Welcome to Mrs. Sharp`s Classroom

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Transcript Welcome to Mrs. Sharp`s Classroom

Welcome to Physical Science
w/ Mrs. Brown!
Unit 4-Force &
Lesson 1-Force
What Is a Force?
 Force and motion are closely linked.
 A force is a push or a pull.
 There are different forces that act on objects, and
most, but not all, forces will change the position of
an object.
 Some forces do not change an object’s motion.
That’s because other forces may be acting on the
object at the same time.
 Some of the forces acting on the object may cancel
each other out
Magnitude and Direction
 A force has:
 Magnitude, or size-measured in an SI unit called a newton
 Direction
 In the United States, scientists commonly use pound to
describe the unit of force. This unit can be converted to
newtons with this equation: 1 pound = 4.45 newtons.
 A force also has direction.
 Described using the words up, down, forward, backward,
or using the points on a compass for north, south, east, and
Force Diagrams
 Net force-the sum of all forces acting on an object.
When the net force on an object is zero, the object’s
motion does not change. So, an object at rest will only
start to move if the net force acting on it is not zero.
Only a net force different from zero changes the motion
of an object.
 Balanced Forces- the net up/down force is equal to
 Unbalanced forces-those with a net force that is not
equal to zero.
Lesson 2-Gravitational
What Is Gravity?
 Earth’s gravitational pull has constant influence,
preventing us and everything on earth from floating off
into space.
 The earth is not the only object that exerts gravitational
 Gravity is a force of attraction that is universal.
 Every mass exerts a pull on every other mass.
Mass Versus Weight
 The words mass and weight are
sometimes used to mean the same thing,
but they are very different.
 The mass of an object is how much matter
is in the object. An object’s mass on earth
is the same as it would be anywhere else
in the universe.
 Weight, however, is a measure of
gravitational force, and an object’s weight
can change depending on where the
object is in the universe.
Distance and Gravity
 The effect of earth’s gravitational force on us is more
noticeable than our effect on the earth because earth is
so massive.
 But the sun is even more massive than earth.
 Although the sun is more massive than earth, it is much
farther away.
 Gravitational force between objects decreases as objects
move farther apart.
 Gravitational force is stronger between objects that are
closer together.
The Law of Universal
Newton greatly expanded our understanding of
gravity and its importance in the universe.
 He defined the law of universal gravitation with
these main points:
1. Every object in the universe exerts a
gravitational force on every other object.
2. The size of the gravitational force depends on
the masses of the objects.
3. The size of the gravitational force depends on
the distance between objects
Lesson 3-Motion
Let’s see how science measures and describes the
relationships between motion, speed, and distance.
Different Kinds of Motion
 3 of the most common types of motion are:
 translational motion
 rotational motion
 vibrational motion
Describing Position
 When we talk about a moving object, we
talk about where it started and where it is
going. Therefore, in order to describe the
motion of an object, we must first identify
its position. Often, the easiest way to
describe an object's position is to compare
its position to the position of other
Distance vs. Displacement
 Distance is how far an object travels, it
does not take the direction of motion into
 Displacement describes how far and in
what direction an object has moved
relative to its starting point.
Calculating Displacement
 You can use this mathematical formula to
calculate displacement:
 d2 – d1 =Δd = displacement
Δd is read “delta d” or “the change in d.”
Remember, displacement
indicates distance traveled in a specific
Calculating Changes in Time
 Translational motion involves a change
in position and also a change in time.
 Just like distance, time also needs a
reference point, a coordinate system, and
a system of units, such as seconds.
 t2 – t1 = Δt = change in time
Lesson 5-Speed &
In this lesson, you’ll find out exactly what these two
words mean, and how to calculate the speed and
velocity of any moving object.
 So we have good idea of
what speed means:
 Some things move fast-high rate of speed
 Some things move slowly-low rate of speed.
 Average Speed-distance an object
travels divided by the time it takes to
travel that distance.
 Speed takes into account both the distance
traveled and the time it takes to get there
Calculating Average Speed
Average speed is represented by the formula: s = d / t
 In this formula,
s represents average speed
d represents total distance
t represents total time
 So, if you walk 12 miles in 3 hours, d = 12 miles
and t = 3 hours. Your speed is:
 s=d/t
s = 12 miles/ 3 hours
s = 4 mph (miles per hour)
Velocity & Calculating
Average Velocity
 Velocity-rate that an object moves in a certain
 Velocity is always represented as a direction in relation to
a reference point (50 km/hour east)
 Average velocity is represented by the formula: v =
Δx / Δt
 V-represents average velocity
Δx-represents the object’s change in position, or
Δt-represents change in time
 To calculate an object’s average velocity, you need to
know where the object started, the point where it ended,
and how long it took to get there.
Lesson 6-Measuring
Speed & Velocity
In this lesson, we will use the formulas for speed and
velocity to learn more about the movement of
everyday objects.
Speed & Velocity Formulas
 To measure the average speed of an object, you need
to know the total distance it has traveled over a period
of time.
 To measure the average velocity of an object, you
need to know its displacement over a period of time.
 s=d/t
average speed = total distance / total time
 Total distance does not take into account direction.
 v = Δx / Δt
average velocity = displacement / change in time
 Displacement is the change in position of an object in
relation to a reference point, so it always includes a
Velocity Includes Direction
 The measurement of velocity always includes
direction of motion, such as north, south, east, or
 If the movement of an object is in one dimension
(such as along a straight line), velocity can be
described as either positive (+) or negative (–) in
relation to a reference point.
 Speed does not include direction. It always has a
positive value.
 No matter which direction it rolls, the marble’s
speed, however, is always positive
Rearranging the Formula for
You can rearrange the formula for average velocity to solve
for displacement or change in time.
 If you know an object’s average velocity over a period of
time, you can calculate how far it moved using the
following formula:
 Δx = v x t
displacement = average velocity x change in time
 If you know an object’s average velocity and
displacement, you can solve for change in time using the
following formula:
 Δt = Δx / v
change in time = displacement / average velocity
Lesson 8: Acceleration
Let’s look at how acceleration is defined, how it’s
measured, and how it relates to our understanding of
 Acceleration-how quickly velocity changes;
acceleration is occurring when objects speed up, slow
down, or change direction
 Acceleration is the rate of change of velocity.
 We say that an object is accelerating if its velocity
changes divided by time
 involves both speed and direction.
 changes when speed changes.
 changes when direction changes, even if speed remains
Acceleration and Gravity
 If you drop an object, it will fall to the
ground due to gravity.
 As it falls, it moves faster and faster.
 Acceleration due to gravity=9.8
meters/per second/per second (m/s/s) or
9.8 m/s2.
 9.8 m/s2 (approximately 22 mph) faster every
second it falls.
Calculating Acceleration
 The average acceleration of a moving object can be
calculated using the formula a = Δv / Δt.
a represents average acceleration
Δv represents change in velocity
Δt represents change in time or elapsed time
(the time the change in velocity takes)
 Change in velocity can be calculated by subtracting
the initial velocity (vi) of a moving object from its final
velocity (vf ).
 The formula for the change in velocity is:
Δv = (velocityf - velocityi)
Positive Acceleration &
 Imagine a car pulling away from a stop sign-the car’s
change in velocity (velocityf – velocityi) and its
acceleration both have positive values. When an object
moves in one dimension (along a straight line) in the
positive direction, and its velocity increases over time (it
speeds up), its acceleration is positive.
 Now imagine that same car slowing down as it
approaches a stop sign. Its final velocity is less than its
initial velocity. Therefore, if the velocity was in the
positive direction, it became less positive. Its change in
velocity and its acceleration have negative values.
 Deceleration is a decrease in velocity over time. When
deceleration occurs in motion along a line, the
acceleration and velocity point in opposite directions.
That means if the velocity is positive, deceleration
corresponds to negative acceleration
Lesson 9: Newton's First
Law of Motion
Newton’s three Laws of Motion describe our countless
interactions with forces and motion every day. In this
lesson, you will explore Newton’s First Law of Motion.
Newton’s First Law of Motion
 In the 1600’s, Sir Isaac Newton described
his three laws of motion.
Newton’s First Law of Motion:
A body at rest will remain at rest unless
acted on by an external, unbalanced
force. (Part 1) A body in motion will
remain in motion unless acted on by an
external, unbalanced force. (Part 2)
What does this mean? Let’s look at
both parts:
Objects at Rest and in Motion
& Inertia
Part 1: You must apply a force to
get an object moving.
All objects have inertia: the mass of
an object is a measure of its inertia.
 The bigger an object is (the more mass it has),
the more force it takes to move it.
Inertial Forces
 Inertia is involved in most of the activities that you do.
 When you are riding in something and it starts moving, you
feel like you are being pushed backward. This is because
your body tends to remain still relative to the ground, while
object you riding in is moving underneath you.
 A similar thing occurs when you come to a sudden stop in
the same object. This time your body continues to move
forward while the object under you stops. So, you feel as
though you are being pushed forward. These forces you feel
are not real forces; they are called inertial forces. There is
no actual force pushing you backward when the object
starts moving, or pushing you forward when the object
suddenly stops.
Part 2: You must apply a force to
get an object to stop moving once it
is going.
Friction: the force that opposes motion,
surface to surface force, or surface to
environment (like air).
Slippery ice has
much less friction
than the ground.
Lesson 10: Mass and
In this lesson, you will explore how scientists define
mass and weight. You will learn what mass and weight
mean, how they are measured, and how they can be
Mass and Weight
Mass and weight are different:
Mass is the amount of matter an object
 Measured in grams (g)
 Mass of an object is the same no matter where in the
universe you are.
Weight is the amount of gravitational
force on an object
Mass and Inertia
 The mass of an object is a measure of the
object’s inertia.
 The greater the object’s mass, the greater
its inertia.
 So, a more massive object is harder to move
from rest, or to change the motion of, than a
less massive object.
Exploring Mass
 A balance compares the mass of the
object being measured with known
masses by comparing the forces from
gravity that each produces.
 The mass of an object is the same no
matter where in the universe it is
measured. This is because the amount of
matter in an object does not change by
just moving it.
What mass vs. weight looks like…
Exploring Weight
 Will your weight differ on earth and on the
 Yes. The gravitational force acting on you is not
the same on earth as it is on the moon. That’s
why your weight of the is different.
 Weight, unlike mass, will be different for the
same object in two places where gravity
 An object can even have a slightly different weight
at different locations on earth. This is because the
earth’s gravitational pull varies a little
Weight Is a Force
 Since weight is the pull of gravity on an object, it is a force
and mass is simply a quantity of matter and is not a force.
 Scientists define weight using the following formula:
 Weight = mass x acceleration due to gravity
 In this formula:
w represents weight
m represents mass
g represents acceleration due to gravity
 (g = 9.8 m/s/s (meters per second squared) or 9.8 m/s2
 Measured in Newtons (N)
 Weight changes depending on gravitational pull
Lesson 11: Newton's
Second Law of Motion
You will learn what Newton described about the motion
of an object when an unbalanced force does act on it.
You will explore how force, acceleration, and mass are
Newton's Second Law of
When an unbalanced force acts on an
object, the object will be accelerated. The
acceleration will be proportional to the
force, and will be in the same direction as
the force.
Acceleration Depends on
Force and Mass
Let’s explore Newton’s Second Law of Motion on and
 Increasing the force of a push on something
increases its acceleration
 Decreasing the force would decrease the
 The same force acting on a smaller mass produces
a larger acceleration.
 Similarly, a larger mass would be accelerated less.
Expressing Force as a
 Newton developed a mathematical formula to describe
the relationship between force, mass, and acceleration.
 The formula is: F = ma.
F = unbalanced force (N)
m = mass (kg)
a = acceleration (m/s2)
 The on-screen display shows how to determine the
amount of force needed to accelerate a 2-kg mass at 3
m/s2. Take a few minutes to study how the problem is
solved. Remember that the units of a newton are
equivalent to kg m/s2.
***Note that in this lesson, only average force and
acceleration examined. It is very difficult to examine forces
and accelerations that occur in an instant in time
Lesson 12: Newton's
Third Law of Motion
What happens when one object exerts a force on another
object? How does the motion of each object change?
To find out we will explore Newton’s 3rd Law!
Let’s remember some of the
important concepts so far in this
 A force is a push or a pull.
 An unbalanced force changes an object’s motion.
 Objects have inertia and resist forces that try to
change their motion.
 Friction is the force between two surfaces that
opposes motion.
 Unbalanced forces cause objects to accelerate
according to the equation: F = ma
Forces Occur in Pairs
 Imagine a Spring.
 The reason it behaves as it does is because forces
always occur in pairs.
 Whenever you are pushing on a spring, the spring is
pushing back on you.
 Likewise, if you are pushing on a wall, the wall is
pushing back on you! Although you can’t see this the
way you can see the spring being compressed, it’s true.
Newton described force pairs in his Third Law of
Newton’s Third Law of
For every action, there is an
equal but opposite reaction
 A force is a push or a pull.
 A push or a pull involves more than one object.
 One object does the pushing (or pulling), while the
other object gets pushed (or pulled)
 In other words…If one object exerts a force on a second
object, the second object exerts a force on the first
object that is equal in magnitude and opposite in
 Some action pairs are hard to notice
 Sitting on a chair
 Pushing against a wall
Computing Acceleration &
Actions and Reactions
Acceleration Formula: a = F/m
Where: a-acceleration, F-force & m-mass.
**Another way of stating Newton’s Third Law of Motion is
that for every action, there is an equal and opposite
 When two objects interact, the force exerted by the first
object is called the action force.
 The force exerted by the second object is called the
reaction force.
Lesson 13: Buoyant
Have you ever wondered why a rubber duck can float in your bathtub,
but a rock cannot?
In this lesson, you will explore the force exerted by fluids. You will
learn why some objects float, while others sink.
Buoyant Force
 Buoyant force is the force exerted on an
object which is immersed in a fluid.
 A fluid is any substance that flows:
 Gases and liquids are fluids
Buoyant Force and Weight
In order for an object to float in a
fluid, its weight (which is a force)
must be less than the buoyant force
exerted by the fluid.
 K12 example: a rubber duck and a rock in a
swimming pool
 The weight of the rubber duck is less than the
buoyant force of the water, so the duck floats.
 The weight of the rock is greater than the
buoyant force of the water, so the rock sinks.
Fluid Pressure
Fluid Pressure Facts:
(K12 example: Iceberg)
 When an object is placed in a fluid, pressure is exerted
on all its sides.
 This pressure is called fluid pressure.
 Pressure measures how concentrated a force is on a
particular area.
 The fluid pressure is the force that the fluid exerts on a
surface divided by the area of the surface.
 The pressure of all fluids increases as depth increases.
Archimedes’ Principle
How can we determine the amount of buoyant force acting
on an object?
 In the third century BC, a Greek
named Archimedes determined a way to
measure buoyant force, known
as Archimedes’ principle.
 According to this principle, when an object
is placed in a fluid, the buoyant force
acting on it is equal to the weight of fluid
that the object displaces.
 K12 example: rock in cylinder
Measuring Buoyant Force
 First, an object is lowered into a container
of water.
 As the object moves further down into the
container, water is displaced and flows
into a catch bucket.
 Finally, when the object is resting on the
bottom of the container, you can see the
total amount of water that has been
displaced by the object.
Measuring Buoyant Force
 Density=mass divided by volume or:
D = m / V
 Why do large ships float? Large volume in the
open hull makes the ship less dense than
Air Exerts Buoyant Force
Air Force Facts:
 Air is a fluid that exerts buoyant force.
 However, since the density of air is so low,
few substances float in it.
**A notable exception to this is helium. Air
is about seven times denser than helium.
So, party and parade balloons filled with
helium float in the air. The buoyant force
exerted by the air drives them upward
We will move on
to Unit 10