Ch. 3 & 4
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. 3 & 4
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
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
ConcepTest 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.
ConcepTest 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? ...
ConcepTest 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
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.
Ch. 3 & 4
Motion & Forces
V. Nonlinear Motion
Projectile Motion
 Circular Motion
 Free-fall

A. Projectile Motion

Projectile
 any object thrown
in the air
 acted upon only
by gravity
 follows a
parabolic path
called a trajectory
 has horizontal and vertical velocities
PROJECTILE MINI-LAB
A. Projectile Motion

Projectile Velocities

Horizontal and vertical velocities are
independent of each other!
A. Projectile Motion

Horizontal Velocity
 depends on inertia
 remains constant

Vertical Velocity
 depends on gravity
 accelerates
downward at
9.8 m/s2
ConcepTest


A moving truck launches a ball vertically
(relative to the truck). If the truck maintains a
constant horizontal velocity after the launch,
where will the ball land (ignore air resistance)?
A) In front of the truck
B) Behind the truck
C) In the truck
C) In the truck. The
horizontal velocity of the
ball remains constant
and is unaffected by its
vertical motion.
Animation from “Multimedia Physics Studios.”
B. Circular Motion

Centripetal Acceleration
 acceleration toward the center of a
circular path
 caused by centripetal force
B-BALL DEMO
PLATE DEMO
B. Circular Motion

On the ground...
 friction provides centripetal force
B. Circular Motion

In orbit...
 gravity provides centripetal force
ROUND LAB
B. Circular Motion

In orbit...
 Which satellites travel faster?
Near-Earth Satellites
Geostationary Satellites
B. Circular Motion

Extra Credit
 Use the NASA Liftoff
web site to find the
International Space
Station (“STATION”)
in the sky this weekend.
 liftoff.msfc.nasa.gov/realtime/Jpass/20/
 Write a brief description of your sighting time, appearance, & other observations.
C. 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
C. Free-Fall

Weightlessness
 surroundings are falling at the same
rate so they don’t exert a force on
the object
Go to Space Settlement Video Library.
C. Free-Fall
Space Shuttle Missions
Go to CNN.com.
Go to NASA.
NASA’s KC-135 - “The Vomit Comet”
ConcepTest 1
TRUE or FALSE:
An astronaut on the Space Shuttle
feels weightless because there is no
gravity in space.
FALSE!
There is gravity which is causing the
Shuttle to free-fall towards the Earth.
She feels weightless because she’s
free-falling at the same rate.
ConcepTest 2
Describe the path of a marble as it
leaves the spiral tube shown below.
It will travel in a straight line since the
tube is no longer exerting a net force
on it.
Ch. 3 & 4
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?
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 kg)
v=?
v = 2.25 m/s
p

m v
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
C. Conservation of Momentum

A 5-kg cart traveling at 4.2 m/s strikes a
stationary 2-kg cart and they connect.
Find their speed after the collision.
BEFORE
Cart 1:
p = 21 kg·m/s
m = 5 kg
v = 4.2 m/s
Cart 2 :
m = 2 kg
v = 0 m/s
p=0
pbefore = 21 kg·m/s
AFTER
Cart 1 + 2:
m = 7 kg
v=?
p
m v
v=p÷m
v = (21 kg·m/s) ÷ (7 kg)
v = 3 m/s
pafter = 21 kg·m/s
C. Conservation of Momentum

A 50-kg clown is shot out of a 250-kg
cannon at a speed of 20 m/s. What is
the recoil speed of the cannon?
BEFORE
AFTER
Clown:
m = 50 kg
v = 0 m/s
p=0
Clown:
p = 1000 kg·m/s
m = 50 kg
v = 20 m/s
Cannon:
m = 250 kg
v = 0 m/s
p=0
Cannon: p = -1000 kg·m/s
m = 250 kg
v = ? m/s
pbefore = 0
pafter = 0
C. Conservation of Momentum

So…now we can solve for velocity.
GIVEN:
WORK:
p = -1000 kg·m/s v = p ÷ m
m = 250 kg
v = (-1000 kg·m/s)÷(250 kg)
v=?
v = - 4 m/s
p
(4 m/s backwards)
m v
Ch. 3 & 4
Motion & Forces
VII. Forces in Fluids
Archimedes’ Principle
 Pascal’s Principle
 Bernoulli’s Principle

A. Archimedes’ Principle

Fluid
 matter that flows
 liquids and gases

Buoyancy
 the ability of a fluid to exert an
upward force on an object
immersed in it
A. Archimedes’ Principle

Bouyant Force
 upward force exerted by a fluid on an
immersed object
 bouyant force > weight
balloon rises
 bouyant force < weight
balloon sinks
 bouyant force = weight
balloon floats
A. Archimedes’ Principle

Archimedes’ Principle
 the bouyant force on an object in a
fluid is equal to the weight of fluid
displaced by the object
Not
More
water
needs
water
is
to displaced
betodisplaced
in order
in order
cancel
to to
Veryenough
little
water
needs
be displaced
intoorder
cancel weight
weight
 ball sinks.
 ball floats lower
in the water.
on surface.
View Buoyancy JAVA Applet.
View animations produced by students at Poly Prep Country Day School in Brooklyn, New York.
B. Pascal’s Principle

Pascal’s Principle
 pressure applied to a fluid is
transmitted unchanged throughout
the fluid
View hydraulics explanation.
F1
P
A1
F2
A
A2
B. Pascal’s Principle

A car weighing 1000 N sits on a 250 m2 platform.
What force is needed on the 10 m2 plunger to
keep the car from sinking?
GIVEN:
WORK:
Platform:
F = 1000 N
A = 250 m2
Plunger:
F=?
A = 10 m2
1000 N =
250 m2
F2
F1 F2

A1 A2
10 m2
(1000N)(10m2)=(250 m2)F2
F2 = 40 N
C. Bernoulli’s Principle

Bernoulli’s Principle
 as the velocity of a fluid increases,
the pressure exerted by the fluid
decreases
 EX:airplane lift, curve balls
C. Bernoulli’s Principle
Airplane lift
View airplane wings explanation.
Curve Ball
C. Bernoulli’s Principle
Funnel Demos
View funnel explanation.
View inverted funnel explanation.
C. Bernoulli’s Principle

Venturi Effect
 fluids flow faster through narrow
spaces causing reduced pressure
 EX: garden sprayer, atomizer,
carburetor
C. Bernoulli’s Principle
Venturi Effect - Atomizers
View atomizer explanation.