2.1 Force
Net Force
A force is a push or a pull.
A force of some kind is always required to change the state
of motion of an object.
The combination of all forces acting on an object is called
the net force. The net force on an object changes its
motion.
The scientific unit of force is the newton, abbreviated N.
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
2.1 Force
Tension and Weight
A stretched spring is under a “stretching force” called
tension.
Pounds and newtons are units of weight, which are
units of force.
2.3 Support Force
What forces act on a book lying at rest on a table?
• One is the force due to gravity—the weight of the book.
• There must be another force acting on it to produce a net
force of zero—an upward force opposite to the force of gravity.
The upward force that balances the weight of an object on a surface
is called the support force.
A support force is often called the normal force.
2.4 Equilibrium for Moving Objects
The state of rest is only one form of equilibrium.
An object moving at constant speed in a straight-line path is also in a
state of equilibrium. Once in motion, if there is no net force to
change the state of motion, it is in equilibrium.
Assessment Questions
1.
When you hold a rock in your hand at rest,
the forces on the rock
a.
are mainly due to gravity.
b.
are mainly due to the upward push of
your hand.
c.
cancel to zero.
d.
don’t act unless the rock is dropped.
Assessment Questions
1.
When you hold a rock in your hand at rest,
the forces on the rock
a.
are mainly due to gravity.
b.
are mainly due to the upward push of
your hand.
c.
cancel to zero.
d.
don’t act unless the rock is dropped.
Answer: C
Assessment Questions
4.
When a desk is horizontally pushed across a floor at a steady speed
in a straight-line direction, the amount of friction acting on the desk is
a.
less than the pushing force.
b.
equal to the pushing force.
c.
greater than the pushing force.
d.
dependent on the speed of the sliding crate.
Assessment Questions
4.
When a desk is horizontally pushed across a floor at a steady speed
in a straight-line direction, the amount of friction acting on the desk is
a.
less than the pushing force.
b.
equal to the pushing force.
c.
greater than the pushing force.
d.
dependent on the speed of the sliding crate.
Answer: B
Assessment Questions
5.
When Nellie hangs at rest by a pair of ropes, the tensions in the
ropes
a.
always equal her weight.
b.
always equal half her weight.
c.
depend on the angle of the ropes to the vertical.
d.
are twice her weight.
Assessment Questions
5.
When Nellie hangs at rest by a pair of ropes, the tensions in the
ropes
a.
always equal her weight.
b.
always equal half her weight.
c.
depend on the angle of the ropes to the vertical.
d.
are twice her weight.
Answer: C
6.1 Force Causes Acceleration
Recall from the previous chapter that the combination of forces
acting on an object is the net force.
• Acceleration depends on the net force.
• To increase the acceleration of an object, you must
increase the net force acting on it.
• An object’s acceleration is directly proportional to the net
force acting on it:
acceleration ~ net force
(The symbol ~ stands for “is directly proportional to.”)
6.3 Newton’s Second Law
Newton’s second law describes the relationship among an
object's mass, an object's acceleration, and the net force on an
object.
6.4 Friction
Friction is a force and affects motion:
• Friction acts on materials that are in contact with each
other.
• It always acts in a direction to oppose relative motion.
• When two solid objects come into contact, the friction is
mainly due to irregularities in the two surfaces.
6.5 Applying Force-Pressure
The amount of force per unit of area is called pressure.
When the force is perpendicular to the surface area,
P is the pressure and A is the area over which the force acts.
Pressure is measured in newtons per square meter, or pascals
(Pa). One newton per square meter is equal to one pascal.
6.5 Applying Force-Pressure
The driving force per nail is not enough to puncture the skin.
CAUTION: Do not attempt this on your own!
6.7 Falling and Air Resistance
A feather and a coin fall with equal accelerations in a vacuum, but
very unequally in the presence of air.
When falling in air, the coin falls quickly while the feather flutters
to the ground.
The force due to air resistance diminishes the net force acting on
the falling objects.
6.7 Falling and Air Resistance
Speed and Area
You experience the force due to air resistance when you
stick your hand out of the window of a moving car.
• If the car moves faster, the force on your hand
increases.
• If instead of just your hand, you hold your physics book
out the window with the large side facing forward, the
air resistance force is much larger than on your hand at
the same speed.
Assessment Questions
2.
When a net force acts on an object, its acceleration depends on the
object’s
a.
initial speed.
b.
mass.
c.
volume.
d.
weight.
Assessment Questions
2.
When a net force acts on an object, its acceleration depends on the
object’s
a.
initial speed.
b.
mass.
c.
volume.
d.
weight.
Answer: B
Assessment Questions
3.
A cart is pushed and undergoes a certain acceleration. Consider how
the acceleration would compare if it were pushed with twice the net
force while its mass increased by four. Then its acceleration would be
a.
one quarter.
b.
half.
c.
twice.
d.
the same.
Assessment Questions
4.
Friction is a force like any other force and affects motion. Friction
occurs in
a.
solids sliding over one another.
b.
fluids.
c.
air.
d.
all of these
Answer: D
Assessment Questions
5.
When you stand on one foot instead of two, the pressure you exert
on the ground is
a.
half.
b.
the same.
c.
twice.
d.
quadruple.
Assessment Questions
5.
When you stand on one foot instead of two, the pressure you exert
on the ground is
a.
half.
b.
the same.
c.
twice.
d.
quadruple.
Answer: C
7.1 Forces and Interactions
A hammer exerts a force on the nail and drives it into a
board.
• There must also be a force exerted on the hammer to
halt it in the process.
• Newton reasoned that while the hammer exerts a
force on the nail, the nail exerts a force on the
hammer.
• In the interaction, there are a pair of forces, one
acting on the nail and the other acting on the hammer.
7.2 Newton’s Third Law
Newton’s third law describes the relationship between two
forces in an interaction.
• One force is called the action force.
• The other force is called the reaction force.
• Neither force exists without the other.
• They are equal in strength and opposite in direction.
• They occur at the same time (simultaneously).
7.4 Action and Reaction on Different Masses
In the interaction between the boulder and Earth, the boulder
pulls up on Earth with as much force as Earth pulls down on the
boulder.
• The forces are equal in strength and opposite in direction.
• The boulder falls to Earth and Earth falls to the boulder, but
the distance Earth falls is much less.
7.4 Action and Reaction on Different Masses
F represents both the action and reaction forces; m (large), the
mass of the cannon; and m (small), the mass of the cannonball.
Do you see why the change in the velocity of the cannonball is
greater compared with the change in velocity of the cannon?
7.4 Action and Reaction on Different Masses
think!
A tug of war occurs between boys and girls on a polished floor
that’s somewhat slippery. If the boys are wearing socks and the
girls are wearing rubber-soled shoes, who will surely win, and
why?
7.7 Action Equals Reaction
If a sheet of paper is held in midair, the heavyweight
champion of the world could not strike the paper with a force
of 200 N (45 pounds).
The paper is not capable of exerting a reaction force of 200
N, and you cannot have an action force without a reaction
force.
If the paper is against the wall, then the wall will easily assist
the paper in providing 200 N of reaction force, and more if
needed!
Assessment Questions
1.
A force interaction requires at least a(n)
a.
single force.
b.
pair of forces.
c.
action force.
d.
reaction force.
Assessment Questions
1.
A force interaction requires at least a(n)
a.
single force.
b.
pair of forces.
c.
action force.
d.
reaction force.
Answer: B
Assessment Questions
2.
Whenever one object exerts a force on a second object, the second
object exerts a force on the first that is
a.
opposite in direction and equal in magnitude at the same time.
b.
in the same direction and equal in magnitude a moment later.
c.
opposite in direction and greater in magnitude at the same time.
d.
in the same direction and weaker in magnitude a moment later.
Assessment Questions
2.
Whenever one object exerts a force on a second object, the second
object exerts a force on the first that is
a.
opposite in direction and equal in magnitude at the same time.
b.
in the same direction and equal in magnitude a moment later.
c.
opposite in direction and greater in magnitude at the same time.
d.
in the same direction and weaker in magnitude a moment later.
Answer: A
Assessment Questions
4.
When you jump vertically upward, strictly speaking, you cause Earth
to
a.
move downward.
b.
also move upward with you.
c.
remain stationary.
d.
move sideways a bit.
Assessment Questions
4.
When you jump vertically upward, strictly speaking, you cause Earth
to
a.
move downward.
b.
also move upward with you.
c.
remain stationary.
d.
move sideways a bit.
Answer: A
Assessment Questions
5.
A system undergoes acceleration only when acted on by a(n)
a.
net force.
b.
pair of forces.
c.
action and reaction forces.
d.
internal interactions.
Assessment Questions
5.
A system undergoes acceleration only when acted on by a(n)
a.
net force.
b.
pair of forces.
c.
action and reaction forces.
d.
internal interactions.
Answer: A
True or False?
1. The amount of matter in an object is called its weight.
2. The combination of all the forces on an object is called the net force.
3. Inertia is the reluctance any material object has to change in its state of motion.
4. Objects move only when a force is exerted.
5. The acceleration of an object is inversely proportional to the net force acting on it.
6. Air resistance is caused by friction between the air and an object moving through
air.
True or False?
1.
2.
3.
5.
6.
The amount of matter in an object is called its weight.
The combination of all the forces on an object is called
the net force.
Inertia is the reluctance any material object has to change
in its state of motion.
4.
Objects move only when a force is exerted.
The acceleration of an object is inversely proportional to
the net force acting on it.
Air resistance is caused by friction between the air and an
object moving through air.
True or False?
7.
The speed of an object dropped in air will continue to
increase without limit until it strikes the ground.
8.
When one object exerts a force on a second object, the
second object always exerts the same amount of force back on
the first object.
9.
A rocketship is pushed forward by gases that are forced
out the back of the ship.
10.
In order to make a cart move forward, a horse must pull
harder on the cart than the cart pulls on the horse.
11.
If a bicycle and a parked car have a head-on collision, the
force of impact is greater on the bicycle.
12.
The earth and moon pull on each other with equal
amounts of force.