Download I. Newton`s Laws of Motion

Ch. 11
Motion & Forces
I. Newton’s Laws of Motion
“If I have seen far, it is because I have stood
on the shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)
A. Newton’s First Law
 Newton’s
First Law of Motion
 An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force.
B. Newton’s Second Law
 Newton’s
Second Law of Motion
 The acceleration of an object is
directly proportional to the net
force acting on it and inversely
proportional to its mass.
F = ma
C. Newton’s Third Law
 Newton’s
Third Law of Motion
 When one object exerts a force
on a second object, the second
object exerts an equal but
opposite force on the first.
Ch. 11
Motion & Forces
II. Describing Motion
Motion
 Speed & Velocity
 Acceleration

Newton’s First Law
 Newton’s
First Law of Motion
 An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force
force.
A. Motion
 Problem:
 Is your desk moving?
 We
need a reference point...
 nonmoving point from which
motion is measured
A. Motion
 Motion
 Change in position in relation to
a reference point.
Reference point
Motion
A. Motion: Displacement
 The
distance an object has been
moved from one position to
another.
A. Motion
Problem:
 You are a passenger in a car
stopped at a stop sign. Out of the
corner of your eye, you notice a
tree on the side of the road begin
to move forward.
 You have mistakenly set yourself
as the reference point.
B. Speed & Velocity
 Speed
d
 rate of motion
v t
 distance traveled per unit time
distance
speed 
time
B. Speed & Velocity
 Instantaneous
Speed
 speed at a given instant
 Average
Speed
total distance
avg. speed 
total time
B. Speed & Velocity
 Problem:
 A storm is 10 km away and is
moving at a speed of 60 km/h.
Should you be worried?
 It depends
on the
storm’s
direction!
B. Speed & Velocity
 Velocity
 speed in a given direction
 can change even when the
speed is constant!
C. Acceleration
vf - vi
a t
 Acceleration
 the rate of change of velocity
 change in speed or direction
a
v f  vi
t
a:
vf:
vi:
t:
acceleration
final velocity
initial velocity
time
C. Acceleration
 Positive
acceleration
 “speeding up”
 Negative
acceleration
 “slowing down”
D. Calculations
Your neighbor skates at a speed of 4 m/s.
You can skate 100 m in 20 s. Who skates
faster?
GIVEN:
WORK:

d = 100 m
t = 20 s
v=?
d
v t
v=d÷t
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster!
D. Calculations
A roller coaster starts down a hill at 10 m/s.
Three seconds later, its speed is 32 m/s.
What is the roller coaster’s acceleration?
GIVEN:
WORK:

vi = 10 m/s
t=3s
vf = 32 m/s
vf - vi
a=?
a t
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2
D. Calculations
Sound travels 330 m/s. If a lightning bolt
strikes the ground 1 km away from you,
how long will it take for you to hear it?
GIVEN:
WORK:

v = 330 m/s
t=d÷v
d = 1km = 1000m
t = (1000 m) ÷ (330 m/s)
t=?
t
=
3.03
s
d
v t
D. Calculations
How long will it take a car traveling 30 m/s
to come to a stop if its acceleration is
-3 m/s2?
GIVEN:
WORK:

t=?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
vf - vi
a t
t = -30 m/s ÷ -3m/s2
t = 10 s
E. Graphing Motion
Distance-Time Graph
A
B

slope = speed

steeper slope =
faster speed

straight line =
constant speed

flat line =
no motion
E. Graphing Motion
Distance-Time Graph
A



B

Who started out faster?
 A (steeper slope)
Who had a constant speed?
 A
Describe B from 10-20 min.
 B stopped moving
Find their average speeds.
 A = (2400m) ÷ (30min)
A = 80 m/min
 B = (1200m) ÷ (30min)
B = 40 m/min
E. Graphing Motion
Distance-Time Graph
400

Acceleration is
indicated by a
curve on a
Distance-Time
graph.

Changing slope =
changing velocity
Distance (m)
300
200
100
0
0
5
10
Time (s)
15
20
E. Graphing Motion
Speed-Time Graph
3

slope = acceleration
 +ve = speeds up
 -ve = slows down

straight line =
constant accel.

flat line = no accel.
(constant velocity)
Speed (m/s)
2
1
0
0
2
4
6
Time (s)
8
10
E. Graphing Motion
Speed-Time Graph
Specify the time period
when the object was...
 slowing down
 5 to 10 seconds
 speeding up
 0 to 3 seconds
3
Speed (m/s)
2

1
0
0
2
4
6
Time (s)
8
10

moving at a constant
speed
 3 to 5 seconds
not moving
 0 & 10 seconds
Ch. 3 & 4
Motion & Forces
III. Defining Force
Force
 Newton’s First Law
 Friction

A. Force

Force
 a push or pull that one body exerts
on another
 What forces are being
exerted on the football?
Fkick
Fgrav
A. Force

Balanced Forces
 forces acting on
an object that
are opposite in
direction and
equal in size
 no change in
velocity
A. Force

Net Force
 unbalanced forces that are not
opposite and equal
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
B. Newton’s First Law
 Newton’s
First Law of Motion
 An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force.
B. Newton’s First Law

Newton’s First Law of Motion
 “Law of Inertia”

Inertia
 tendency of an object to resist any
change in its motion
 increases as mass increases
Concept Test 1
TRUE or FALSE?
The object shown in the diagram must
be at rest since there is no net force
acting on it.
FALSE! A net force does not
cause motion. A net force
causes a change in motion,
or acceleration.
Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.
Concept Test 2
You are a passenger in a car and not
wearing your seat belt.
Without increasing or decreasing its
speed, the car makes a sharp left turn,
and you find yourself colliding with the
right-hand door.
Which is the correct analysis of the
situation? ...
Concept Test 2
1. Before and after the collision, there
is a rightward force pushing you
into the door.
2. Starting at the time of collision, the
door exerts a leftward force on you.
3. both of the above
4. neither of the above
C. Friction

Friction
 force that opposes motion between
2 surfaces
 depends on the:
• types of surfaces
• force between the
surfaces
C. Friction

Friction is greater...
 between rough surfaces
 when there’s a greater
force between the
surfaces
(e.g. more weight)

Pros and Cons?
Ch. 3 & 4
Motion & Forces
IV. Force & Acceleration
Newton’s Second Law
 Gravity
 Air Resistance
 Calculations

A. Newton’s Second Law

Newton’s Second Law of Motion
 The acceleration of an object is
directly proportional to the net force
acting on it and inversely
proportional to its mass.
F = ma
A. Newton’s Second Law
F
a
m
F = ma
F
m a
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
B. Gravity

Gravity
 force of attraction between any two
objects in the universe
 increases as...
• mass increases
• distance decreases
B. Gravity
Who experiences more gravity - the
astronaut or the politician?
 Which exerts more gravity the Earth or the moon?

less
distance
more
mass
B. Gravity

Weight
 the force of gravity on an object
W = mg
W: weight (N)
m: mass (kg)
g: acceleration due
to gravity (m/s2)
MASS
WEIGHT
always the same
(kg)
depends on gravity
(N)
B. Gravity

Would you weigh more on Earth
or Jupiter?
 Jupiter because...
greater mass
greater gravity
greater weight
B. Gravity

Accel. due to gravity (g)
 In the absence of air
resistance, all falling objects
have the same acceleration!
 On Earth: g = 9.8 m/s2
W
g
m
elephant
g
W
m
feather
Animation from “Multimedia Physics Studios.”
C. Air Resistance

Air Resistance
 a.k.a. “fluid friction” or “drag”
 force that air exerts on a moving
object to oppose its motion
 depends on:
• speed
• surface area
• shape
• density of fluid
C. Air Resistance

Terminal Velocity
 maximum velocity reached
by a falling object
F
 reached when…
air
Fgrav = Fair
 no net force
 no acceleration
 constant velocity
Fgrav
C. Air Resistance

Terminal Velocity
 increasing speed  increasing air
resistance until…
Fair = Fgrav
Animation from “Multimedia Physics Studios.”
C. Air Resistance

Falling with air resistance
 heavier objects fall faster
because they accelerate
to higher speeds before
reaching terminal velocity
Fgrav = Fair
 larger Fgrav
 need larger Fair
 need higher speed
Animation from “Multimedia Physics Studios.”
D. Calculations

What force would be required to
accelerate a 40 kg mass by 4 m/s2?
GIVEN:
WORK:
F=?
m = 40 kg
a = 4 m/s2
F = ma
F
m a
F = (40 kg)(4 m/s2)
F = 160 N
D. Calculations

A 4.0 kg shotput is thrown with 30 N of
force. What is its acceleration?
GIVEN:
WORK:
m = 4.0 kg
F = 30 N
a=?
a=F÷m
F
m a
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
D. Calculations

Mrs. J. weighs 557 N. What is her
mass?
GIVEN:
WORK:
F(W) = 557 N
m=?
a(g) = 9.8 m/s2
m=F÷a
F
m a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
Concept Test

Is the following statement true or false?
 An astronaut has less mass on the
moon since the moon exerts a weaker
gravitational force.
 False! Mass does not depend on
gravity, weight does. The astronaut has
less weight on the moon.
Motion & Forces
V. Free-Fall Motion
Free-fall
 Circular Motion
 Free-fall

A. Free-Fall

Free-Fall
 when an object is influenced only
by the force of gravity

Weightlessness
 sensation produced when an object
and its surroundings are in free-fall
 object is not weightless!
CUP DEMO
A. Free-Fall

Weightlessness
 surroundings are falling at the same
rate so they don’t exert a force on
the object
Motion & Forces
VI. Action and Reaction
Newton’s Third Law
 Momentum
 Conservation of Momentum

A. Newton’s Third Law
 Newton’s
Third Law of Motion
 When one object exerts a force
on a second object, the second
object exerts an equal but
opposite force on the first.
A. Newton’s Third Law

Problem:
 How can a horse
pull a cart if the cart
is pulling back on
the horse with an equal but
opposite force?
 Aren’t these “balanced forces”
resulting in no acceleration?
NO!!!
A. Newton’s Third Law

Explanation:
 forces are equal and opposite but
act on different objects
 they are not “balanced forces”
 the movement of the horse
depends on the forces acting on
the horse
A. Newton’s Third Law

Action-Reaction Pairs

The hammer exerts
a force on the nail
to the right.

The nail exerts an
equal but opposite
force on the
hammer to the left.
A. Newton’s Third Law

Action-Reaction Pairs
The rocket exerts a
downward force on the
exhaust gases.
 The gases exert an
equal but opposite
upward force on the
rocket.

FG
FR
A. Newton’s Third Law

Action-Reaction Pairs

Both objects accelerate.

The amount of acceleration
depends on the mass of the object.
F
a 
m

Small mass  more acceleration

Large mass  less acceleration
JET CAR CHALLENGE
CHALLENGE:
Construct a car that will travel as far as
possible (at least 3 meters) using only
the following materials.
scissors
 tape
 4 plastic lids
 2 skewers

2 straws
 1 balloon
 1 tray

How do each of Newton’s Laws apply?