (ma) 16

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Force Problems: What force is needed to give an object with a mass of 25.0 kg an acceleration of 15.0 m/s 2 ? Given:Unknown: m = 25.0 kg a = 15.0 m/sec² F = ? Formula F = ma F = (25 kg)(15 m/sec²) F = 375 kgm/sec² = 375 N 17

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Newton’s Third Law "Every action has an equal and opposite reaction."

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Newton ’ s Third Law of Motion 34 … to every action force there is an equal and opposite reaction force

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35 Action- Reaction forces act on different objects

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36 Review Newton’s Laws Clip

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Forces that oppose motion 18

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1919 -Friction is a force that opposes motion between two surfaces that are touching each other. Forces that oppose motion Friction Song 2 Friction Song 1

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-Air resistance is the force air exerts on a moving object Forces that oppose motion

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Friction What Causes Friction? Microscopic, electrostatic interactions between contacting surfaces Three types of friction: 1.Static Friction 2.Sliding Friction 3.Rolling Friction 20

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Types of Friction Types of friction: 1. Static – force required to overcome inertia of a stationary object 21

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Static Friction (F S ) 22

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To decrease friction, surfaces can be lubricated with a liquid such as oil. 3. Rolling – force required to keep an object rolling at a constant speed 2. Sliding – force required to keep an object sliding at a constant speed Types of Friction (cont’.) 23

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FRICTION LAB

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CLIP The amount of force required to overcome static friction is always greater than the force required to overcome sliding or rolling friction. 24

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GRAVITY!! TheThe force of gravity is an attractive force between objects. amount of GRAVITATIONAL FORCE between objects depends upon their: 1-1- Masses (greater mass means more gravity) 2-2- Distance apart (greater distance means less gravity) *All objects fall toward the earth at the same rate of acceleration, regardless of their masses! 25

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Calculating the Gravitational Force 26

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27 Gravitation

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Weightlessness Gravity in Gravity in Space 28 Moon Moon Hammer & Feather

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Free fall and Terminal Velocity The opposing force of air resistance increases with speed. Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance. This is the highest speed an object can reach. Different objects have different terminal velocities. 29

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Weight and Gravity 30 W e i g h t i s t h e f o r c e o f g r a v i t y b e t w e e n t h e e a r t h a n d a b o d y o n i t s s u r f a c e. W e i g h t ≠ m a s s ! W e i g h t i s m e a s u r e d i n N e w t o n s ( N ). BECAUSE IT’S A FORCE! W =m x g

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C o u l d y o u r w e i g h t c h a n g e b y s i m p l y g o i n g t o t h e t o p o f a m o u n t a i n ? 31

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Weight When the mass of an object and the acceleration due to gravity are known, the weight of an object can be calculated. 32 Ex. Mass = 100kg Acc.= 9.81 m/s2 Weight = 981 N W = (g)(m)

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Example Problem What is your weight if your mass 4.52 kilograms? Given:Unknown: m = 4.52 kgW = ? g = a = 9.8 m/sec² Formula: W = mg WW = (4.52kg)(9.8 m/sec²) WW = 44.296 kgm/sec² = 44.296 N 33

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Projectile Motion Projectiles follow a curved path because of the Earth’s gravitational pull. They have 2 types of motion that are independent of each other: 1. Horizontal motion 2. Vertical motion 37

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Projectile Motion Which will hit the ground first? A ball that is dropped straight down or a ball that is “shot” out horizontally? * An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!!!! 38

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Another Example of Projectile Motion 39

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Circular Motion An object moving in a circular motion at the same speed is accelerating toward the center because its direction is constantly changing. Centripetal acceleration is acceleration toward the center of a curved path. 40 'Centripetal' comes from the Latin word for 'center seeking' only centripetal is the real force Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line. Roller Coaster Clip

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Circular Motion cont.’ Centripetal force is the force that causes a moving object to move in a curved path Without a centripetal force, an object in motion continues along a straight-line path. With a centripetal force, an object in motion will be accelerated and change its direction. 41

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How do rollercoasters work!?! Amusement Park Forces 43

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