Newton s First Law of Motion

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1 Newton s First Law of Motion

2 Learning Target Target 1: Use Newton s Laws of Motion to describe and predict motion Explain, draw and interpret force vector diagrams Predict direction and magnitude of motion using force vector diagrams

3 Learning Target Target 2: The only reason to learn algebra is to do physics Calculate the net force acting on an object Calculate the distance, speed and time of an object (D=RT) Rate, Speed, and velocity are all calculated the same way using this formula

4 Galileo s Concept of Inertia For 2000 years, the Aristotelian theory that heavy objects fall faster than lighter ones prevailed Galileo was the first to actually test the theory with an experiment

5 Galileo s Concept of Inertia I do not feel obliged to believe that the same God who has endowed us with sense, reason and intellect has intended us to forgo their use. -- Galileo Galileo Galilei: Old dead Italian guy = Very Smart

6 Galileo s Concept of Inertia Galileo Galilei was born in Pisa, Italy on February 15, 1564 Studied medicine, mathematics, physics and philosophy Galileo pioneered modern experimental scientific method (some say invented it) Described and supported the heliocentric theory of the solar system For which the Roman Catholic Church s Inquisition convicted him of heresy Forced him to recant & was sentenced to life imprisonment

7 Galileo s Concept of Inertia Used the telescope to discover the moons of Jupiter, sunspots and craters on the moon Laid the groundwork for Einstein s theory of relativity

8 Galileo s Concept of Inertia Legend says he dropped two objects from the leaning tower of Pisa More likely, he used balls rolling down inclines

9 Galileo s Concept of Inertia As the ball rolls down an incline it picks up speed (accelerates) due to gravity As the ball rolls up an incline it loses speed (accelerates) due to gravity Acceleration is any change in speed or direction Initial Position on the incline = Final Position on the incline

10 Galileo s Concept of Inertia Initial Position Final Position Initial Position = Final Position Initial Position Final Position Initial Position = Final Position

11 Galileo s Concept of Inertia Initial Position Final Position? Initial Position = Final Position A ball rolling level does so neither against or with gravity It neither speeds up nor slows down So what stops it?

12 Galileo s Concept of Inertia Galileo reasoned that the ball would stop due to loss of kinetic energy to friction Therefore: The ball would never stop if there was no friction

13 Newton s 1 st Law What you think you know so far Part 1: An object at rest stays at rest unless an outside force acts upon it Part 2: An object in motion stays in motion unless an outside force acts upon it

14 Newton s 1 st Law But even at rest, aren t there many forces acting on you? Aren t these forces outside you?

15 Newton s 1 st Law Gravity is the force that pulls you towards the center of the Earth It is always on (even in weekends and holidays) and operates only in one direction

16 Newton s 1 st Law Support Force (normal force) is the force that supports an object against gravity If gravity were left unchecked, then you would crash though the chair, the floor, the crust, the mantle, outer core and finally come to rest in the inner core

17 Newton s 1 st Law So the phrase unless an outside force acts upon it doesn t seem to make a lot of sense

18 Newton s 1 st Law What we need to change for Part 1: Force is a push or pull Net force is the total sum of forces acting on an object

19 Newton s 1 st Law So we change Part 1, adding net in front of force An object at rest stays at rest unless a net force acts upon it When the net force = 0, we don t move Only when the net force > zero do we move

20 Newton s 1 st Law What we need to change for Part 2: Force is a push or pull Net force is the total sum of forces acting on an object

21 Newton s 1 st Law So we change Part 2, adding net in front of force An object in motion stays in motion unless a net force acts upon it When the net force = zero, we stay in motion Only when the net force > zero do we unmotion Hmmm. But what kind of motion?

22 Newton s 1 st Law So we change Part 2 again: If an object is in straight-line motion, it will continue in straight-line motion unless acted upon by a net force No, it is still not quite right

23 Newton s 1 st Law So let s think a little When we know both speed and direction of an object we know its velocity Constant velocity = unchanging speed and direction (constant speed and direction) Neither slows down or speeds up or changes direction Now we are on to something!

24 Newton s 1 st Law So we change Part 2 yet again: If an object is in constant velocity, then it will remain in constant velocity unless acted upon by a net force Brilliant!

25 Newton s 1 st Law Newton s First Law: The Law of Inertia If an object is at rest, then it will remain at rest unless a net force acts upon it Static equilibrium If an object is in constant velocity, then it will remain in constant velocity unless a net force acts upon it Dynamic equilibrium

26 Newton s 1 st Law Even fewer words: Objects in equilibrium remain in equilibrium unless a net force acts upon them

27 Newton s 1 st Law So, apparently, an object really only wants to do one thing Remain in equilibrium (static or dynamic) Unless a net force acts on an object, it will stay in equilibrium Net force < zero, objects do not leave equilibrium

28 Newton s 1 st Law So why do we need a net force? To get out of equilibrium! To start moving To stop moving TO change speed Slow down or speed up Change direction

29 Newton s 1 st Law A hockey puck is set in motion across a frozen pond. If ice friction and air resistance are ignored, then the force required to keep the puck sliding at constant velocity is

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32 Galileo: Speed and Velocity Before G-Money, things moving were described as simply fast or slow G-Money defined speed scientifically as the distance traveled per unit of time Speed = distance/time

33 Galileo: Speed and Velocity When we know both speed and direction of an object, we know its velocity Traveling 60 km/h to the north Such a quantity that specifies magnitude and direction is a vector quantity

34 Galileo: Speed and Velocity Constant speed = steady speed Neither slows nor speeds up Constant velocity = steady speed and direction Neither slows down, speeds up, or changes direction

35 Learning Target Target 1: Use Newton s Laws of Motion to describe and predict motion Explain, draw and interpret force vector diagrams Predict direction and magnitude of motion using force vector diagrams

36 Learning Target Target 2: The only reason to learn algebra is to do physics Calculate the net force acting on an object Calculate the distance, speed and time of an object (D=RT) Calculate the acceleration of an object (A=F/M)