Transcript Chapter One, Unit F - Warren County Schools

Forces and Motion
Science Question of the Day
 What causes the noise when you crack your joints?
Gravity
 Gravity is a specific kind of force.
 A force is simply the push or a pull on an object.
 Can you think of some examples of pushes and pulls
on objects?
 Take my soccer ball for example…
 How about my yo-yo…
Gravity
 Kicking the soccer ball I am applying a force quickly to
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an object.
Forces can also be added over time.
What are some examples of forces applied over time?
Think about my car…
Or an elevator…
Gravity
 Some forces are always acting on us.
 Gravity is an example of a force that is always acting
upon us no matter where we are!
Gravity
 Insert video on gravity here.
Gravity
 Each piece of matter in the universe no matter how
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large or how small pulls on one another.
This force is called gravitation.
We see this most on Earth with how objects fall to the
ground like my bouncy ball.
Our gravity keeps everything on the earth, even the
oceans!
If we didn’t have gravity or we had a weaker
gravitational force our water and some of our
atmosphere might drift off into space!
Gravity
 Even though gravity is what keeps us on the Earth, it is
a pretty weak force.
 My bouncy ball and soccer ball both have gravitational
force.
 Do you think if I put them on the ground they will roll
toward one another?
 Let’s find out…
Gravity
 What do you know about that? They didn’t roll
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together.!
Why not?
It’s not because of their size. Size has nothing to do
with gravitational force.
It depends of the mass of the objects and the distance
between them.
Think about the planets. Jupiter is huge, but it’s
gravitational pull is not stronger than say Uranus. It’s
all about the mass, baby!
Mass and Gravity
 The more an object weighs (weight is relative, but we’ll
use it here since we are all Earthlings, I think…) the
more gravitational pull it has.
 Remember we said that some objects can be very
small, but weigh a lot while some objects can be very
large and weigh a small amount.
 Take a marble and a beach ball for example. Which has
more mass?
Distance and Gravity
 The closer you are to something the more gravitational
pull you will feel.
 We notice this when we think about the sun and the
Earth.
 The sun is about 330,000 times more massive than the
Earth, but we feel the Earth’s gravitational pull more
because we are closer to it than the sun.
 Did you know that if you are standing on a mountain
top you have less gravitational pull from the Earth
than you do if you are standing at sea level?
Can gravity be measured?
 Yes! Gravity can be measured in a unit called newtons.
 Any ideas as to why it is called a newton?
 When we step on the scale we are measuring the force
of gravity on ourselves.
 We can express our weight in newtons just as we can in
pounds as newtons.
Time for a laugh…
 When NASA first started sending up astronauts, they
quickly discovered that ballpoint pens would not work
in zero gravity. To combat the problem, NASA
scientists spent a decade and $12 billion to develop a
pen that writes in zero gravity, upside down,
underwater, on almost any surface including glass and
at temperatures ranging from below freezing to 300 C.
The Russians used a pencil.
 Get it? The Russians… Wow, that was funny!
Time for a little practice…
 Read “Gravity” and complete questions 1-6 for
homework.
 The teacher asked little Johnny, “What is the definition
of infinity?”
 Little Johnny replied, “Tonight’s homework
assignment.”
Science Question of the Day
 Why does pepper make you sneeze?
Speed and Velocity
 When we apply force to an object, you are putting the
object into motion.
 We can observe the motion in many ways:
 Did the object move quickly? Slowly?
 How far did the object go?
 Which direction did it move?
There are many ways in which we can observe motion.
Two ways we are going to look at today are speed and
velocity.
Speed
 Let’s think about a pitcher and a baseball.
 How fast is he or she really pitching and how can we
determine this?
 How far is the pitcher’s mound from home plate?
 How long did it take for the baseball to go from the
pitcher’s glove to home plate?
 The relationship between the distance and the time is
the speed.
Speed
 To calculate the speed, you must divide the distance by the
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time.
Let’s try that in the classroom.
How far am I from my volunteer?
How many seconds did it take for the ball to reach my
volunteer?
Let’s do some math!
We just used a ratio in real life! Ms. White would be so
proud!
It is a comparison of the distance moved to time that
elapsed during the motion.
Speed
 We could even take this further and figure out how far
my ball could travel in an hour.
 How many feet are in a mile?
 5,280 feet
 What equation could we make in order to determine
how many miles per hour my ball traveled?
Speed
 Speed is difficult to calculate at a constant.
 For example, the wind could affect how quickly my ball is
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moving or the track in which it moves.
If I’m driving a car, I may speed up or slow down depending
on traffic patterns.
This is why we determine the average speed of objects.
Here’s an example: A trip from here to Disney World is 756
miles. This trip can be made in 12 hours and 49 minutes.
What average speed must we go to get there in this amount
of time?
Speed
 Insert movie here.
Velocity
 Another way to observe motion is Velocity.
 Velocity is the speed of an object going in a particular
direction.
 Suppose two cars pass each other in opposite lanes of
an east/west highway. Both cars are traveling at 75
mph, so both have the same speed. But the cars are
going in different direction, so they have different
velocities.
 One car has a velocity of 75 mph heading east and the
other is 75 mph heading west.
Velocity
 Velocity is simply speed with a direction.
 Think about a race car on a circular track.
 The speed may stay constant, but the velocity changes
when the car changes direction.
Momentum
 Could you tell the difference between a bowling ball
that was painted like a soccer ball and a real soccer ball
rolling down a bowling lane?
 When they hit the pins you could! The bowling ball
would knock the pins over, but the soccer ball would
only move a couple of pins.
 This is because the bowling all is heavier and would
have more momentum.
Momentum
 Momentum is the measure of how hard it is to slow
down or stop an object that is in motion.
 Think about football players. Is it easier to stop the
quarterback or the lineman?
 Duh, the quarterback because he is smaller and
lighter.
 You can determine the momentum of an object by
multiplying the mass times the velocity.
Momentum
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Let’s figure out the momentum of the bowling ball.
The ball weighs 8 lbs.
It is traveling at 3 mph.
What is it’s momentum?
How about the soccer ball?
It weighs 1 lb.
It is also traveling at 3 mph.
What is it’s momentum?
Obviously the bowling pin the linebacker in this
example.
Conservation of Momentum
 When the bowling ball struck the pins it created a
collision.
 There is a law of science that states that and object’s
momentum before a collision stays the same as
momentum after the collision if no other forces act on
the object.
 We can see this by the bowling pins absorbing the
force of the bowling pin and scattering in the lane.
 This is called the law of the conservation of
momentum.
Time for a little practice…
 Read “Speed and Velocity” and answer questions 1-6
for homework tonight.
Science Question of the Day
 What is static electricity and how does it work?
The Laws of Motion
 There are several laws that were discovered by
scientists thousands of years ago that describe how
objects move.
 We all know about Newton and gravity, but some you
may not have heard of are Galileo, Aristotle, and
Ptolemy.
 Over the next few days, we will discover Three Laws of
Motion.
First Law of Motion
 An object at rest tends to stay at rest, while an object in
motion tends to stay in motion in a straight line until
an outside force acts on it.
 What does this mean?
 Objects do not change their velocity unless some force
acts on them.
 Example: A hockey puck sitting on the ice does not
move until something makes it move.
 The puck will continue to travel in the same direction
across the ice until a stick changes it or friction slows it
down.
Friction
 Friction is a force that opposes motion whenever two
surfaces rub against each other.
 Friction is only present when other forces such as
gravity or muscle power are present and act to move an
object against something else.
 Friction is what causes objects that are rolling to stop
once the energy that keeps them moving is less than
the energy that was required to make them move.
 Let’s roll this ball and see what happens.
Inertia
 Objects tend to resist a change in motion.
 For example, when you are in your car and you go
around a curve, your body tends to want to go in the
opposite direction. This is because of inertia.
 This is the property that makes it hard to push a car
when it is parked, or hard to stop it once it is moving.
 Sometimes the first law is called the law of inertia, but
inertia is not a law; it is a property.
Balanced & Unbalanced Forces
 Before we can learn about the Second Law of Motion,
we must first learn about Balanced and Unbalance d
Forces.
 Which one of these pictures if balanced? Unbalanced?
Balanced Forces
 We have been learning about forces such as gravity,
inertia, friction, etc.
 Most of the time, object have more than one force
acting on them at a time.
 A ball rolling across the floor is experiencing all three
of the above stated forces.
 Balanced forces are forces that are acting equally on an
object, but are acting in opposite directions.
Balanced Forces
 When the forces are balanced, either the object does
not move (like the teddy bear in the picture), or it
moves at a constant velocity. This is called the net
force.
 Think about a skier. What forces of motion are acting
on him?
Balanced Forces
 The skier continues down the slopes because the forces
of gravity, inertia, and friction are all balanced.
 Once he reaches the bottom of the slopes, the force of
friction will overtake the forces of gravity and inertia,
and the skier will eventually stop.
 This is when Unbalanced Forces take over.
Unbalanced Forces
 Unbalanced forces cause a change in velocity making
the object speed up, slow down, or change direction.
 In a game of tug of war, two teams are pulling on the
rope. If they are both pulling with equal force, than
the forces are balanced and the net force is zero; the
rope does not move.
 However, if a team pull harder, then the rope starts to
move in their direction.
 This is an unbalanced force.
Unbalanced Forces
 Think about the following scenarios. How are
unbalanced forces acting on the objects?
 Speeding up a car
 Slowing down a car
 Changing the direction of a car
Here’s a hint: friction!
Forces
 Insert movie on forces here.
Time for a little practice…
 Read “Balanced and Unbalanced Forces” and answer
questions 1-5 for homework due tomorrow.
Science Question of the Day
 How and why do cats purr?
Second Law of Motion
 This law explains how an unbalanced force changes
the motion of objects. The force of an object is equal to
its mass times acceleration
 When unbalanced forces act on objects it changes the
velocity of the object (its speed, direction, or both).
 The rate of this change is called acceleration.
 This law states:
 The greater the force, the greater is the acceleration, or
change in velocity.
 The greater the mass, the smaller is the acceleration or
change in velocity.
Second Law of Motion
 So basically, the more force applied, the faster it will
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go. The less force applied, the slower it will go.
And, the more mass you have in an object, the more
force is needed for acceleration,
Take a miniature car for example vs. a real one.
I can push the miniature with one finger with very
little effort, and it shoots forward.
If I tried to do that with my car I would probably break
my finger.
Second Law of Motion
 The direction in which force is applied is the direction
in which the object will move.
 For example, if an object is already moving and we
push it from behind it will simply speed up.
 Let’s have a little race.
Second Law of Motion
 If the force is applied from the opposite direction the
object is moving, then it will slow down the object.
 For example, if we were to play bumper office chairs
(and we won’t for safety sake), and I were to push my
chair into someone else’s chair then it would slow
down or stop their motion.
 We see this in front-end collision crash tests.
Third Law of Motion
 For every action force, there is an equal and opposite
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reaction force.
This basically means that forces always come in pairs.
For example, as I am pushing down on this floor with
my feet, the floor is pushing back.
The forces are always equal and opposite.
If the floor did not push back, I would fall right
through.
The Laws of Motion
 Insert movie here.
Time for a little practice…
 You and a partner will be rotating through several
mini-experiments and recording your results over the
next few days.
 All the activities are designed to help you understand
the different laws of motion.
 At the end of each rotation you will fill out your
research guide. This is your final product that will be
due next week.
 Please make sure your put effort into your research
guides as they are worth 100 points!