Download I. Newton`s Laws of Motion - Old Saybrook Public Schools

Unit 2: Motion & Forces
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)
Motion & Forces
Ch 1. 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
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=?
v=d÷t
d
v = 5 m/s
v t
v = (100 m) ÷ (20 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
2
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
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

Distance (m)
300
200
Acceleration is
indicated by a
curve on a
Distance-Time
graph.
100
0
0
5
10
Time (s)
15
20
• C
h
a
n
gi
n
g
sl
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
Bell Ringer
 What
is Speed?
 What
is Acceleration?
Motion & Forces
Ch 1. 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
 Has
the ability to change an
objects motion:
 Starting
 Stopping
 Speeding up
 Slowing down
 Changing direction
A. Force
 May
change an object’s shape
 Forces give energy to an object
 All of the forces acting on an
object together are known as net
forces
A. Force
 Forces
can be represented with
arrows called vectors.
 Vectors show the direction and
magnitude of a force
 Forces are measure in Newtons
(N)
 https://youtu.be/bOIe0DIMbI8
A. Force

Balanced Forces
 forces acting on
an object that
are opposite in
direction and
equal in size
 no change in
velocity
A. Force

Unbalanced Forces
 unbalanced forces that are not
opposite and equal
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
Bell Ringer
 What
is a Force?
 What
unit is force measure in?
 What
is a vector?
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
B. Newton’s First Law
 Objects
do not change their
motion unless a force acts on
them
 An object will NOT start moving
unless a force acts on it
 An object will NOT stop moving
unless a force acts on it.
 An object will NOT change speed
unless a force acts on it
 An object will NOT change
direction unless a force acts on it.
 The more mass an object has, the
more inertia it has. This means that
more mass an object has the harder
it is to move, stop, or change the
speed or direction of the object.

http://www.physicsclassroom.com/mmedia/newtlaws/cci.cfm
 Don’t
let this be you. Wear seat
belts!
 Because of inertia, objects
(including you) resist changes in
their motion. When the car going
80 km/ hour is stopped by the
brick wall, your body keeps
moving at 80 m/hr.
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?
Bell Ringer
 What
is friction?
 What
two things affect friction?
Review
Motion & Forces
Ch 2. Force & Acceleration
Newton’s Second Law
 Gravity
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
Bell Ringer
 What
is gravity?
 What unit of measurement is
force measured in?
 What are the units for: velocity,
acceleration, distance, mass?
C. 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
C. 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
C. Calculations

Ms. S. 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
ConcepTest

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
Ch 3. 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
Examples



As a man exits a canoe,
the canoe moves in the
opposite direction. The
canoe has an equal and
opposite reaction to the
man’s action.
As the paddle is pushed
backward in the water the
canoe moves forward
A swimmer pushes water
back with his arms, but his
body moves forward.
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?
B. Momentum

Momentum
 quantity of motion
p = mv
p
m v
p:
m:
v:
momentum (kg ·m/s)
mass (kg)
velocity (m/s)
B. Momentum
Find the momentum of a bumper car if it
has a total mass of 280 kg and a velocity
of 3.2 m/s.
GIVEN:
WORK:
p=?
p = mv
m = 280 kg
p = (280 kg)(3.2 m/s)
v = 3.2 m/s
p = 896 kg·m/s
p

m v
B. Momentum
The momentum of a second bumper car
is 675 kg·m/s. What is its velocity if its
total mass is 300 kg?
GIVEN:
WORK:
p = 675 kg·m/s
v=p÷m
m = 300 kg
v = (675 kg·m/s)÷(300
v=?
kg)

p
m v
v = 2.25 m/s
C. Conservation of Momentum

Law of Conservation of Momentum
 The total momentum in a group of
objects doesn’t change unless
outside forces act on the objects.
pbefore = pafter
C. Conservation of Momentum

Elastic Collision
 KE is conserved

Inelastic Collision
 KE is not conserved
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.