# Chapter 2 : MOTION - PowerPoint PPT Presentation

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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

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

UNNATURAL MOTION- Requires force

EX: push a book across table

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

As a ball rolls down an incline it speeds up

• up incline,slows

• Reduced angle, ball goes farther

### Inertia

Objects at rest tend to remain at rest.

Moving objects tend to remain moving.

### 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

• Speed something has at any instant

• Ex: speedometer

Average speed= total distance covered

time interval

### 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.

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!!

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?

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

• Changing velocity because direction is changing.

### Other formulas:

V = D/T

D = V x T

T = D/V

V = velocity, D = distance, T=time

### 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:

• 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.

• ) 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

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

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

### 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.

See Board

Newton (N) – unit for force

An arrow represents Force as vector quantities.

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

• 12 N + 8 N = _____

• -20 N + 3 N = _____

• 7 N + 8 N = _____

• 15 N - 10 N = _____

### 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

∑ - 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?

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- state of no change.

An object moving at constant velocity is in dynamic equilibrium.

### 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 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?