chapter 2 : motion

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

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 & directionex: 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:

1. 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.

a. ) 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 boxNewton’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)

Vector Addition:

1) 12 N + 8 N = _____

2) -20 N + 3 N = _____

3) 7 N + 8 N = _____

4) 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?