Transcript I. Newton`s Laws of Motion

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
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.
Motion
 Motion
 Change in position in relation to
a reference point.
Motion
Reference
point: is a
nonmoving
point from
which motion is
measured
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.
Speed & Velocity
 Speed
 rate of motion
 distance traveled per unit time
distance
speed 
time
Speed & Velocity
 Instantaneous
Speed
 speed at a given instant
 Average
Speed
total distance
avg. speed 
total time
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!
Speed & Velocity
 Velocity
 speed in a given direction
 can change even when the
speed is constant!
Acceleration
 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
Acceleration
 Positive
acceleration
 “speeding up”
 Negative
acceleration
 “slowing down”
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
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster!
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
a=?
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2
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
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)
t = -30 m/s ÷ -3m/s2
t = 10 s
Graphing Motion
Distance-Time Graph
A
B

slope = speed

steeper slope =
faster speed

straight line =
constant speed

flat line =
no motion
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
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
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
5
Time (s)
10
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
Motion & Forces
III. Defining Force
Force
 Newton’s First Law
 Friction

Force

Force
 a push or pull that one body exerts
on another
 What forces are being
exerted on the football?
Fkick
Fgrav
Force

Balanced Forces
 forces acting on
an object that
are opposite in
direction and
equal in size
 no change in
velocity
Force

Net Force
 unbalanced forces that are not
opposite and equal
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
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.
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
 Inertia increases as mass increases
Concept Check 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 Check 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 Check 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
Friction

Friction
 force that opposes motion between
two surfaces
 Friction depends on the:
• types of surfaces
• force between the
surfaces
Friction

Friction is greater...
 between rough surfaces
 when there’s a greater
force between the
surfaces
(e.g. more weight)

Pros and Cons?
Motion & Forces
IV. Force & Acceleration
Newton’s Second Law
 Gravity
 Air Resistance
 Calculations

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
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
Newton's 2nd Law of
Motion

If a net force is applied to an object, that
object will accelerate in the direction of the
force.
Force = Mass x Acceleration
(N)
(kg)
(newton)
(m/s/s)
In the example Force is kept constant :
F=MxA
F=MxA
F=MxA
72 = 6 x 12
72 = 12 x 6
72 = 18 x 4
Looking at these what can you say about
the relationship between Mass and
Acceleration. It is Inverse or an opposite
relationship. Mass goes up acceleration goes
down.
 Practice
problem:
If a motorcycle has a mass of 212 kg and
the engine produces 3816 N of force, at
what rate will the motorcycle accelerate?
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 = (40 kg)(4 m/s2)
F = 160 N
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
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
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
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
Gravity

Gravity
 Gravity is a force of attraction
between any two objects in the
universe
 increases as...
• mass increases
• distance decreases
Gravity
Who experiences more gravity - the
astronaut or the politician?
 Which exerts more gravity the Earth or the moon?

less
distance
more
mass
Weight vs. Mass
Weight is a measure of gravitational pull, which is
a force. So if F = M x A, then weight also = M x A
More specifically though, the acceleration for
weight is due to gravity, so you can automatically
put 9.8 m/s/s in the equation.
So the equation ends up being:
Weight = Mass x 9.8
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)
Gravity

Would you weigh more on Earth
or Jupiter?
 Jupiter because...
greater mass
greater gravity
greater weight
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.”
Air Resistance

Air Resistance
 a.k.a. “fluid friction” or “drag”
 Is a force that air exerts on a
moving object to oppose its motion
 Air resistance depends on:
•
•
•
•
speed
surface area
shape
density of fluid being moved through
Air Resistance

Terminal Velocity
 maximum velocity reached
by a falling object
F
 Is reached when…
air
Fgrav = Fair
 no net force
 no acceleration
 constant velocity
Fgrav
Air Resistance

Terminal Velocity
 increasing speed  increasing air
resistance until…
Fair = Fgrav
Animation from “Multimedia Physics Studios.”
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.”
Examples of air resistance:
 Car
aerodynamics
 Spoilers
on
racecars
 Buses
 Hand
surfing
Motion & Forces
V. Nonlinear Motion
Projectile Motion
Free-fall

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
Projectile Motion



Horizontal and vertical
velocities are independent
of each other!
Horizontal Velocity
 depends on inertia
 remains constant
Vertical Velocity
 depends on gravity
 accelerates downward at
9.8 m/s2
Projectiles

A projectile has both horizontal force and
vertical force applied to it. One makes it want
to go up and down, the other wants it to go
sideways.
The combination of the 2 forces creates the
curved path.
Projectiles
Regardless of the shape, the path
followed by a projectile is called its
trajectory.
Golf Ball Trajectory
Projectiles
In real life, projectiles are effected by air
resistance (the force which air exerts on a
moving object) and therefore may not
follow a perfectly curved path.
Like the flight of a golf ball.
Concept Check


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.”
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
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.
Free-Fall
Space Shuttle Missions
Go to CNN.com.
Go to NASA.
NASA’s KC-135 - “The Vomit Comet”
Concept Check 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.
Motion & Forces
VI. Action and Reaction
Newton’s Third Law
 Momentum
 Conservation of Momentum

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.
Newton's 3rd Law of
Motion
 For every force, there is a force
equal in size, but opposite in
direction.
 Ex.:
-hitting a wall
-rocket propulsion
-the kick of a gun
-an arrow being shot
-hard hit in football
-swimming
-pogo stick
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!!!
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.
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
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
F 1 = F2
or
M1 x A1 = M2 x A2
Practice problem:
If Larry kicks a .5 kg ball with 90 N
of force, at what rate will the ball
accelerate?
Practice Problem

A 12 gauge shotgun fires a slug which has a
mass of .15 kg and accelerates at 250 m/s/s.
What is the force of the recoil (kick) of the gun
when it is fired?
Momentum
 Momentum
is inertia in motion.
 It is determined by how big an
object is and how fast it is
moving.
p (momentum) = m x v
(kg x m/s)
A stationary object has zero
momentum.
Practice problem:
A car has a mass of 1050 kg and
has 5250 kg x m/s of momentum as
it drives down the road. What is the
velocity of the car?
Practice Problem
 Is
it possible for a little Volkswagon
to have more momentum than a
Mack truck?
Explain.
The Law of Conservation of
Momentum states that the total
amount of momentum of an object
or group of objects does not change
(unless acted on by a net force).
Think of it like a class poker day in Mrs.
Shaw’s Science. Everybody brings their
money, and some of that money will change
hands throughout the course of the hour.
But from beginning to end, the total amount
of money at the beginning of the hour will be
the same as at the end of the hour.
 Examples





Of Momentum:
Newton's Cradle
Dog pile
Jar of marbles dropping
Pool break
Car wreck
Practice Problems
1. Kenny drops an object out of a window. If gravity exerts a
force of 147 N on the object, what is the mass of the object?
2. A baseball player hits a high fly ball to the outfield. If the
ball is in the air for a total of 6 seconds, how fast was the ball
traveling downward when the outfielder caught it?
3. In problem #3, how fast was the ball traveling upward when
it left the bat? Explain.
4. Two bumper cars collide with each other. The first car has a
mass of 124 kg (car and driver), while the second car has a total
mass of 148 kg. When the cars collide, the first is knocked
backwards with a rate of acceleration of 4.77 m/s/s. At what
rate of acceleration was the other car knocked backwards?
5. Which will have more momentum, a softball (.42 kg) thrown at
45 m/s, or a baseball (.35 kg) thrown at 55 m/s? Show work.
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?
 End
here
Ch. 3 & 4
Motion & Forces
VII. Forces in Fluids (Ch. 8.5)
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.