Download Chapter 12 Forces and Motion

Pretest
Chapter 12
1. What is relative motion?
2. What is the difference between distance and displacement?
3. How is average speed calculated?
4. On a distance-time graph, what does the slope represent?
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Pretest (continued)
Chapter 12
5. What is velocity?
6. How is acceleration related to velocity?
7. A backpack falls out of an open window. The backpack starts
from rest and hits the ground 1.0 second later with a velocity
of 9.8 m/s. What is the average acceleration of the backpack?
a. 9.8 m/s
c. 9.8 m/s2
b. 9.8 m
d. all of the above
8. How are mass and weight different?
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Interest Grabber
Section 12.1
Objects in Free Fall
What factors affect a falling object? Perform the following simple
activity to begin learning about the forces that act on falling
objects.
1. Stand beside your desk. Hold a sheet of notebook paper
level at eye level. Release the sheet of paper and watch it
fall. Describe the motion of the paper.
2. Hold a sheet of notebook paper that has been crumpled
into a tight ball at eye level. Release the crumpled paper
and watch it fall. Describe the motion of the paper.
3. How do the motions of the flat sheet of paper and the
crumbled ball of paper compare? What forces do you
think are acting on each sheet of paper?
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Reading Strategy
Section 12.1
Relating Text and Visuals
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a. Yes
e. Yes
i. No
b. No motion
f. No motion
c. Yes
g. Yes
j. Potted plant
accelerates.
d. No motion
h. No motion
Chapter 12
Forces
Unit 3 Chapter 12
&
Motion
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12.1 Forces
• A force is a push or a pull that acts on an object.
• A force can cause a resting object to move, or it can
accelerate a moving object by changing the object's
speed or direction.
• Force is measured by the stretch of a spring in newtons, abv.
as N.
• 1 newton is equal to 1 kilogram-meter per second squared
(1 N = 1 kg·m/s2).
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Representing
Forces
• You can use an arrow (vector) to represent the direction and
strength of a force.
• The direction of the arrow represents the direction of the
force.
• The length of the arrow represents the strength, or magnitude,
of the force.
• You can combine force vectors by vector addition to show the
resultant vector or the net force of how forces combine.
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Combining Forces
• The net force is the overall force acting on an object after all
the forces are combined.
• When the forces on an object are balanced, the net force
is zero and there is no change in the object's motion.
• When an unbalanced force acts on an object, the object
accelerates in the direction of the largest combined force.
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Combining Forces
(Vectors)
• Forces acting in opposite directions can also combine to
produce an unbalanced force.
• The net force equals the size of the larger force minus the
size of the smaller force.
• Two unequal forces act in opposite directions but combine to
produce an unbalanced net force.
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Combining Forces Acting
on an Object
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Figure 4
Combining Forces Acting
on an Object
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Figure 4
Combining Forces Acting
on an Object
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Figure 4
Combining Forces Acting
on an Object
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Figure 4
Combining Forces Acting
on an Object
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Figure 4
Combining Forces Acting
on an Object
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Figure 4
Friction
• Friction is a force that opposes the motion of objects that
touch, as they move past each other.
• There are four main types of friction: static friction,
sliding friction, rolling friction, and fluid friction.
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Friction - 2
• Static friction is the friction force that acts on objects that are
not moving.
• Sliding friction is a force that opposes the direction of motion
of an object as it slides over a surface .
• Rolling friction is the friction force that acts on rolling objects
like wheels.
• Fluid friction opposes the motion of an object through a fluid,
the higher the viscosity, the more resistance to flow.
• Air resistance comes under fluid friction.
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Gravity
• Gravity is a force of attraction that acts between any two
masses.
• Earth's gravitational force exerts a force of attraction on every
other object that is near Earth.
• Gravity can act over large distances.
• Earth's gravity acts downward toward the center of Earth.
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Falling Objects
• On Earth, both gravity and air resistance affect the motion of a
falling object.
• Gravity causes objects to accelerate downward, whereas air
resistance acts in the direction opposite to the motion and
reduces acceleration.
• Terminal velocity is the constant velocity of a falling object when
the force of air resistance equals the force of gravity.
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Projectile Motion
• When you throw a ball forward, you'll notice that it actually
follows a curved path.
• This curved path (a parabola) is an example of projectile
motion.
• The combination of an initial forward velocity and the
downward vertical force of gravity causes the ball to
follow a curved path.
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Interest Grabber
Section 12.2
Describing Motion
Imagine you are carrying a bowl of soup on a food tray as
you walk toward your favorite table. Suddenly, someone
walks in front of you. You stop abruptly to avoid a collision.
1. Describe the motion of the soup in the bowl immediately
after your abrupt stop.
2. Explain why you think the soup in the bowl behaved the
way it did.
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Reading Strategy
Section 12.2
Building Vocabulary
a.
b.
c.
d.
e.
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Inertia is the tendency of an object to resist a change in its motion
Mass
Mass is the amount of matter an object contains as measured by its inertia.
Weight
Weight is the force of gravity acting on an object.
12.2 Newton’s Laws
• Newton built on the work of scientists such as Galileo.
• Newton summarized his study of force and motion in several
laws of motion.
• Previous errors held back progress in the study of motion for
most of recorded. history, but the plague had forced Newton
to leave college and study on his own.
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Newton’s 1st Law
• According to Newton's first law of motion, the state of
motion of an object does not change as long as the net
force acting on the object is zero.
• In other words, an object in motion stays in motion, & an
object at rest stays at rest, unless acted upon by an outside
force.
• Inertia is the tendency of an object to resist a change in its
motion.
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Newton’s 2nd Law
• An unbalanced force causes an object's velocity to change.
• According to Newton's second law of motion, the
acceleration of an object is equal to the net force acting
on it divided by the object's mass.
• Acceleration = Net force/Mass or a = F/m
• Mass is a measure of the inertia of an object and depends on
the amount of matter the object contains.
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Newton’s Second Law
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Section 12.2
Newton’s Second Law
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Section 12.2
Newton’s Second Law
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Section 12.2
Newton’s Second Law
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Section 12.2
Effects of a Force on
Acceleration
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Figure 13
Effects of a Force on
Acceleration
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Figure 13
Effects of a Force on
Acceleration
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Figure 13
Effects of a Force on
Acceleration
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Figure 13
Effects of a Force on
Acceleration
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Figure 13
Effects of a Force on
Acceleration
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Figure 13
Combining Laws 1
&2
• Weight is the force of gravity acting on an object.
• Formula for weight is w = mg (g = 9.8 m/s2) or
Weight = mass X acceleration of gravity.
• Formula for force is F = ma or
Force = mass X acceleration.
Mass is a measure of the inertia of an object; weight is a measure
of the force of gravity acting on an object.
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Interest Grabber
Section 12.3
Changing Motion
Did you know that billiard balls can be used to study
the transfer of energy between objects?
1. Describe the motion of the balls in Figure A.
2. Figure B shows the motion of the billiard balls after
impact. Explain why the motion of the balls changes in
Figure B.
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Reading Strategy
Section 12.3
Summarizing
a. kg•m/s
b. mass (or velocity)
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c. velocity (or mass)
12.3 Newton’s 3rd
Law
• Forces always exist in pairs.
• According to Newton's third law of motion, whenever one
object exerts a force on a second object, the second
object exerts an equal and opposite force on the first
object.
• These two forces are called action and reaction forces.
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Momentum
• Momentum is the energy of motion that an object has.
• The momentum for any object at rest is zero.
• An object has a large momentum if the product of its
mass and velocity is large.
• A bullet fired at high velocity has a high momentum even
though it is of low mass.
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Momentum - 2
• Momentum is measured in units of kilogram-meters per
second kg•m/s.
• Momentum increases as an objects speed increases.
• You can calculate momentum by multiplying an object's mass
(in kilograms) and its velocity (in meters per second).
• Momentum = mass x velocity. M = mv
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Momentum Is
Conserved
• Collisions obey the law of conservation of momentum if the
system is closed.
• A closed system means other objects and forces cannot enter
or leave a system.
• According to the law of conservation of momentum, if no
net force acts on a system, then the total momentum of the
system does not change.
• In a closed system, the loss of momentum of one object
equals the gain in momentum of another object. I.e.
momentum is conserved
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Conservation of
Momentum
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Figure 17A and 17B
Conservation of
Momentum
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Figure 17A and 17B
Conservation of
Momentum
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Figure 17A and 17B
Conservation of
Momentum
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Figure 17A and 17B
Conservation of
Momentum
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Figure 17C
Conservation of
Momentum
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Figure 17C
Interest Grabber
Section 12.4
Comparing Forces
No matter where you are in the universe, certain types of forces are
present. You are already familiar with two of these forces—electric
force and magnetic force.
1. Describe the behavior of two bar magnets that are
positioned so that their north and south poles are nearly
touching.
2. Describe a common behavior of clothes when they are
removed from a clothes dryer.
3. How are these two forces the same? How are they different?
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Reading Strategy
Section 12.4
Comparing and Contrasting
a. Neutrons and proton
b. Very short (decreases rapidly beyond the diameter of a few protons)
c. Very strong (100 times stronger than electrical repulsion force)
d. All particles
e. Short
f. Weaker than the strong force
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12.4 Universal
Forces
• The four universal forces are the electromagnetic, strong
nuclear, weak nuclear, and gravitational .
• All the universal forces act over a distance between particles
of matter (particles don’t need to touch each other).
• Each of these forces is affected by the distance between the
particles of matter.
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Electromagnetic
Force
• Electromagnetic force is associated with charged particles.
• Electric force and magnetic force are the only forces that
can both attract and repel.
• Electric & magnetic force are two different aspects of the
electromagnetic force.
• Electric forces act between charged objects & particles like
electrons & protons.
• Opposites attract (+ & - , N&S) & like ones repel (N&N, S&S,
+ & +, - & -).
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Nuclear Forces
• Two forces, the strong nuclear force and the weak
nuclear force, act within the nucleus to hold it together.
• The strong nuclear force overcomes the electric force of
repulsion that acts among the protons in the nucleus.
• The weak nuclear force is involved in certain types of
radioactive processes.
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Gravitational Force
• Gravitational force is an attractive force that acts between
any two masses.
• Newton's law of universal gravitation states that every
object in the universe attracts every other object.
• The gravitational force between two objects is proportional to
their masses and decreases rapidly as the distance between
the masses increases.
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Gravitational Forces Acting on
Masses at Different Distances
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Figure 21
Gravitational Forces Acting on
Masses at Different Distances
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Figure 21
Gravitational Forces Acting on
Masses at Different Distances
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Figure 21
Gravitational Forces Acting on
Masses at Different Distances
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Figure 21
Earth, Moon, &
Tides
• Earth’s gravitational attraction keeps the moon in a nearly
circular orbit.
• Centripetal force is a center-directed force that continuously
changes the direction of an object to make it move in a circle.
• The gravitational pull of the moon causes two bulges in the
Earth’s oceans.
• The Earth rotates once each day beneath these bulges
causing two high tides and two low tides.
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Forces Acting on
the Moon
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Figure 22
Pretest Answers
Chapter 12
1. What is relative motion?
Relative motion is movement in relation to a frame of
reference.
2. What is the difference between distance and displacement?
Distance is the length of a path between two points.
Displacement is the direction from the starting point and
the length of a straight line from the starting point to the
ending point.
3. How is average speed calculated?
Total distance is divided by total time.
4. On a distance-time graph, what does the slope represent?
The slope represents the speed.
Click the mouse button to display the answers.
Pretest Answers
Chapter 12
(continued)
5. What is velocity?
Velocity is speed with direction.
6. How is acceleration related to velocity?
Acceleration is change in velocity, that is, any change in
speed, direction, or both.
7. A backpack falls out of an open window. The backpack starts
from rest and hits the ground 1.0 second later with a velocity
of 9.8 m/s. What is the average acceleration of the backpack?
a. 9.8 m/s
b. 9.8 m
c. 9.8 m/s2
d. all of the above
8. How are mass and weight different?
Mass is a measure of inertia; weight is the measure of the
force of gravity acting on an object.
Click the mouse button to display the answers.
Interest Grabber
Section 12.1
Answers
1. Stand beside your desk. Hold a sheet of notebook paper level at eye
level. Release the sheet of paper and watch it fall. Describe the
motion of the paper.
The paper flutters slowly to the ground.
2. Hold a sheet of notebook paper that has been crumpled into a tight
ball at eye level. Release the crumpled paper and watch it fall.
Describe the motion of the paper.
The crumpled sheet of paper falls straight to the ground.
3. How do the motions of the flat sheet of paper and the crumbled ball
of paper compare? What forces do you think are acting on each
sheet of paper?
The flat sheet of paper fluttered slowly to the ground whereas the
crumpled ball of paper fell more quickly to the ground and followed a
straight-line path. Do not assess students on correctly identifying the
two opposing forces on the paper; accept any reasonable response.
The two opposing forces are gravity and air resistance.
Interest Grabber
Section 12.2
Answers
1. Describe the motion of the soup in the bowl immediately
after your abrupt stop.
The soup in the bowl continues forward and spills over
the bowl’s front edge onto the food tray.
2. Explain why you think the soup in the bowl behaved the
way it did.
The soup’s motion is explained by momentum. The
momentum of the soup keeps it moving forward after the
forward motion of the food tray has been halted.
Interest Grabber
Section 12.3
Answers
1. Describe the motion of the balls in Figure A.
The racked balls in Figure A are motionless. The cue ball
moves in a straight line and at high speed toward the
racked balls.
2. Figure B shows the motion of the billiard balls after
impact. Explain why the motion of the balls changes in
Figure B.
The high-speed cue ball strikes the motionless balls
and transfers momentum and kinetic energy to
them, causing them to move.
Interest Grabber
Section 12.4
Answers
1. Describe the behavior of two bar magnets that are positioned
so that their north and south poles are nearly touching.
The opposite poles of the bar magnets attract each other. If
the magnets are close enough, they will move together.
2. Describe a common behavior of clothes when they are
removed from a clothes dryer.
Clothes removed from a clothes dryer often stick together.
3. How are these two forces the same? How are they different?
Both forces are associated with charged particles. Both forces
attract. The magnets are pulled together by magnetic forces,
whereas the clothes cling together because of electric forces.
Chapter 12
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Self-grading assessment
For links on forces, go to www.SciLinks.org and enter
the Web Code as follows: ccn-2121.
For links on mass, go to www.SciLinks.org and enter the
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For links on Newton’s laws, go to www.SciLinks.org and
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