Transcript Chapters One and Two - elementaryscienceteachers

Sections
1. Describing and
Measuring Motion
2. Acceleration
3. The Nature of Force
4. Force, Mass, and
Acceleration
5. Friction and Gravity
6. Action and Reaction
What is Motion?
An object is in motion
when its distance from
another object is
changing.
Motion depends on your
reference point (an object
used for comparison in
determining if something
is in motion).
Describing Distance and Speed
The SI unit for distance is the meter.
Speed is calculated as:
Distance/time
Average Speed
Since most objects in motion do not
travel at a constant speed, we
calculate an average speed which is
calculated as:
Total distance/Total time
Average Speed (cont.)
If Lance Armstrong cycles a total of
889 Km in the Tour De France in 49
hours, What is his average speed?
Velocity
When you know the speed and the
direction of an object, you know its
velocity.
Graphing Motion
Slope refers to the steepness of a
line on a graph. When graphing
motion (distance vs. time) the
steeper the slope, the faster the
motion.
Slope = rise/run
Graphing Motion
Graphing Motion
Time
1 min
2 min
3 min
4 min
5 min
6 min
7 min
8 min
9 min
10 min
Boy #1 Distance
Boy #2 Distance
.2 Km
.4 Km
.6 Km
.8 Km
.8 Km
.8 Km
.9 Km
1.1 Km
1.3 Km
1.6 Km
.2 Km
.4 Km
.6 Km
.8 Km
1.0 Km
1.2 Km
1.4 Km
1.6 Km
1.8 Km
2.0 Km
Acceleration
Acceleration refers to the rate at
which velocity changes…a change in
velocity over time.
Acceleration (cont.)
Speeding up
Changing direction
Slowing down
Acceleration (cont.)
Acceleration (cont.)
If a roller coaster is traveling
at 7 m/s at its highest point,
and 3 seconds later its speed
has increased to 28 m/s,
what is its average
acceleration?
Graphing Acceleration
The Nature of Force
A force is a push or a pull. Forces are
described by magnitude and
direction.
Forces
Forces can add together (when
acting in the same direction), or
cancel each other out (when acting
in opposite directions).
+ =0
+
=
Forces
The overall force acting on an object after
all of the forces have been added together
is called the net force. Unbalanced forces
always result in motion.
Forces
Equal forces acting on an object are
called balanced forces. Balanced
forced do not result in motion…
Newton’s 1st Law of Motion
Inertia is the tendency of an object
to resist change in its motion.
“An object at rest will stay at rest,
and an object in motion will stay in
motion, unless acted upon by an
unbalanced force.”
Inertia: An everyday experience
Inertia and Mass
Mass refers to the amount of matter
in an object. The more mass an
object has, the more inertia it has…
Newton’s Second Law
“The net force on an object is equal to
the product of its acceleration and its
mass.”
Force
(N)
= Mass
(Kg)
X Acceleration
(m/s2)
OR
Acceleration(m/s )
2
= Force
/mass
(N)
(Kg)
Newton’s Second Law
If the wagon has a
mass of 62 Kg, and
the girl is pulling it
with a force of 170
N, what is the rate
at which the wagon
is accelerating?
Newton’s Second Law
What force is required
for this 275 Kg
motorcycle to
accelerate at a rate of
18 (m/s2)?
Friction
The force that one surface exerts on
another is called Friction.
Even objects that appear smooth have
many irregularities…which get caught on
the irregularities of the opposing surface,
and resist motion..
Friction
Friction always acts in the opposite
direction of the force/motion.
Friction: An Everyday Experience
Friction: Three types
Sliding friction
Rolling Friction
Fluid Friction
Gravity
Gravity is the force that pulls objects
toward each other. The more
massive the object, the more
gravitational pull it has.
Freefall
When the only force acting on an
object is gravity (ignoring air
resistance) it is said to be in “freefall.”
Acceleration Due to Gravity
Objects in freefall
will accelerate at
9.8m/s2. All objects
accelerate at the
same rate
regardless of mass
(again, ignoring air
resistance).
Acceleration Due to Gravity
If the person is in
freefall for 3.8
seconds, what is
his approximate
speed just before
the bungee cord
slows his fall?
Projectile motion
An object that is thrown horizontally
is called a “projectile”.
Projectile Motion
When an object is
simply dropped
from the same
height as another
object that is
thrown, they will
hit the ground at
the same time!
Air Resistance
Objects falling
through the air
experience a type of
fluid friction called
“air resistance.”
Air Resistance
Terminal Velocity
When air resistance is so great that an
object no longer accelerates in freefall,
the object has reached “terminal
velocity.”
Weight or Mass?…..
– Mass refers to the amount of matter an
object contains, while weight refers to
the gravitational pull on an object.
– Gravitational pull on an object can be
measured in newtons. Since F=M x A,
one calculates weight (N) by multiplying
mass (Kg) times the acceleration due to
gravity (9.8 m/s2).
What is the
approximate
gravitational force
acting on this
motorcycle if it has
a mass of 225 Kg?
Think…..
How much would I weigh on the moon?
Because the moon has one sixth the gravity of Earth, you would
weigh six times less than what you weigh on Earth. This is why the
astronauts were able to move easily in their heavy space suits. In
the photo to the right, John Young, commander of the Apollo 16
lunar landing mission, jumps up from the lunar surface as he
salutes.
Universal Gravitation
The law of universal gravitation
states that the force of gravity acts
between all objects in the universe.
Newton’s 3rd Law of Motion
“ If one object exerts force on
another object, then the second
object exerts a force of equal
magnitude in the opposite direction
on the first object.”
Newton’s 3rd Law affects everything!
Momentum
In assessing what
Newton referred to
as “quantity of
motion” we now
call “momentum”.
Momentum equals
mass X velocity
Momentum
What has more momentum, a 12,000 Kg
jet traveling at 700 m/s, or a 295,000 Kg
cruise ship traveling at 6 m/s?
Conservation of Momentum
The Law of Conservation of Momentum
states the total momentum of any
group of interacting objects remains
the same. (in the absence of outside
forces)
Conservation of Momentum