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Physics 10 Chapter 3
1-D Motion
Natural Motion
•Objects have a proper place
•Objects seek their natural place
•The heavier the object,
the faster it falls.
Violent Motion
•Imposed motion
•A Moving object must have a
force acting on it.
Galileo Challenged The Dogma
Of Natural Motion
The natural motion of
a body is to remain in
whatever state of
motion it is in unless
acted upon by net
external forces.
Galileo Challenged The Dogma
Of Natural Motion with
Experiments
Galileo’s Motion Studies
What causes the ball
to speed up?
gravity
What causes the ball
to slow down?
gravity
Does the speed
change?
No!
Galileo’s Inclined Planes
If I have a long horizontal plane without friction,
how far will the ball go to reach the same height?
Once the ball rolls down the
incline, what keeps it moving?
Inertia
The tendency of an object to resist a change in motion.
Inertia
Inertia
Inertia
Inertia Question
A quick pull will break which string?
A slow and steady pull will break which string?
The Earth Revolves around the sun at
30,000 m/sec!!! (~67,000 miles/hr)
The Earth Rotates on its axis at
464 m/s (1000 miles/hr)!
The Solar System moves through the Galaxy at
220,000 m/s ( 500,000 miles/hr)
How come we can’t feel these motions?
Cuz we are moving right along with it!
Mass is a Measure of Inertia
Kilogram: KG
Mass & Weight
Mass is how much stuff an object has – inertia.
Weight is due to the force of gravity and depends on the mass
and the planet!
F = ma
W = mg
g = 10m / s
2
Mass & Weight
W = mg
g = 10m / s
2
Calculate your weight in N.
Calculate your mass in Kg.
1 lb = 4.45 N
Weight and Force
W = mg
g = 10m / s 2
The hanging weight has a mass of 1 kg. What is its
weight in Newtons?
a) 0.1 N
100 N
b) 1 N
c) 10 N
d)
What is the reading on each of the two scales?
a) 1 N
b) 5 N
c) 10 N
d) 10 N
Mass on the Moon
Gravity on the surface of the moon is only 1/6 as strong
as on the Earth. What is the mass of a 12-kg object on
the moon? What is the weight on the moon?
Isaac Newton
(1642 -1727)
1687 Published Principia
•Invented Calculus
•3 Laws of Motion
•Universal Law of Gravity
Newton’s First Law
(Law of Inertia)
An object will remain at rest or
in a constant state of motion
unless acted upon by net forces.
Newton’s First Law
(Law of Inertia)
∑F = 0
An object will remain at rest or in a
constant state of motion unless acted upon
by net external forces.
Push or a Pull
The unit is a Newton (N)
Contact Forces
Tension Forces
Support Forces
Friction Forces
Net Force
Direction of applied forces matters!!!
Opposite forces cancel!
∑F = 0
Net Forces
Forces up equal the forces down.
Forces sideways cancel too.
T1
T2
W
Static Equilibrium
∑F = 0
If the Net Force is zero, then the object remains at
rest – in static equilibrium.
T1
T2
W
Dynamic Equilibrium
∑F = 0
If the Net Force is zero, then the object moves at a
constant speed – in dynamic equilibrium.
W
Newton’s First Law
If
∑ F = 0 => No Change in Motion
Dynamic Equilibrium
Static Equilibrium
Net Force Question
The person weighs 250 N and the tension in each rope is 200N.
What is the weight of the staging?
a)
b)
c)
d)
50 N
100 N
150 N
200 N
Two 2 N weights are pulling on the
spring scale as shown. The reading
on the scale is:
a) 0 N
b) 1 N
c) 2 N
d) 4 N
Galileo’s Motion Studies
gave us…
Definitions:
Δx
v=
Δt
v0 + v f
v=
2
Δv
a=
Δt
Speed
How fast How far is changing.
The rate at which an object moves.
distance
Velocity
Velocity is a vector. It has both magnitude (speed) and direction.
Instantaneous velocity is the velocity at any instant.
distance
Speed & Velocity
total distance traveled Δd
Average Speed =
=
Δt
total time
m
[speed ] = ,
s
[distance] = m
•Speed: Scalar (magnitude)
•Velocity: Vector (magnitude AND direction)
•Unit is m/s
• s and v often interchanged …..
Sense of Speed
1m / s ≈ 2.25mi / hr
1m / s = 3.6km / hr = 2.24mi / hr
10m / s = 36km / hr = 22.4mi / hr
20m / s = 72km / hr = 44.8mi / hr
30m / s = 108km / hr = 67.2mi / hr
1km / hr ≈ 0.62mi / hr
Speed Question
What is its average speed? (in m/s)
total distance traveled
Average Speed =
total time
Distance from Speed
total distance traveled
Average Speed =
total time
Average Speed x total time= total distance traveled
d = vt
If you travel an average of 80
km/hr for 15 minutes, how far have
you traveled in that time?
Acceleration
How fast How fast is changing.
The rate at which the speed is changing.
change in velocity Δv
=
a=
change in time
Δt
Speeding up
Slowing down
Speeding up or slowing down?
Acceleration is in the direction of
the net Force but not necessarily
in the direction of velocity.
Velocity is always in the direction of the motion!
Velocity from Acceleration
change in velocity Δv
=
a=
change in time
Δt
Δv = aΔt
v = at
If you start from rest and accelerate at
a rate of 5m/s every second (5 m/s2)
for 10 seconds, what will be your
final speed?
v = at = 5m / s 2 x10 s = 50m / s
Quicky Question
An automobile enters a freeway on-ramp at 15.0m/s and accelerates
uniformly up to 25.0 m/s in a time of 10.0s.
a) What is the automobile’s average velocity?
Which equation?
vave =
v0 + v f
2
15m / s + 25m / s
=
= 20m / s
2
Quicky Question
An automobile enters a freeway on-ramp at 15.0m/s and accelerates
uniformly up to 25.0 m/s in a time of 10.0s.
b) What is the distance traveled in this amount of time?
Which equation?
d = vt
= (20m / s )(10 s ) = 200m
d = 200m
Quicky Question
An automobile enters a freeway on-ramp at 15.0m/s and accelerates
uniformly up to 25.0 m/s in a time of 10.0s.
c) What is the automobile’s acceleration?
Which equation?
Δv v f − vi
25m / s − 15m / s
2
a=
=
=
= 1m / s
Δt
Δt
10 s
Quizzy Questions
A car moves with an average speed of 40 km/h for 15
minutes. How far does it travel?
a) 60 km
b) 10 km c) 30 km
d) 30 km/h
A car can go from rest to 90 km/hr in 10 s. What is the
acceleration of the car?
a) 60 km/hr/s b) 9km/hr/s c) 30 km/hr/s d) 3 km/h/s
Hill Question
a)
b)
c)
On which of these hills does the ball roll down with increasing
speed and decreasing acceleration? a)
b)
c)
Tracks Question
Two balls are released simultaneously from rest at the left
end of equal length tracks A and B as shown. Which ball
reaches the end of its track first?
Free Fall
In the absence of air resistance,
all objects fall due to the force of
gravity with an acceleration “g”:
g = 9.8m / s ~ 10m / s
2
2
Free Fall
In a vacuum, all objects fall with the same
acceleration due to gravity: 10 m/s2.
Falling from Rest
v f = v 0 + gt
1 2
Δ y = v 0 t + gt
2
Take
down
as +y:
+
Estimate :
a = g ~ 10m / s
2
v = at = 10t
1 2
2
Δy = at = 5t
2
How FAR is not
How FAST!
v ≠ Δy !
v0 = 0
v = 10m / s
Δy = 5m
v = 20m / s
Δy = 20m
v = 30m / s
Δy = 45m
v = 40m / s
Δy = 80m
v = 50m / s
Δy = 125m
What Goes Up Must Come Down
Someone standing at the
edge of a cliff throws one
ball straight up and one
straight down at the same
speed. Ignoring air
resistance, which ball
strikes the ground with the
greatest speed?
Free Fall: Throwing UP
What is the speed at the top of the
path?(assume g = 10m/s2 )
ZERO!
What is the acceleration at the top?
g=10m/s2
What is the speed at the same
height on the way down?
-30 m/s
+y
Free Fall Symmetry
Which ball has the greatest speed when it hits the ground?
SAME!
The gravitational field is symmetric
(in the absence of air resistance.)
Symmetry of The G-Field
Quizzy Questions
If you throw a ball straight up in the air at 30 m/s,
how fast is it going 1 second later?
10 m/s
b) 5 m/s
c) 20 m/s d) 0 m/s
What is the acceleration of a falling object that has
reached its terminal velocity?
a) 10 m/s2 b) 5 m/s2
c) zero
d) unknown
Free Fall Summary
For an object without an initial velocity and without air resistance.
a = g = 10m / s
v = 10t
2
d = 5t
2
If there is an initial velocity, then add it!
v(t ) = vi + 10t
d (t ) = vi t + 5t
Do Aunt Minnie problems in the Practice Book.
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