Chapter 2 motion
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Chapter 2 : MOTION. p.16 in your book!. Aristotle (384-322 BC) Objects have a proper “place” And strive to get there. NATURAL MOTION - No force required ex: boulder “falls down” smoke “goes up” Thought heavier objects fall faster than lighter objects.

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Chapter 2 : MOTION

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Chapter 2 motion

Chapter 2 : MOTION

p.16 in your book!


Chapter 2 motion

Aristotle (384-322 BC)

Objects have a proper “place” And strive to get there.

  • NATURAL MOTION - No force required

    ex: boulder “falls down”

    smoke “goes up”

    Thought heavier objects fall faster than lighter objects


Chapter 2 motion

UNNATURAL MOTION- Requires force

EX: push a book across table


Chapter 2 motion

Galileo-

  • Objects drop at same rate (except for air friction)

    “Leaning Tower of Pisa “ experiment

  • If no friction…no forces required to keep moving objects moving. EX:Satellites


Chapter 2 motion

As a ball rolls down an incline it speeds up

  • up incline,slows

  • Reduced angle, ball goes farther


Inertia

Inertia

Objects at rest tend to remain at rest.

Moving objects tend to remain moving.


Speed

Speed

  • How fast something is moving: the rate at which distance is covered.

    Speed= Distance

    Time

    EX: mph (mi/hr) , km/hr, cm/hr

    / = “per” = divided by ex: 100km/hr


1 instantaneous speed

1. Instantaneous speed

  • Speed something has at any instant

  • Ex: speedometer


Chapter 2 motion

Average speed= total distance covered

time interval


Example we drive 100 km in a time of 2 hrs

Example: we drive 100 km in a time of 2 hrs.

Av sp= Total distance covered = 100km =50km

time interval 2hrs hr

Trip could have variations in speed -average speed!


Another example we walk to mcdonalds 2 0km away it takes 40 minutes

Another example: we walk to McDonalds : 2.0km away & it takes 40 minutes.

Av speed = Total distance covered

Time interval

Av speed = 2.0km / 40 min

= 0.05 km /min

But… stopped for traffic,tied shoe,ran across the road… YOU GET THE IDEA!!


Chapter 2 motion

Velocity – includes speed & direction

ex: 60km/ hr North

This is a Vector Quantity- includes direction & magnitude.

What is the difference between constant speed & velocity?


How can a racecar have constant speed but it s velocity is changing

How can a racecar have constant speed but it’s velocity is changing?

  • Constant speed- doesn’t speed up or slow down.

  • Changing velocity because direction is changing.


Other formulas

Other formulas:

V = D/T

D = V x T

T = D/V

V = velocity, D = distance, T=time


Interpreting distance vs time graphs

Interpreting Distance vs. Time graphs:

See board:

  • Speed vs. Velocity

  • What is Slope?

  • What is ______ doing?

    • Car “a”

    • Car “b”

    • Car “c”

    • Car “d”


Lets try some problems

Lets try some problems:

  • Av speed of bike that travels

    150 m in 15 secs

  • V = D / T

  • V = 150 m /15 s

  • V = 10m/s


2 you ran an av speed of 3 km hr for 1 hr

# 2 : You ran an av. Speed of 3 km/hr for 1 hr.

  • ) How far did you go?

  • D=VxT

    3km/hr • 1 hr

    D = 3km

    b. At this rate, how far in 2 hrs? 10 hrs?

    3km/hr • 2 hr = 6km

    3km/hr • 10 hr = 30km


2 4 motion is relative

2.4 Motion Is relative

  • Right now :Your speed is zero relative to Earth,

    But.. 30 km / s relative to the sun.


Isaac newton

Isaac Newton

  • P. 22 green box

    Newton’s 1st Law “THE LAW OF INERTIA”

  • Every object continues in a state of rest, or in a state of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.

    “the table cloth trick”

    “penny & index card inquiry”


Net force combination of all forces that act on an object

Net Force – combination of all forces that act on an object.

See Board

Newton (N) – unit for force

An arrow represents Force as vector quantities.

  • Arrows length represents magnitude (how much) and direction (which way)


Vector addition

Vector Addition:

  • 12 N + 8 N = _____

  • -20 N + 3 N = _____

  • 7 N + 8 N = _____

  • 15 N - 10 N = _____


2 7 equilibrium for objects at rest

2.7 Equilibrium for objects at rest

Spring scale & block example on board

Attracted to the Earth with a force of __ N.

Weight of object (downward force)= tension in rope (upward force).

The block is at rest, so net force is Zero.


Mechanical equilibrium f 0

Mechanical equilibrium : ∑ F = 0

∑ - sum

F- force

Objects at rest have equal & opposite forces acting on them.

  • Sum of upward vectors= sum of downward vectors

  • Static Equilibrium


Why don t we fall through the floor

Why don’t we fall through the floor?

Support Force or “normal force”.- the upward force

EX: book on desk : weight & gravity

∑ F = 0

What is the net force on a bathroom scale when a 110 lb person stands on it?

A: Zero. Scale is at rest. Scale reads support force which has same magnitude as weight.


Equilibrium for moving objects

Equilibrium for moving objects

Equilibrium- state of no change.

An object moving at constant velocity is in dynamic equilibrium.


Some questions for you

Some Questions for you…

  • Give an example of something moving when a net force of zero acts on it?

  • If we push a crate at a constant velocity, how do we know how much friction acts on the crate compared to our pushing force?

  • Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left on the board


Some questions for you1

Some Questions for you…

  • Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left below? One day he decides to anchor his chair to a nearby flagpole – why did Harry end up taking vacation early?


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