Download Ch. 12 Notes - leavellphysicalscience

Chapter 12 Forces and
Motion
Section 12.1 Forces
Section 12.2 Newton’s First and Second Laws
of Motion
Section 12.3 Newton’s Third Law of Motion
and Momentum
Section 12.1 Forces
What is a Force?
Def.-a push or a pull that acts on an object
Key Concept: A force can cause a resting object
to move, or it can accelerate a moving object by
changing the object’s speed or direction
Ex. Figure 1 pg. 356; man walking in gusty
wind- wind (force) can change his speed or
direction
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Section 12.1 Forces
Measuring Force
Most of the time force is easy to measure.
Figure 2; fruit acts as a force (weight) on the spring in a
scale
 Units of Force
Force is measured in Newtons (N)
1 Newton-the force that causes a 1 kg mass to
accelerate at a rate of 1 m/s2 or is in other words
1kg*m/s2
Newton: after Sir Isaac Newton (scientist) who explained
how force, mass, and acceleration are related
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Section 12.1 Forces
Representing Force
Use an arrow to represent the direction and
strength of a force.
Direction of arrow=direction of force; the length
of arrow=the strength/magnitude of the force
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Section 12.1 Forces
Combining Forces
You can combine force arrows to show the result
of how forces combine.
Forces in the same direction add together;
forces in opposite directions subtract from one
another.
Net force-the overall force acting on an object
after all the forces are combined
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Section 12.1 Forces
Balanced Forces
Key Concept: When the forces on an object are
balanced, the net force is zero and there is no
change in the object’s motion.
Ex. Tug of war and arm wrestling
An unlimited number of individual forces can act
on an object and still produce a net force of
zero.
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Section 12.1 Forces
Unbalanced Forces
An unbalanced force is a force that results when the net
force acting on an object is not equal to 0.
Key Concept: When an unbalanced force acts on an
object, the object accelerates.
Forces acting in opposite directions can also combine to
produce an unbalanced force (winners of tug of war).
The net force equals the size of the larger force minus
the size of the smaller force.
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Combining
Forces Acting on
an Object
Figure 4
Section 12.1 Forces
Friction
**All moving objects are subject to friction.
Def.-a force that opposes the motion of objects
that touch as they move past each other
**Without friction, surfaces would be more
slippery than ice.
Friction acts at the surface where objects are in
contact (that includes solid objects that are
directly touching each other and also objects
moving through a liquid or a gas)
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Section 12.1 Forces
Friction
Key Concept: There are four main types of
friction: static friction, sliding friction, rolling
friction, and fluid friction.
 Static Friction
Def.-the friction that acts on objects that are
moving
**It always acts in the direction opposite to that
of the applied force (Taking steps, push off a
step, between your shoes and the ground)
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Section 12.1 Forces
Sliding Friction
Def.-A force that opposes the direction of motion
of an object as it slides over a surface
**Sliding friction is less than static friction, so
less force is needed to keep an object moving
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Section 12.1 Forces
Rolling Friction
** When a round object rolls across a flat floor, both the
object and the floor are bent slightly out of shape.
Def.-the friction force that acts on rolling objects ( is
about 100 to 1000 times less than the force of static or
sliding friction)
Ex. Ball bearings (Figure 6 pg. 360)-used to reduce
friction (rolling friction replaces sliding friction) inline
skates, automobiles, bicycles, skateboards
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Section 12.1 Forces
Fluid Friction
Water and a mixture of gases such as air are known as
fluids.
Def.-the force that opposes the motion of an object
through a fluid
Increases as the speed of an object moving through the
fluid increases
Air resistance-fluid friction acting on an object moving
through the air
At higher speeds, air resistance can be a significant
force.
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Section 12.1 Forces
Gravity
Def.-a force that acts between any two masses
Is an attractive force (pulls objects together)
**Gravity holds us to the ground.**
Does not require objects to be in contact for it to
act on them
Key Concept: Earth’s gravity acts downward
toward the center of Earth.
Upward force usually balances the downward
force of gravity. (Book on desk; boulder on cliff)
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Section 12.1 Forces
Falling Objects
Both gravity and air resistance affect the motion
of a falling object.
Key Concept: Gravity causes objects to
accelerate downward, whereas air resistance
acts in the direction opposite to the motion and
reduces acceleration. **Fig. 8 flying squirrel
Large surface area maximizes the force of air
resistance
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Section 12.1 Forces
Falling Objects
Falling objects accelerate and gain speed=increase in air
resistance
If an object falls for a long time, the upward force of air
resistance becomes equal to the downward force of
gravity.
Forces acting on the object are balanced, acceleration is
0, and the object continues to fall at a constant velocity.
Terminal velocity-the constant velocity of a falling object
when the force of air resistance equals the force of
gravity (2 objects w/ same mass fall at same rate)
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Section 12.1 Forces
Projectile Motion
Ex. A ball that is thrown, follows a curved path
Def.-the motion of a falling object (projectile)
after it is given an initial forward velocity
Air resistance and gravity are the only forces
acting on a projectile.
Key Concept: 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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Section 12.2 Newton’s First and
Second Laws of Motion
Aristotle, Galileo, and Newton
**It took about 2000 years to develop the
understanding of the relationships between force
and motion.
 Aristotle
Incorrectly proposed that force is required to
keep an object moving at constant speed
Error held back progress in the study of motion
for almost 2000 years
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Section 12.2 Newton’s First and
Second Laws of Motion
Galileo
Studied how gravity produces constant
acceleration
Concluded that moving objects not subjected to
friction or any other force would continue to
move indefinitely
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Section 12.2 Newton’s First and
Second Laws of Motion
Newton
Built on the work of other scientists (like Galileo)
Published his results many years later in a book
entitled Principia (first had to define mass and
force)
Then introduced his laws of motion
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Section 12.2 Newton’s First and
Second Laws of Motion
Newton’s First Law of Motion
**Newton summarized his study of force and
motion in several laws of motion.
Key Concept: 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.
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Section 12.2 Newton’s First and
Second Laws of Motion
Newton’s First Law of Motion
Unless an unbalanced force acts, an object at
rest remains at rest, and an object in motion
remains in motion with the same speed and
direction.
Ex. Ball (at rest is kicked; slows down from
friction between the ball and the ground)
First law aka the law of inertia.
Def.-the tendency of an object to resist a change
in its motion
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Section 12.2 Newton’s First and
Second Laws of Motion
Newton’s Second Law of Motion
**How do unbalanced forces affect the motion
of an object?
An unbalanced force causes an object’s velocity
to change (the object accelerates).
The more force used, the more acceleration
there is.
Newton: the acceleration of an object depends
on its mass
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Section 12.2 Newton’s First and
Second Laws of Motion
Newton’s Second Law of Motion
Mass-a measure of the inertia of an object and
depends on the amount of matter the object
contains
Key Concept: 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.
Ie. Doubling an object’s mass will cut its
acceleration by half.
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Section 12.2
Newton’s Second Law
Section 12.2 Newton’s First and
Second Laws of Motion
Newton’s Second Law of Motion
The acceleration of an object is in the same
direction as the net force.
Newton’s 2nd law also applies when a net force
acts in the direction opposite to the object’s
motion. (The force causes a deceleration that
reduces the speed)
Ex. Seat belt, volleyball
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Figure 13
Effects of a Force on
Acceleration
Figure 13
Effects of a Force on
Acceleration
Figure 13
Effects of a Force on Acceleration
Section 12.2 Newton’s First and
Second Laws
Weight and Mass
**Mass and weight are not the same, but are related.
Weight-the force of gravity acting on an object
An object’s weight is the product of the objects mass
and acceleration due to gravity acting on it.
Weight=Mass x Acceleration due to gravity, or W=mg;
g=9.8 m/s2
(F)orce or (W)eight is expressed in Newtons;
Acceleration due to grativty (a or g) is expressed in m/s2
Mass is expressed in kilograms.
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Section 12.2 Newton’s First and
Second Laws of Motion
Weight and Mass
Mass and weight are proportional; doubling
mass, doubles the object’s weight
Key Concept: 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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Section 12.3 Newton’s Third Law
of Motion and Momentum
Newton’s Third Law
**A force can’t exist alone. Forces always exist
in pairs.
Key Concept: 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.
The two forces are called: action and reaction.
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Section 12.3 Newton’s Third Law
of Motion and Momentum
Action and Reaction Forces
Action force-the force exerted by the first object
Reaction force- the force exerted by the second
object
Both forces are equal in size and opposite in
direction
Ex. Pushing on a wall
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Section 12.3 Newton’s Third Law
of Motion and Momentum
Action-Reaction Forces and Motion
Not all action and reaction forces produce
motion (pushing a wall).
 Action-Reaction Forces Do Not Cancel
**Net force is not zero with action reaction
forces.
b/c action and reaction forces do not act on the
same object (swimmer in water)
Only when equal and opposite forces act on the
same object do they result in a net force of 0.
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Section 12.3 Newton’s Third Law
of Motion and Momentum
Momentum
Def.-the product of an objects mass and its
velocity
**An object with more momentum is hard to
stop.
Key Concept: An object has a large momentum
if the product of its mass and velocity is large.
Momentum for any object at rest is 0.
Momentum= Mass x Velocity (kg * m/s)
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Section 12.3 Newton’s Third Law
of Motion and Momentum
Conservation of Momentum
What happens when objects collide?
Under certain conditions, collisions obey the law
of conservation of momentum.
***Conservation of momentum means that
momentum does not increase or decrease.
If a system is closed the momentum is
conserved
Closed system-other objects and forces cannot
enter or leave a system
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Section 12.3 Newton’s Third Law
of Motion and Momentum
Conservation of Momentum
Objects within the system can exert forces on
one another.
Law of conservation of momentum-law stating
that the total momentum of a system does not
change if no net force acts on the system
Key Concept: In a closed system, the loss of
momentum of one object equals the gain in
momentum of another object—momentum is
conserved.
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Figure 17A and 17B
Conservation of
Momentum
Figure 17C
Conservation of
Momentum