Transcript Chapter 4

Chapter 4
Forces and the Laws of
Motion
Changes in Motion
Objectives:
1. Describe how force affects the motion of an
object.
2. Interpret and construct free-body diagrams.
The Force is Strong With You
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Force is an action exerted on an object which may change
the object's state of rest or motion.
Force may cause a change in an object's velocity with
respect to time (acceleration.)
Forces can cause objects at rest to move, cause objects in
motion to stop, and cause objects to change direction.
The Force is Strong With You
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The SI unit of force is the Newton (named for Sir Isaac
Newton.)
The amount of force that, when acting on 1 kg of mass,
produces an acceleration of 1 m/s2.
1 Newton = 1 kg ▪ 1 m/s2
Weight is a measure of the gravitational force exerted on an
object.
1 lb = 4.448 N
1 N = 0.225 lb
Contact Forces
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Contact forces result from the physical contact between 2
objects.
Pushing, pulling, throwing, catching
Field Forces
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Field forces do not involve physical contact between
objects, rather they are forces that act at a distance.
Gravitational force, electrical forces (attraction or repulsion)
Masses create gravitational fields in the space around them.
Charged objects create electromagnetic fields in the space
around them.
Field Forces
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All macroscopic contact forces occur as a result of
microscopic field forces.
All contact can be examined on a microscopic atomic level
and related to the interactions of atomic particles.
Force Diagrams (Draw a Good
Picture!)
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Force is a vector since it has both a magnitude and a
direction.
Force diagrams are diagrams that show force as vectors.
A force vector's tail is attached to the center of gravity of the
object.
A force vector points in the direction of the force and its
length is proportional to the magnitude of the force.
Free-Body Diagrams
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A free-body diagram is a representation of a single object
and all the forces acting on that object.
The forces exerted by the object isolated on other objects
are not included in a free-body diagram because they do not
affect the motion of the object.
Free-body diagrams are constructed and analyzed like
vector diagrams.
Concept Check
A truck pulls a trailer on a flat stretch of road. The
forces acting on the trailer are the force due to
gravity (250 000 N downward), the force exerted
by the road (250 000 N upward), and the force
exerted by the cable connecting the trailer to the
truck (20 000 N to the right.) The forces acting on
the truck are the force due to gravity (80 000 N
downward), the force exerted by the road (80 000
N) upward, and the force exerted by the car's
engine (26 400 N to the right.) Draw and label
free-body diagrams of both the trailer and the
truck.
Newton's First Law
Objectives:
1. Explain the relationship between the motion of
an object and the net external force acting on the
object.
2. Determine the net external force on an object.
3. Calculate the force required to bring an object
into equilibrium.
Inertia
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Inertia is the tendency of an object to resist being moved or,
if the object is moving, to resist a change in speed or
direction.
Galileo theorized about the nature of an object to maintain
its state of rest or motion.
Newton furthered Galileo's conclusions by developing
Newton's first law of motion
– An object at rest remains at rest, and an object in
motion continues in motion with a constant velocity
(that is, constant speed in a straight line) unless the
object experiences a net external force.
Inertia
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Inertia, therefore, is the tendency of an object not to be
accelerated.
Newton's first law thus states that when the net external
force on an object is zero, the object's acceleration (or the
change in the object's velocity) is zero.
Net Force
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A car is traveling at a constant velocity.
The net external force on the car is zero.
There are many forces acting on the car (Fforward, Fresistance,
Fgravity, Fnormal)
The individual external forces acting on the car are not zero,
but the net external force is zero.
An external force is a force that acts on an object as a result
of the interaction between the object and its environment.
Net Force
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All four forces acting on the car are external forces.
The net force is the sum of all the forces acting on the car.
Net force is a single force whose external effects on a rigid
body are the same as the effects of several actual forces
acting on the body.
When all external forces acting on an object are known, the
net force can be found.
Concept Check
A gust of wind blows an apple from a tree. As the
apple falls, the gravitational force on the apple is
2.25 N downward, and the force of the wind on the
apple is 1.05 N to the right. Find the magnitude
and direction of the net force on the apple.
Concept Check
2.48 N at 25.0˚counterclockwise from straight
down.
Equilibrium
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Equilibrium is the state in which the net force on an object
is zero.
An object that is at rest or moving with a constant velocity is
in equilibrium.
To determine whether an object is in equilibrium, determine
the net force.
If the net force is zero, the object is in equilibrium.
If there is a net force, a second force equal and opposite to
the net force will put the body in equilibrium.
Concept Check
Can an object be in equilibrium if only one force
acts on the object?
Concept Check
No. Either no force or two or more forces are
required for equilibrium.
Newton's Second and Third
Laws
Objectives:
1. Describe an object's acceleration in terms of its
mass and the net force acting on it.
2. Predict the direction and magnitude of the
acceleration caused by a known net force.
3. Identify action-reaction pairs.
Newton's Second Law
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Consider a situation in which you are trying to push a heavy
piece of furniture across the room.
If you try to move it by yourself, the acceleration will be so
small it will take a long time to notice a change in velocity.
If you get your friend to help you, the acceleration will be
larger and the furniture will soon be moving very quickly.
Newton's Second Law
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A heavier piece of furniture accelerates less than a lighter
piece of furniture.
It requires less force to accelerated a low-mass object than
a high-mass object.
Newton Second Law
– The acceleration of an object is directly proportional
to the net forces acting on the object and inversely
proportional to the object's mass.
– ΣF=ma
– Net force = mass x acceleration
Concept Check
Space-shuttle astronauts experience accelerations of
about 35 m/s2 during takeoff. What force does a
75 kg astronaut experience during an acceleration
of this magnitude?
Concept Check
2600 N
Concept Check
An 8.5 kg bowling ball initially at rest is dropped
from the top of an 11 m building. The ball hits the
ground 1.5 s later. Find the net force acting on the
falling ball.
Concept Check
83 N
Newton's Third Law
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A single isolated force cannot exist.
Forces always exist in pairs.
Newton's third law of motion
– If two objects interact, the magnitude of the force
exerted on object 1 by object 2 is equal to the
magnitude of the force simultaneously exerted on
object 2 by object 1, and these two forces are
opposite in direction.
– For every action, there is an equal and opposite reaction.
Newton's Third Law
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The forces two interacting objects exert on each other form
an action-reaction pair.
The action force is equal in magnitude and opposite in
direction from the reaction force.
Action and reaction forces occur simultaneously.
Action and reaction forces do not cancel each other such
that an object does not move since each force acts on
different objects.
Newton's Third Law
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The motion of an object is affected only by the forces acting
on that object.
To determine whether an object will accelerate, draw a freebody diagram of the object and determine whether there is a
net force acting on the object.
Action-reaction forces are equal and opposite, but either
object may still have a net force acting on it.
Newton's Third Law and Field
Forces
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Newton's third law also applies to field forces.
Consider the gravitational force exerted by the Earth on an
object.
The object will also exert a force on the Earth.
Since the mass of the Earth is greater than the mass of the
object, the acceleration of the Earth towards the object will
be negligible when compared with the acceleration of the
object towards the Earth.
Everyday Forces
Objectives:
1. Explain the difference between mass and weight.
2. Find the direction and magnitude of normal
forces.
3. Describe air resistance as a form of friction.
4. Use coefficients of friction to calculate frictional
force.
Weight
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The weight of an object is calculated by Fg= mag.
Since the acceleration due to gravity (ag) varies with location
of an object in the universe:
– Weight is a measure of the gravitational force exerted on
an object; its value can change with the location of the
object in the universe.
Normal Force
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The force of gravity is constantly acting on all objects on
Earth, since objects are not continuously falling towards the
center of the Earth there must be a force opposing the force
of gravity to keep them in equilibrium.
This force is the normal force
– A force that acts on a surface that is perpendicular to the
surface.
– Normal means perpendicular.
Normal Force
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The normal force is always perpendicular to the contact
surface, not necessarily directly opposite the force due to
gravity.
In the absence of other forces, Fn is equal and opposite to
the component of Fg that is perpendicular to the contact
surface.
If the angle Θ is the angle between the normal force and the
vertical line and also the angle between the contact surface
and the horizontal, then Fn = magcosΘ.
Friction
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Static friction is the force that resists the initiation of sliding
motion between two surfaces that are in contact and at rest.
As long as the object does not move, the force of static
friction is equal and opposite to the component of the
applied force that is parallel to the surface.
When the applied force is as great as it can be without
causing the object to move, the force of static friction has
reached its maximum value (Fs,max)
Friction
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Frictional forces occur because of microscopic interactions
between contact surfaces.
When viewed at the microscopic level, surfaces only contact
at a few points.
At these points, surfaces adhere together because of the
electrostatic forces between molecules of the two surfaces.
Friction
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Kinetic friction is the force that opposes the movement of
two surfaces that are in contact and are sliding over each
other.
Kinetic friction is less that the maximum static friction.
F=Fapplied-Fk
Friction
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Heavier objects require more force to slide across the same
surface.
This is so because the force of friction is proportional to the
normal force of a surface on an object.
Note that the normal force is equal and opposite to the
vertical component of the gravitational force of an object on
a surface.
Friction
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Friction can be approximated if we know the normal force
and Fs,max.
This is the coefficient of friction
– The ratio of the magnitude of the force of friction between
two objects in contact to the magnitude of the normal
force with which the objects press against each other.
– μs =Fs, max
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Fn
Concept Check
A 24 kg crate is set into motion on a horizontal
floor. Once the crate is in motion, a horizontal
force of 53 N keeps the crate moving with a
constant velocity. Find the coefficient of kinetic
friction, between the crate and the floor.
Concept Check
0.22
Concept Check
Two students are sliding a 225 kg sofa at a constant
speed across a wood floor. One student pulls with
a force of 225 N at an angle of 13˚ above the
horizontal. The other student pushes with a force
of 250 N at an angle of 23˚ below the horizontal.
What is the coefficient of friction between the sofa
and the floor?
Concept Check
0.25
Air Resistance is a Form of
Friction
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Whenever an object moves through a fluid medium, such as
air or water, the fluid provides a resistance to the object's
motion.
When the magnitude of the force applied equals the
magnitude of air resistance, the net force is zero and the
object moves at a constant speed.
When an object in free-fall accelerates, its velocity
increases.
As the velocity increases, the resistance of the air to the
object's motion increases.
Air Resistance is a Form of
Friction
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When the upward force of air resistance is equal to the
downward gravitational force, the net force is zero and the
object moves at a constant velocity.
The object has reached terminal velocity.
Four Fundamental Forces
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Electromagnetic forces-interactions between protons and
neutrons. Acts over long ranges.
Strong nuclear forces-strongest of the forces.
Weak nuclear forces
Gravitational forces-weakest of the forces. Acts over long
ranges.
All four are field forces.