Download Forces - Riverdale Middle School

Chapter Introduction
Lesson 1
Motion
Lesson 2
Forces
Lesson 3
Types of
Forces
Lesson 4
Newton’s
Laws
Chapter Wrap-Up
How do forces
change the motion
of objects?
Motion
Essential Question
• How is an object’s motion related
to speed, velocity, and
acceleration?
Motion
Distance
Speed
Position
Velocity
Motion
Acceleration
What is motion?
• When you explain to your friend how to
get to your house, you most likely will tell
them direction – the way to go to get
there.
• You most likely will tell them distance.
– Distance is the length between two places.
What is motion? (cont.)
• Using direction and distance, can help
you identify the position of something.
– Position is the location of an object
compared with things around it.
What is motion? (cont.)
• A complete description of your position
includes a distance, a direction, and a
reference point – a starting point that can
be used to locate a place or thing.
What is motion? (cont.)
• Motion is a change in an object’s position
compared to fixed objects around it.
• The man in the boat is not in motion
compared to his fishing pole. He is in motion
compared to the buoy.
What is motion? (cont.)
• When riding your bike, in relation to the bike,
you are not moving.
• However, the trees and houses you pass
appear to move backward. This is known as
apparent motion – when things appear to
an observer to be moving but are not
actually changing position.
– You use apparent motion to determine what
direction and how fast you are going.
What is speed?
• Speed is how fast an object’s position
changes with time at any given moment.
• Units of speed are expressed in units of
distance per unit of time.
– Ex. Meters per second (m/s) or miles per
hour (mph).
What is speed? (cont.)
• Constant speed is a change of
position in which the same distance is
traveled each second.
– Ex. Look at the figure below. The same
distance is traveled in the same amount
of time.
What is speed? (cont.)
• Most moving objects do not travel at the
same speed at all times.
– Ex. In a car, you are stopping, starting and
passing other cars.
• When the car’s speed changes, it moves a
different distance each period of time.
What is speed? (cont.)
• Because objects do not travel at the
same speed at all times, it is useful to
calculate an average speed.
• Average speed is the total distance
traveled divided by the total amount of
time.
What is speed? (cont.)
• Graphs that show
comparisons
between distance
and time are called
distance-time
graphs.
• Constant speed is
shown as a straight
line on a distancetime graph.
What is speed? (cont.)
• You can use
distance-time
graphs to compare
the motion of two
different objects.
• The steeper line
indicates a faster
speed.
What is speed? (cont.)
• When an object is
not traveling at
constant speed,
the graph is not a
straight line but a
curved line.
What is velocity?
• Velocity is a description of a moving
object’s speed and direction.
– Ex. If a car is traveling south with a speed
of 70mph, its velocity would be 70mph
south.
What is velocity? (cont.)
• The velocity of an object can be
represented by an arrow.
• A greater speed is shown by a longer
arrow.
• The arrow points in the direction of the
object’s movement.
What is velocity? (cont.)
• Velocity changes when:
– the speed of an object changes
– the direction that the object moves changes
– or both the speed and the direction change
What is velocity? (cont.)
• Speed and velocity are often confused
for the same thing but they are not.
– Speed tells how fast something is going.
– Velocity tells how fast something is going
and in what direction.
What is acceleration?
• Acceleration is a change in the
velocity of an object over time.
• An object accelerates when its velocity
changes as a result of:
– increasing speed.
– decreasing speed.
– or a change in direction.
What is acceleration? (cont.)
• Acceleration has a direction and can be
represented by an arrow.
• The length of the arrow indicates the amount
of acceleration.
• The direction of the arrow depends on
whether velocity increases or decreases.
What is acceleration? (cont.)
• Acceleration is a change in velocity
divided by the time interval during
which the velocity changes.
What is acceleration? (cont.)
• When an object speeds up, the
acceleration is in the same direction as
the motion and is positive.
– Ex. Object goes from 0m/s to 6m/s in 3 sec.
acceleration = (6-0) ÷ 3
=6÷3
= 2m/s2
What is acceleration? (cont.)
• When an object slows down, the
acceleration is in the opposite direction
as the motion and is negative.
– Ex. Object goes from 6m/s to 0m/s in 3 sec.
acceleration = (0-6) ÷ 3
= -6 ÷ 3
= -2m/s2
• Which could be stated as deceleration at
2m/s2
How can you graph acceleration?
• A speed-time graph shows how speed
changes over time.
• A speed-time graph has time on the
horizontal axis—the x-axis, and speed
on the vertical axis—the y axis.
How can you graph acceleration?
(cont)
• An object at rest is not moving, so its
speed is always zero.
How can you graph acceleration?
(cont)
• For an object moving at constant speed,
the speed-time graph is a horizontal line.
How can you graph acceleration?
(cont)
• The line on the speed-time graph for an
object that is speeding up has an
upward slope.
How can you graph acceleration?
(cont)
• The line on the speed-time graph for an
object that is slowing down has a
downward slope.
The total distance traveled divided
by the total time taken to travel that
distance is known as which of
these?
A. speed
B. constant speed
C. instantaneous speed
D. average speed
How is constant speed
represented on a distance-time
graph?
A. a wavy line
B. a crooked line
C. a straight line
D. a curved line
A hot-air balloon rises at a rate of
3km/h. What is its velocity?
A. 3km/h, upward
B. 3mph, upward
C. 3 km/h
D. 3km/h/h
Forces
Essential Question
• What effect do forces have on
objects?
Forces
Force
Net force
Contact force
Balanced
forces
Noncontact
force
Unbalanced
forces
What are forces?
• Force is a push or pull exerted by one object
on another, possibly causing a change in
motion.
• Forces can cause:
– A moving object to accelerate or decelerate.
– An object to start moving.
– An object to change direction.
– An object to stop moving.
What are forces? (cont.)
• Contact force is a force that is exerted
only when objects are touching.
– Ex. baseball bat hitting a baseball or
fingers striking keyboard of computer
• Noncontact force is a force exerted
when objects are not touching.
– Ex. Magnetic force, like a compass needle or
gravitational force, Earth around the
Sun
How do forces affect each other?
• Net force is the combination of all
forces acting on an object.
• The net force depends on the directions
of the forces applied to an object.
• Because forces have direction, you
have to specify a reference direction
when you add forces.
How do forces affect each other? (cont.)
• A force moving in the reference
direction is positive, and a force in the
opposite direction is negative.
• When the forces applied to an object
act in the same direction, the net force
is the sum of the individual forces.
How do forces affect each other? (cont.)
• When forces act in opposite
direction on an object, the net force
is still the sum of the forces.
• The net force is the sum of the
positive and negative forces.
What are balanced and unbalanced
forces?
• Balanced forces are forces that act
together on an object without changing
its motion.
– Ex.
• Unbalanced forces are forces that do
not cancel each other out and that
cause an object to change its motion.
– Ex.
Which refers to forces that
combine and form a net force that
is not zero?
A. balanced forces
B. inertia
C. net force
D. unbalanced forces
When equal forces act on an object
in opposite directions, what is the
net force on the object?
A. zero
B. one
C. equal
D. balanced
Which changes when unbalanced
forces act on an object?
A. color
B. mass
C. motion
D. weight
Types of Forces
Essential Question
• What are the forces that act upon
objects?
Types of Forces
Gravity
Elastic force
Friction
Tension force
Air resistance
What is gravity?
• Gravity is the force of attraction
among all objects.
• Sir Isaac Newton developed the law of
universal gravitation in the late
1600s.
• The law states that all objects are
attracted to each other by a
gravitational force.
What is gravity? (cont.)
• The strength of force depends on the
mass of each object and the distance
between them. Page ___
What is gravity? (cont.)
• Weight is the gravitational force
exerted on an object.
• Near Earth’s surface, an object’s
weight is the gravitational force exerted
on the object by Earth.
• Because weight is a force, it is
measured in Newtons.
What is gravity? (cont.)
• An object’s weight is proportional to its
mass.
• Near Earth’s surface, the weight of an
object in newtons is about ten times its
mass in kilograms.
What is friction?
• Friction is a force that opposes the
motion of an object in contact with a
surface.
• There are 4 types of friction.
– static friction
– sliding friction
– rolling friction
– fluid friction
What is friction? (cont.)
• Static friction prevents surfaces
from sliding past each other.
– Ex. Keeps a book from starting to
move across a table as a breeze blows
through an open window.
– Up to a limit – An applied force can be
strong enough to overcome some static
friction.
What is friction? (cont.)
• Sliding friction opposes the motion
of surfaces sliding past each other.
– Ex. Pushing a box across a floor.
• Rolling friction opposes the motion
of a wheel turning along a surface.
– Ex. Skateboard and the street, slows
the skateboarder down.
What is friction? (cont.)
• Fluid friction is friction between a
surface and a fluid—any material,
such as water or air, that flows.
– Air resistance the force that opposes
the motion of an object through a gas.
• Ex. Leaves falling to the ground.
What is friction? (cont.)
• What causes friction between
surfaces?
– When the microscopic dips and bumps on
one surface catch the dips and bumps on
another surface, the microscopic
roughness slows sliding.
What is friction? (cont.)
• Lubricants
decrease
friction and with
less friction, it is
easier for
surfaces to slide
past each other.
What are elastic forces?
• Elastic force is a force exerted by a
material when it is stretched or
compressed.
• Tension force is a pulling force
applied to an object that can make
an object stretch.
– Ex. Stretching a rubber band.
The force from your fingers to pull
the rubber band is a tension force.
The force from the rubber band on
your fingers is an elastic force.
What are elastic forces? (cont.)
• Compression force is a squeezing force
applied to an object that can make an
object shrink.
– Ex. Squeezing a rubber ball in your palm.
• The push back from the ball is an elastic force.
• Normal force is a force exerted by an
object that is perpendicular to the surface
of the object.
– Ex. You sitting in a chair.
• Gravity is pulling you down
• Normal force from the chair is pushing up on you.
How can you identify forces on
an object?
• Many forces can be acting on an
object at the same time. These
forces can also act in different
directions.
– Ex. Sliding a book across a table.
• There are both horizontal and vertical
forces acting on the book.
Which refers to gravitational force
exerted on an object?
A. contact force
B. gravity
C. mass
D. weight
Which is proportional to an
object’s weight?
A. gravitational force
B. length
C. mass
D. noncontact force
Which is a force that opposes the
movement between two touching
surfaces?
A. net force
B. lubricant
C. gravity
D. friction
Newton’s Laws
Essential Question
• What are Newton’s three
fundamental laws of motion?
Newton’s Laws
Newton’s first law of motion
Newton’s second law of motion
Newton’s third law of motion
Action force
Reaction force
What is Newton’s first law of
motion?
• Sir Isaac Newton studied how forces
affect the motion of objects. He
developed three rules known as
Newton’s laws of motion.
What is Newton’s first law of
motion? (cont.)
• Newton’s first law of motion is a
physical law stating that an object at
rest tends to stay at rest and an object
moving in a straight line at a constant
speed tends to continue moving that
way.
What is Newton’s first law of
motion? (cont.)
• Newton’s first law of motion only applies to
balanced forces acting on an object.
• When unbalanced forces act on an object,
the object’s velocity changes.
– Ex. Soccer ball still on the ground will stay
there, because the vertical and horizontal
forces are all balanced, until someone kicks it,
then there is an unbalanced force acting on it.
What is Newton’s first law of
motion? (cont.)
• Newton’s first law of motion is sometimes
called the law of inertia.
• Inertia is the tendency of a moving object to
keep moving in a straight line.
– Ex. If a crash test dummy is traveling in a test car
and the brakes are pressed hard, what happens
to the dummy?
• The dummy will be thrown forward because it is
traveling at the same speed as the car. The
unbalanced force that stopped the car was applied
to the car and not the dummy.
What is Newton’s second law of
motion?
• Newton’s first law of motion shows that
a net force is needed for an object to
accelerate.
• Newton’s second law of motion shows
how much acceleration this net force
will cause.
What is Newton’s second law of
motion? (cont.)
• Newton’s second law of motion is a
physical law that states that an object’s
acceleration depends on the object’s mass
and the amount of net force applied to it.
What is Newton’s second law of
motion? (cont.)
• The direction of acceleration is the same
as the direction of the net force.
• Acceleration is expressed in meters per
second squared (m/s2), mass is in
kilograms (kg), and force is in newtons
(N).
What is Newton’s second law of
motion? (cont.)
• Momentum is a measure of how hard it
is to stop a moving object.
• Momentum is the product of an object’s
mass and velocity.
What is Newton’s second law of
motion? (cont.)
• Momentum is useful for studying the motion of
colliding objects.
– Ex. A train and a truck are traveling toward each other
at the same speed. Which will have more
momentum? Why?
• The train will have more momentum because it has
more mass.
– Can the truck ever have more momentum than the
train?
• Yes, if the truck’s speed is great enough.
What is Newton’s second law of
motion? (cont.)
• According to the law of conservation
of momentum, the total momentum of
a group of objects stays the same
unless outside forces such as friction
act on the objects.
What is Newton’s third law of
motion?
• Newton’s third law of motion relates forces
between two objects.
• Newton’s third law of motion is a physical
law that states that for every action there is
an equal and opposite reaction.
– Ex. Space shuttle lifting off.
• Burning fuel produces hot gases. The shuttle
pushes on the gases and the gases push on the
shuttle causing it to lift off.
What is Newton’s third law of
motion? (cont.)
• A force pair is the forces two objects apply
to each other.
• In a force pair, one force is called the
action force and the other force is called
the reaction force.
What is Newton’s third law of
motion? (cont.)
• If the forces of a force pair always act in
opposite directions and are always the
same strength, why don’t they cancel each
other?
– Because each force acts on a different object.
– Ex. The shuttle on the gas (action force)
The gas on the shuttle (reaction force)
What is weightlessness?
• Gravitational force exists among all
objects in the universe.
• The force of gravity causes an astronaut
to fall towards Earth but at the same time
he is moving forward and no force is
slowing down the acceleration.
– This is what keeps him/her on a circular
path orbiting Earth and is why he/she does
not feel the force of gravity or feels
weightless.
What is weightlessness? (cont.)
• True weightlessness is a term used to
describe the condition of any object that
has moved to a distant region and is in a
position where there is no detectable
gravitational pull.
• However, there is no place in the universe
where gravitational pull does not exist and
an object is truly without weight.
Which law explains that every
force has a reaction force in the
opposite direction?
A. Newton’s third law of motion
B. Newton’s second law of motion
C. Newton’s first law of motion
D. Law of conservation of momentum
Which is a measure of how hard it
is to stop a moving object?
A. velocity
B. momentum
C. inertia
D. elastic collision
Visual Summary
Chapter Review
Standardized Test Practice
An object’s motion
changes if an
unbalanced force
acts on it.
Lesson 1: Motion
• Speed is how fast an object’s position
changes with time.
• Velocity changes when speed, direction, or
both speed and direction change.
• Acceleration is a change in velocity over time.
An object accelerates when it speeds up,
slows down, or changes direction.
Lesson 2: Forces
• A force is a push or a pull exerted by one
object on another.
• The net force is the sum of all forces
acting on an object.
Lesson 3: Types of Forces
• The law of universal gravitation states
that all objects are attracted to each
other by gravity.
• Friction can stop or slow down objects
that are sliding past each other.
Lesson 4: Newton’s Laws
• According to Newton’s first law of motion,
the motion of an object is not changed by
balanced forces acting on it.
• According to Newton’s second law of
motion, an object’s acceleration is the net
force on the object divided by its mass.
• Newton’s third law of motion states that
forces come in pairs, and these two forces
are equal in strength and opposite in
direction.
Which refers to a force that is
applied when two objects touch?
A. contact
B. gravity
C. mass
D. noncontact
The tendency of an object to
resist a change in its motion is
called what?
A. velocity
B. terminal velocity
C. net force
D. inertia
Which term refers to forces that
combine with a net force of zero?
A. unbalanced
B. terminal
C. centripetal
D. balanced
If acceleration is in the direction
of motion, which of these is true?
A. It is positive.
B. It is negative.
C. The object is at rest.
D. The speed-time graph has a
downward slope.
Where is speed located on a
speed-time graph?
A. The horizontal axis (the x-axis).
B. The vertical axis (the y-axis).
C. Speed is not indicated on a
speed-time graph.
D. The speed is the slope of the line.
Which describes the magnetic
force?
A. contact
B. gravitational
C. noncontact
D. static
Which term refers to the process
of changing position?
A. displacement
B. reference point
C. motion
D. velocity
If the net force acting on a moving
object is in the direction that the
object is moving, the object does
what?
A. stops
B. speeds up
C. slows down
D. changes direction
Which of these results in a
change of velocity?
A. the speed of an object changes
B. the direction an object moves
changes
C. both the speed and direction of an
object changes
D. all of the above
Which term refers to a measure of
how much the velocity of an
object changes in a unit of time?
A. direction
B. speed
C. acceleration
D. velocity