Download Unit 2

Chapter 13
Forces and
Motion
List of chapter 13
vocabulary words:
(nine words total)
•
•
•
•
•
•
•
•
•
Speed
Velocity
Acceleration
Force
Gravity
Friction
First law of motion
Second law of motion
Third law of motion
Target Goals:
• “I can…..”
____define speed,
velocity, and
acceleration.
___calculate the
average speed.
What is motion?
• You can look around you and see
that there are things in motion:
• Ex: (Your teacher walking across the room;
your friend writing with a pencil).
• Even if you don’t see anything
moving, motion is still occurring all
around you.
• (Ex: air particles moving, the Earth circling
the sun, blood traveling in your body)
Motion
• An object is considered to be in
motion if it constantly changes
position.
What is motion?.....
• When you want to know the motion
of an object, you simply watch the
object.
• But you are actually watching the
object in relation to another object
that appears to be “staying in place”.
The object staying in place is called
the reference point.
Something to think
about….
• Suppose you look out of the window and
see a truck parked next to a tree. A few
hours later, you look out the window and
that the truck is parked further down the
street about 200 yards away from the tree.
What is your reference point in this
scenario?
Speed Depends on
Distance and Time
• When you are moving, your position is
changing. How quickly your position
changes depends on the speed.
• Speed is the rate at which an object
moves
• Speed depends on 2 things:
a) distance traveled
b) the time taken to travel that distance
Determining Speed…
• The SI unit for speed is m/s, or
meters per second.
Other ways to calculate
speed include:
kilometers per hour (km/h)
Feet per second (ft/s)
Miles per hour (mph)
Brain pop review….
• "Calculating
Speed, Distance,
Time”
• After the brain
pop, Grab a
Whiteboard, Dry
erase marker,
and cleaning
rag!!
Speed Depends on Distance and Time,
continued
• Determining Average Speed- Average
speed equals the total distance divided by
the total time.
AVERAGE
SPEED
=
TOTAL DISTANCE
TOTAL TIME
Hint: “Bottom---out”
“Top--------in”
Riddle me this…..
• Imagine that two
birds leave the same
tree at the same
time. They both fly at
10km/h for 5 min, 12
km/h for 8 min, and
5km/h for 10 min.
Why didn’t they end
up in the same
place?
Direction matters….
• The birds went
in different
directions!!!
• Their speeds
were the same,
but they had
different
velocities.
Velocity: Direction
Matters
• The speed of an object in a
“particular direction” is called
velocity.
• BE CAREFUL! Do not confuse speed &
velocity; they do not mean the same thing!
• What is the difference between speed and
velocity?
Velocity is the speed plus the “direction”
in which the object is moving.
Example of Speed vs.
Velocity
• Speed: A car traveling 60 mph
• Velocity: A car traveling 60
mph east
• Don’t get speed and velocity confused:
(Direction is added to the object’s
velocity.) EX: When you use a compass to
determine the direction in which you are walking 5
mph.
Target Goals…..
• “I can……..”
• ______Define
acceleration.
• _____ Define Force.
• _____ Describe how
forces changes an
object’s motion
(Newton’s laws).
Acceleration:
(---has a slightly different meaning in
science)
• The “rate” at which “velocity changes”
over time is called acceleration.
• ** An object accelerates if its speed,
direction, or both change.
Did acceleration occur?
If so, … “Why?”
• Scenario 1:You are riding your bike at 9km/h. Ten
minutes later, your speed is 6km/h.
• (Yes!! Acceleration occurred because speed
decreased)
• Scenario 2:You ride your bike around the block at a
constant speed of 11km/h.
• (Yes!! Acceleration occurred because direction
changed)
• Scenario 3:You ride your bike in a straight line at a
constant speed of 10km/h.
• No!! Acceleration did not occur because neither
speed nor direction changed.)
Are you accelerating if…
•
•
•
•
1.
2.
3.
4.
You pick up the pace walking? Yes!
A car turns on a race track? Yes!
You slow down to a steady walk? Yes!
An object is moving in a circle? Yes!
• What about you standing still? NO!!
Acceleration according
to Tim and Moby
• BrainpopAcceleration
After the
brain pop
complete the
review quiz!!
Practice problem:
• Calculate the acceleration in the following
problems:
•
Remember:
Acceleration = Final speed – starting speed
time
1) Barney sat waiting anxiously at the red light.
Realizing he was late for the bike competition
Barney raced across campus at a speed of 80 m/s
only to be pulled over by the bike patrol 20
seconds later. Calculate his acceleration.
Try one more……
• Bert traveled on his bicycle at a speed of
45 m/s. Ten seconds later Bert slowed
down to 15m/s to avoid hitting the turtle
that was crossing his path.
• Calculate Bert’s acceleration.
Check your answer…
1) Zach kicked a stationary soccer ball as hard as he
human possibly could. The soccer ball went as many
meters as it could in just 12 seconds. The ball had a
speed of 60m/s. Calculate his acceleration.
60m/s-----0m/s
12 seconds
= 5m/s/s
Answer:
• Zac hopped in a golf cart at the local country club and
tried to take off with it. He was speeding at 88 m/s when
he noticed an old lady with a walker on the sidewalk
ahead. In just 6 seconds, he quickly make a hairpin turn
at a speed 46m/s. Calculate his acceleration.
46m/s---88m/s_____
6 seconds
= -7m/s/s
Something to think about….
• “Where do you see a force
happening in the room right
now? Which object is exerting
the force, and which is
receiving it?”
The answer…..
• Copy and answer the statement below:
• “Where do you see a force
happening in the room right
now? Which object is exerting
the force, and which is
receiving it?”
• ANSWER: GRAVITY AND
FRICTION!!!
What is force?
• In science, a force is a push or
a pull and always acts in a
certain direction
• Ex. if you push something, the force is in the
direction of the push
• A force is anything that makes an
object accelerate. A force can change
the acceleration of an object. This
acceleration can be a change in the
speed or direction of the object.
Mass vs. Weight
• Mass is the amount of matter in an object,
so will it ever change?
• Mass is measured with a balancer.
• Weight, however, is the measure of the
amount of gravitational force exerted on
an object.
• Weight is measured with a spring scale
Weight…..
 Weight is also a force. The SI
unit for weight is measured in
Newtons.
Weight Is not the same as mass!
Mass is measured in kilograms.
Even if the mass of an object doesn’t
change its weight will change if its
distance from Earth changes.
Something to think
about…
• Does this
astronaut have
the same mass
on Earth as he
does on the
moon?
• Does he have
the same
weight?
Forces cont’d
• It is not always easy to tell
what is exerting a force or what
is receiving a force. For
example, you cannot see what
exerts the force that pulls
magnets to refrigerators.
Assignment….
• BrainPOP:
"Force"
• After the brain
pop, complete the
review quiz and
the “A Weighty
Problem”
handout. Paste
this onto ISN
page 211.
Target Goals….
• I can describe
how forces
change an
object’s motion
(Newton’s Three
Laws of Motion)
Newton’s
Laws of Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction
• 1st Law – An object at rest will stay at
rest, and an object in motion will stay in
motion at constant velocity, unless
acted upon by a force.
1st Law
• Once airborne,
unless acted
on by a force (
such as gravity
and air
resistance ), it
would never
stop!
1st Law
• Unless acted
upon by a
force, this golf
ball would sit
on the tee
forever.
1st Law (law of inertia)
• Inertia is the tendency
of an object to resist
changes in its velocity:
whether in motion or
motionless.
• Basically, an object will
“keep doing what it
was doing” unless
acted on by a force.
These pumpkins will not move
unless acted on by a force.
So remember: Newton’s 1st law Fir
Is also called the law of inertia.
Inertia is the tendency of an object
to resist change in motion.
• (ex#1: Because of inertia you slide towards the side of a car when the
driver turns a corner.
• (ex#2: Inertia is also what causes a car, bike, or plane to stop
immediately).
Inertia
A property of matter
Mass is a measure of inertia.
The tendency of an object to resist
any change in its motion
The smaller the mass the less the
amount of inertia
The greater the speed the more the
inertia is needed.
Mass and Inertia are
related:
• Changing the motion of the smaller object
will be easier than changing the motion of
a larger object, because the smaller car
has less mass.
• Inertia makes it harder to accelerate a car
than to accelerate a bicycle. It also makes
it easier to stop a moving bicycle than a
car moving at the same speed.
More Examples from Real Life
A powerful locomotive begins to pull a
long line of boxcars that were sitting at
rest. Since the boxcars are so massive,
they have a great deal of inertia and it
takes a large force to change their
motion. Once they are moving, it takes
a large force to stop them.
On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).
Newtons’s 1st Law and
You
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 km/hour.
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always a force acting upon it.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
nd
2
Law
nd
2
Law
The acceleration of
an object depends on
the net force acting on
the object and the
object’s mass
Formula: (F= ma)
In simpler terms:
Second law: The greater
the force applied to
an object, the more
the object will
accelerate depending
on the mass.
It takes more force to
accelerate an object
with a lot of mass than
The player in black had more
to accelerate
acceleration thus he hit with
something with very
a greater amount of force
little mass.
Second law:
The force of an object is equal to the
product of its mass and acceleration,
or
Force is calculated by:
(force = mass x acceleration)
What does F = ma mean?
- Is it harder to throw a basketball or a baseball?
- Why??
A basketball, of course! It has more mass. If you throw
both balls with the same force, the speed of the baseball
will be greater.
To get the basketball to have the same acceleration as
the baseball, you must double the force you apply.
So….if the force applied is the same, as mass increases, the
acceleration of an object decreases
nd
2
Law
• When “mass is in kilograms”
and “acceleration is in m/s/s”,
the unit of force is measured in
Newtons (N).
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
F = ma
98 N = 10 kg x 9.8
m/s/s
9.8 N = 1 kg x 9.8
m/s/s
Remember:
•
F=
M
x A
• Force =
Mass x Acceleration
• (newtons)= (kg) x (m/s/s)
If mass remains constant, doubling the acceleration, doubles the
force. If force remains constant, doubling the mass, halves the
acceleration.
nd
2
Law (F = m x a)
• How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
• Write the formula
• F=mxa
• Fill in given numbers and units
• F = 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800 N
Examples of Newton’s 2nd
Law……(Explain how)..
a) hitting a baseball?
the harder you hit the ball the farther it goes….
b) Gas pedal in a car? Brake pedal in a car??
accelerating or decelerating a car
c) The positioning of football players?
massive players on the line with lighter ( faster to
accelerate) players in the backfield
d) a loaded versus an
unloaded truck?
more force is needed to get
the loaded truck to start
and stop moving.
third law:
For every
action force, there
is an equal and
opposite reaction
force. (Forces are
always paired)
3rd Law
According to
Newton, whenever
objects A and B
interact with each
other, they exert
forces upon each
other. When you sit
in your chair, your
body exerts a
downward force on
the chair and the
chair exerts an
upward force on
your body.
3rd Law
Example:
There are two
forces resulting
from this
interaction – “a
force on the chair
and a force on
your body”. These
two forces are
called action and
reaction forces.
Newton’s 3rd Law in Nature
• Consider the propulsion of
a fish through the water.
A fish uses its fins to push
water backwards. In turn,
the water reacts by
pushing the fish forwards,
propelling the fish through
the water.
• The size of the force on
the water equals the size
of the force on the fish;
the direction of the force
on the water (backwards)
is opposite the direction
of the force on the fish
(forwards).
3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion.
How?
As the birds push down
on the air with their
wings, the air pushes
their wings up and gives
them lift.
Third law continued..
• Consider the flying motion of birds. A bird
flies by use of its wings. The wings of a
bird push air downwards. In turn, the air
reacts by pushing the bird upwards.
• The size of the force on the air equals the
size of the force on the bird; the direction
of the force on the air (downwards) is
opposite the direction of the force on the
bird (upwards).
• Action-reaction force pairs make it
possible for birds to fly.
Other examples of Newton’s Third
Law
How is this ball and
bat demonstrating
Newton’s 3rd law?
The baseball forces
the bat to the left
(an action); the bat
forces the ball to
the right (the
reaction).
3rd Law
• How is your car’s
motion on the way
to school like
Newton’s 3rd law?
• A car is equipped
with wheels which
spin forward. As the
wheels spin forward,
they grip the road
and push the road
backwards.
3rd Law
The reaction of a rocket is an
application of the third law
of motion. Various fuels are
burned in the engine,
producing hot gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom of
the tube. As the gases move
downward, the rocket moves
in the opposite direction.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall?
You slide in the opposite direction (away from
the wall), because you pushed on the wall but
the wall pushed back on you with equal and
opposite force.
Why does it hurt so much when you stub
your toe?
When your toe exerts a force on a rock, the
rock exerts an equal force back on your toe.
The harder you hit your toe against it, the
more force the rock exerts back on your toe
(and the more your toe hurts).
Brain pop: Newton’s
laws (Tim and Moby
Style)
• Brainpop movie:
Newton's Laws
• After the brainpop:
complete the
worksheets:
“Which Law” and
“Brainpop review
worksheet”.