Chapter 10 Forces
Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.
Chapter 10 Forces
Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.
Chapter 10 Forces
Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time
Chapter 10 Forces
Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time
Chapter 10 Forces
Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed
Chapter 10 Forces
Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed
Chapter 10 Forces
Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s,
what is its initial speed?
a. -100 m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s
Chapter 10 Forces
Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s,
what is its initial speed?
a. -100 m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s
Chapter 10 Forces
Section 1: Friction, Gravity, and Elastic Forces
Standard 8.2.a Students know a force has both
direction and magnitude.
Standard 8.2.c Students know when the forces on
an object are balanced, the motion of the object
does not change.
Chapter 10 Forces
What is a Force?
How is a force described?
A force is described by its
magnitude and by the direction
in which it acts.
Force
A push or a pull
Newton (N)
The SI unit for the magnitude,
or strength, of a force.
The combination of all forces
acting on an object.
Net force
Chapter 10 Forces
Combining Force Vectors
The strength and direction of the individual forces determine
the net force.
Chapter 10 Forces
Unbalanced Forces
Unbalanced forces acting on an object result in a net force
and cause a change in the object’s velocity.
Chapter 10 Forces
Balanced Forces
Balanced forces acting on an object do not change the
object’s velocity.
Chapter 10 Forces
Section 1 Quick Quiz
What happens when two forces act in the same direction?
A.They add together.
B.Their sum divided by two is the total force.
C.They cancel each other out.
D.The stronger one prevails.
Answer: A – They add together.
Chapter 10 Forces
Section 1 Quick Quiz
Balanced forces acting on an object
A.sometimes change the object’s motion.
B.always change the object’s motion.
C.are not related to motion.
D.never change the object’s motion.
Answer: D – never change the object’s motion
Chapter 10 Forces
Section 2: Friction, Gravity, and Elastic Forces
Standard 8.2.b Students know when an object is
subject to two or more forces at once, the result is
the cumulative effect of all the forces.
Standard 8.2.d Students know how to identify
separately the two or more forces that are acting on
a single static object, including gravity, elastic
forces due to tension or compression in matter, and
friction.
Chapter 10 Forces
Friction
What factors determine
the strength of the
friction force between
two surfaces?
The strength of the force of
friction depends on the types of
surfaces involved and on how
hard the surfaces push together.
friction
A force that two surfaces exert
on each other when they rub
against each other
Chapter 10 Forces
Types of Friction
Static Friction
•
•
•
Occurs with objects that
are not moving.
You must use extra force
to start the motion of
stationary objects.
Once the object is
moving there is no
longer any static friction.
Sliding Friction
•Occurs when two solid
surfaces slide over each
other.
•Examples:
• Sticky powder on the
soles of ballet slippers.
• Rubber pads on
bicycle brakes.
Chapter 10 Forces
Types of Friction
Rolling Friction
Fluid Friction
•Occurs when an object rolls
across a surface.
•Rolling friction is less than
sliding friction for similar
materials.
•Engineers use ball
bearings to reduce friction in
many products.
•Occurs when a solid object
moves through a fluid.
•Fluid friction is usually less
than sliding friction. Parts of
machines are bathed in oil for
this reason.
•Fluid friction occurs between
people and air.
Chapter 10 Forces
Gravity
What factors affect the
gravitational force
between two objects?
gravity
Law of Universal
Gravitation
mass
The force of gravity between
objects increases with greater
mass and decreases with greater
distance.
A force that pulls objects toward
each other
The force of gravity acts between
all objects in the universe.
The measure of the amount of
matter in an object
The SI unit of mass is the
kilogram (kg).
Chapter 10 Forces
Gravity Between Objects
The force of gravity between objects increases with greater
mass and decreases with greater distance.
Chapter 10 Forces
Mass and Weight
The gravitational force exerted on a person or object at the
surface of a planet is known as weight.
Weight = Mass X Acceleration due to Gravity
Chapter 10 Forces
Mass and Weight
The weight of a person on the Moon is one-sixth of
their weight on Earth. The mass of an object is the
same on the Moon as it is on Earth.
Example:
Weight on Earth 1617 N
Mass on Earth 165 kg
Weight on Moon 270 N
Mass on Moon 165 kg
Chapter 10 Forces
Gravity and Motion
Why do objects
accelerate during free
fall?
In free fall, the force of gravity alone
causes an object to accelerate in the
downward direction.
•All objects in free fall accelerate at
the same rate regardless of their
masses.
•The acceleration due to gravity is
9.8 meters per second for each
second an object is falling.
•An object’s velocity is 9.8 m/sec
after 1 second
•Its velocity is 19.6 m/sec after 2
seconds, etc.
Chapter 10 Forces
Free Fall
The graph shows how the speed
of an object in free fall changes
with time. Use the graph to
answer the following questions.
Chapter 10 Forces
Free Fall
Interpreting Graphs:
What is the speed of the
object at 1 second? At 3
seconds?
9.8 m/s; 29.4 m/s.
Chapter 10 Forces
Free Fall
Calculating:
Calculate the slope of the
graph. What does this
number represent?
The slope is 9.8. The speed
increases by 9.8 m/s each
second.
Chapter 10 Forces
Free Fall
Predicting:
Use the slope that you
calculated in Step 2 to predict
the object’s speed at 6
seconds.
58.8 m/s
Chapter 10 Forces
Free Fall
Drawing Conclusions:
The graph has a constant
slope. What does the slope
tell you about the object’s
motion?
The object’s speed increases
at a constant rate.
Chapter 10 Forces
Air Resistance
Falling objects with a greater surface area experience more
air resistance. Air resistance is an upward force exerted on
falling objects.
Chapter 10 Forces
Projectile Motion
•
•
An object that is thrown is called a projectile.
The path of a projectile has horizontal motion due to
a lateral force and vertical motion due to
gravitational force.
Chapter 10 Forces
Elastic Forces
When is matter
considered to be
elastic?
Two Types of Elastic
Forces
Matter is considered elastic if it
returns to its original shape after
it is squeezed or stretched.
Compression
•An elastic force that squeezes
or pushes matter together.
•Example: A cushion on a couch
compresses due to a person’s
weight until it comes close
enough together to exert an
equal compression force in the
other direction.
Chapter 10 Forces
Elastic Forces
Two Types of Elastic
Forces
Tension
• An elastic force that stretches
or pulls matter.
• Example: The strings on a
guitar.
Chapter 10 Forces
Section 2 Quick Quiz
Which of the following is an example of rolling friction?
A.bike tires on the road as you ride
B.a boat on the water as it sails
C.two hands rubbing together
D.your shoes on a sidewalk as you walk
Answer: A – bike tires on the road as you ride
Chapter 10 Forces
Section 2 Quick Quiz
Which of the following is an example of increasing friction
intentionally?
A. waxing skis
B. throwing sand on an icy driveway
C. oiling a squeaky door
D. adding grease to gears on a bike
Answer: B - throwing sand on an icy driveway
Chapter 10 Forces
Section 2 Quick Quiz
The force of gravity on a person or object on the surface of a
planet is called
A. air resistance.
B. mass.
C. free fall.
D. weight.
Answer: D - weight
Chapter 10 Forces
Section 2 Quick Quiz
The law of universal gravitation states that any two objects in
the universe, without exception,
A.repel each other.
B.combine to provide a balanced force.
C.attract each other
D.create friction.
Answer: C – attract each other
Chapter 10 Forces
Section 2 Quick Quiz
When the only force acting on a falling object is gravity, the
object is said to be
A.stationary.
B.decelerating.
C.in free fall.
D.a projectile.
Answer: C – in free fall
Chapter 10 Forces
Section 2 Quick Quiz
Objects falling through air experience a type of friction called
A.rolling friction.
B.terminal velocity.
C.inertia.
D.air resistance.
Answer: D – air resistance
Chapter 10 Forces
Section 2 Quick Quiz
Which type of elastic force is present in the strings on a
guitar?
A. tension
B. compression
C. friction
D. inertia
Answer: A - tension
Chapter 10 Forces
Section 2 Quick Quiz
Which of the following examples demonstrates a
compression force?
A.plucking a guitar string
B.pulling a wagon
C.dropping an apple
D.squeezing a sponge
Answer: D – squeezing a sponge
Chapter 10 Forces
Section 2 Quick Quiz
What holds up a large suspension bridge, such as the
Golden Gate Bridge?
A.Fluid friction between the bridge’s supports and the water
around them
B.A lack of balance between the weight of the bridge and the
supports below it
C.An upward tension force in the cables that balances the
weight of the bridge
D.The gravitational attraction between the bridge and its
main supports
Answer: C - An upward tension force in the cables that
balances the weight of the bridge
Chapter 10 Forces
Newton’s First and Second Laws
Standard 8.2.e Students know that when the forces
on an object are unbalanced, the object will change
its velocity (that is, it will speed up, slow down, or
change direction).
Standard 8.2.f Students know the greater the mass
of an object, the more force is needed to achieve
the same rate of change in motion.
Chapter 10 Forces
The First Law of Motion
What is Newton’s First Law
of Motion?
An object will remain at rest or
moving at a constant velocity
unless it is acted upon by an
unbalanced force.
•Inertia is the tendency of an
object to resist a change in
motion.
•Another name for Newton’s First
Law of Motion is the Law of
Inertia.
•The greater and object’s mass,
the greater its inertia.
Chapter 10 Forces
The Second Law of Motion
What is Newton’s
Second Law of Motion?
Acceleration depends on the net
force acting on the object and on
the object’s mass.
Acceleration = Net force / Mass
•Acceleration is measured in
meters per second per second.
•Net force is measured in
Newtons (N), and mass is
measured in kilograms (kg).
The above formula can also be
written as:
Net Force = Mass X Acceleration
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier to accelerates at
2.0 m/s2. Calculate the net force that causes this acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.
Calculate the net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
The net force (Fnet) = __
What formula contains the given quantities and the unknown
quantity?
a = Fnet/m or Fnet = m x a
Perform the calculation.
Fnet = m x a = 55 kg x 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.
Calculate the net force that causes this acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required. This does not include the force that
overcomes friction.
Chapter 10 Forces
Calculating Force
Practice Problem
What is the net force on a 1,000-kg object accelerating
at 3 m/s2?
3,000 N (1,000 kg x 3 m/s2)
Chapter 10 Forces
Calculating Force
Practice Problem
What net force is needed to accelerate a 25-kg cart
at 14 m/s2?
350 N (25 kg x 14 m/s2)
Chapter 10 Forces
Section 3 Quick Quiz
The greater the mass of an object,
A.the greater its inertia.
B.the more balanced it is.
C.the more space it takes up.
D.the easier the object starts moving.
Answer: A – the greater its inertia
Chapter 10 Forces
Newton’s Third Law
Standard 8.2.e Students know that when the forces
on an object are unbalanced, the object will change
its velocity (that is, it will speed up, slow down, or
change direction).
Chapter 10 Forces
Newton’s Third Law of Motion
What is Newton’s
Third Law of Motion?
Newton’s third law of motion
states that if one object exerts a
force on another object, then the
second object exerts a force of
equal strength in the opposite
direction on the first object.
Chapter 10 Forces
Newton’s Third Law of Motion
How can you calculate
the momentum of an
object?
A characteristic of a moving object
that depends on both the mass and
the velocity of the object.
Momentum = Mass X Velocity
The unit for momentum is
measured in kilogram-meters per
second (kg-m/s)
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s, or a 4.0-kg sledgehammer swung at 0.9 m/s?
Read and Understand
What information are you given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 0.9 m/s
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Plan and Solve
What quantities are you trying to calculate?
The momentum of each sledgehammer = __
What formula contains the given quantities and the unknown
quantity?
Momentum = Mass x Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 km x 1.5 m/s = 4.5 kg•m/s
Larger sledgehammer = 4.0 km x 0.9 m/s = 3.6 kg•m/s
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Look Back and Check
Does your answer make sense?
It is possible for the 3.0-kg hammer to have more momentum than
the 4.0-kg one because it has a greater velocity.
Chapter 10 Forces
Calculating Momentum
Practice Problem
A golf ball travels at 16 m/s, while a baseball moves at 7
m/s. The mass of the golf ball is 0.045 kg and the mass
of the baseball is 0.14 kg. Which has greater
momentum?
Golf ball: 0.045 kg x 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg x 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.
Chapter 10 Forces
Calculating Momentum
Practice Problem
What is the momentum of a bird with a mass of 0.018 kg
flying at 15 m/s?
0.27 kg•m/s (0.018 kg x 15 m/s = 0.27 kg•m/s)
Chapter 10 Forces
Law of Conservation of Momentum
What is the Law of
Conservation of
Momentum?
The total momentum of any group
of objects remains the same, or is
conserved, unless outside forces
act on the objects.
Chapter-10 Forces
Conservation of Momentum
In the absence of friction, momentum is conserved when two
train cars collide.
Chapter 10 Forces
Section 4 Quick Quiz
How does the momentum of a 4.0-kg bowling ball rolling at
3.0 m/s compare to the momentum of a 6.0-kg bowling ball
rolling at 2.0 m/s?
A. The 4.0-kg bowling ball has more momentum.
B. The 6.0-kg bowling ball has more momentum.
C. There is not enough information to compare the two
bowling balls.
D. The bowling balls have equal momentum.
Answer: D - The bowling balls have equal momentum.
Chapter 10 Forces
Section 4 Quick Quiz
According to Newton’s third law of motion, when a hammer
strikes and exerts force on a nail, the nail
A.disappears into the wood.
B.creates a friction with the hammer.
C.moves at a constant speed.
D.exerts an equal force back on the hammer.
Answer: D - exerts an equal force back on the hammer.
Chapter 10 Forces
Section 4 Quick Quiz
How can you increase the momentum of an object?
A.by decreasing its velocity
B.by increasing its mass
C.by increasing its friction
D.by decreasing its acceleration
Answer: B – by increasing its mass
Chapter 10 Forces
Section 4 Quick Quiz
According to the law of conservation of momentum, when
two objects collide in the absence of friction,
A.only the object with the larger mass continues on.
B.momentum is not lost.
C.velocity increases.
D.velocity decreases.
Answer: B – momentum is not lost
Chapter 10 Forces
Section 4 Quick Quiz
The total momentum of a group of objects is conserved
unless
A.outside forces act on the objects.
B.outside forces do not act on the objects.
C.there are more than two objects.
D.the objects are moving.
Answer: A – outside forces act on the objects
Chapter 10 Forces
Rockets and Satellites
Standard 8.2.e Students know that when the forces
on an object are unbalanced, the object will change
its velocity (that is, it will speed up, slow down, or
change direction).
Chapter 10 Forces
Rockets
How does a rocket lift off?
•
•
A rocket can rise into the air
because the gases it expels with
a downward action force exert
an equal but opposite reaction
force on the rocket.
As long as the upward force,
called thrust, is greater than the
downward pull of gravity there is
an unbalanced force, and the
rocket will accelerate toward
space.
Chapter 10 Forces
Satellites
What keeps a
satellite in orbit?
Satellites in orbit around Earth
continuously fall toward Earth, but because
Earth is curved they travel around it
•A satellite is a falling projectile that keeps
missing the ground.
•A satellite continues to move around the
Earth due to its inertia.
•Gravity continuously changes the
satellite’s direction.
•The speed an object must be thrown in
order to orbit Earth is about 7,900 meters
per second.
Chapter 10 Forces
What Is a Satellite?
A projectile follows a curved path. The horizontal and vertical
motions act independently.
Chapter 10 Forces
What Is a Satellite?
The faster a projectile is
thrown, the farther it travels
before it hits the ground. A
projectile with enough
velocity moves in a circular
orbit. Any force that causes
an object to move in a circle
is called a centripetal force.
Chapter 10 Forces
What Is a Satellite?
Depending on their uses, artificial satellites orbit at different
heights.
Chapter 10 Forces
Section 5 Quick Quiz
The achievement of lifting a rocket off the ground and into
space can be explained by
A.the law of conservation of momentum.
B.Newton’s first law.
C.Newton’s second law.
D.Newton’s third law.
Answer: D – Newton’s third law
Chapter 10 Forces
Section 5 Quick Quiz
What is required for a rocket to lift off into space?
A.mass that is greater than Earth’s
B.thrust that is greater than Earth’s gravity
C.more velocity than friction
D.very little air resistance
Answer: B - thrust that is greater than Earth’s gravity
Chapter 10 Forces
Section 5 Quick Quiz
Any force that causes an object to move in a circle is
called a(n)
A.centripetal force.
B.gravitational force.
C.unbalanced force.
D.balanced force.
Answer: A – centripetal force