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Chapter 12 Forces and Motion
Lesson 12.1 Forces
The wind pushes against the man and his umbrella. The push from the wind is a ________________.
How do forces affect the motion of an object?
A force is a _____________ or a _____________ that acts on an object.
A force can cause a resting object to ______________, or it can _______________________ a moving
object by changing the object’s speed or direction.
Measuring Force
You may have measured forces using a spring scale. The stretch of the spring in the scale depends on the
amount of ________________ (a type of force) acting on it.
Units of Force
One _________________ (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1
meter per second each second (1 m/s2).
1 N = 1 kg•m/s2
Representing Force
________________ can represent a force. The lengths of the arrows show relative amounts of force.
Combining Forces
How do forces affect the motion of an object?
When the forces on an object are balanced, the net force is ________________, and there is no change
in the object’s motion.
When an unbalanced force acts on an object, the object _______________________.
The ___________ force is the overall force acting on an object after all the forces are combined.
• Forces in the same direction ________ together.
• Forces in opposite directions ____________________ from one another.
Balanced Forces
Balanced forces are forces that combine to produce a net force of ________________.
An ___________________________ number of individual forces can act on an object to produce a net
force of zero.
The two groups pull with equal forces in _______________________ directions. The forces combine to
make a net force of zero.
Unbalanced Forces
An ____________________________ force is a force that results when the net force acting on an object
is not equal to zero.
Forces can add together or subtract from one another.
Friction
What are the four main types of friction?
____________________ is the force that opposes the motion of objects that touch as they move past
each other.
There are four main types of friction: static friction, ___________________ friction, rolling friction, and
fluid friction.
Static Friction
Static friction is the friction force that acts on objects that are ____________ moving.
Static friction always acts in the direction ____________________ to that of the applied force.
Sliding Friction
Sliding friction is a force that ________________ the direction of motion of an object as it slides over a
surface.
Sliding friction is _________ than static friction.
A. Static friction–the potted tree does not move.
B. Sliding friction–when the tree moves, sliding friction acts to oppose the direction of motion.
Rolling Friction
____________________ friction is the friction force that acts on rolling objects.
The force of rolling friction is about 100 to 1000 times _______________ than the force of static or
sliding friction.
Ball bearings in these wheels greatly reduce friction by replacing ________________ friction with rolling
friction.
Fluid Friction
• Fluid friction opposes the motion of an object through a __________________.
• Fluid friction acting on an object moving through the air is known as ________ resistance.
• Fluid friction increases as the ________________ of the object moving through the fluid
increases.
Gravity
In what direction does Earth’s gravity act?
Earth’s gravity acts ________________________ toward the center of Earth.
How do gravity and air resistance affect a falling object?
Gravity causes objects to _____________________ downward, whereas air resistance acts in the
direction _______________________ to the motion and reduces acceleration.
Gravity is a force that acts between any two ____________________.
• Gravity is an _______________________ force.
• Gravity can act over _______________ distances.
Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force
on the boulder. The forces are _______________________.
Falling Objects
• Both gravity and air resistance affect the motion of a ________________ object.
• As objects fall to the ground, they _____________________ and gain speed.
• ________________ velocity is the constant velocity of a falling object when the force of air
resistance equals the force of gravity.
This flying squirrel takes advantage of air resistance to slow its fall and increase the distance covered in
the jump.
Projectile Motion
Why does a projectile follow a curved path?
The combination of an initial forward velocity and the downward vertical force of _______________
causes the ball to follow a curved path.
A thrown ball follows a ______________________ path.
______________________ motion is the motion of a falling object (projectile) after it is given an initial
forward velocity.
Air resistance and gravity are the only ____________________ acting on a projectile.
A. Their masses are different, but the blue and green balls fall at the same rate.
B. The yellow ball is a projectile, following a curved path.
Lesson 12.2 Newton’s First and Second Laws of Motion
Galileo’s work helped correct misconceptions about force and motion that had been widely held since
Aristotle’s time.
It took about 2000 years to develop the modern understanding of the relationships between force and
motion.
Aristotle
Aristotle made scientific discoveries through careful _______________________ and logical reasoning.
Aristotle incorrectly proposed that force is required to keep an object moving at
____________________ speed.
Galileo
Galileo Galilei studied how gravity produces ___________________________ acceleration.
• He rolled balls down wooden ramps.
• He concluded that moving objects not subjected to ____________________ or any
other force would continue to move indefinitely.
Newton
Newton built on the work of scientists such as Galileo.
• Newton first defined ____________________ and force.
• He then introduced his laws of motion.
Isaac Newton published his work on force and motion in the book entitled Principia.
Newton’s First Law of Motion
How does Newton’s first law relate change in motion to a zero net force?
According to Newton’s _________________ law of motion, the state of motion of an object does not
change as long as the net force acting on the object is ________________________.
Unless an unbalanced force acts, an object at rest remains at _________________.
Unless an unbalanced force acts, an object in motion remains in ________________ with the same
speed and direction.
________________________ is the tendency of an object to resist a change in its motion.
This crash sequence illustrates inertia. The test dummy __________________________ its forward
motion as the car slows and stops.
Newton’s Second Law of Motion
How does Newton’s second law relate force, mass, and acceleration?
According to Newton’s second law of motion, the acceleration of an object is equal to the net force
acting on it _____________________ by the object’s mass.
The acceleration of an object is ______________________ proportional to the net force acting on it.
The acceleration of an object also depends upon its ________________________.
_____________________ is a measure of the inertia of an object.
The acceleration of an object is always in the same __________________________ as the net force.
When a net force acts in the direction opposite to the object’s motion, the force produces a
______________________________.
Example
An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net
force on the car is 4000 newtons, what is the car’s acceleration?
Read and Understand
What information are you given?
Plan and Solve
What unknown are you trying to calculate? What formula contains the given quantities?
Plan and Solve
Replace each variable with its known value and solve.
Look Back and Check
Is your answer reasonable?
1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of
the cart if the net force on the cart is 60.0 N?
2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N
acts on it in an upward direction?
3. An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction.
What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.)
4. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the
wheelchair? (Hint: Solve Newton’s second law for mass.)
Newton’s Second Law of Motion
Acceleration depends directly on _________________ and inversely on __________________. The same
force causes the single cart to accelerate eight times faster than the chain of eight carts.
Weight and Mass
How are weight and mass related?
Mass is a measure of the ____________________ of an object; weight is a measure of the force of
__________________ acting on an object.
Mass and weight are related but are not the ___________________.
• Mass is the measure of the amount of ________________________ an object contains.
• _________________________ is the force of gravity acting on an object.
• Weight is the ___________________ mass and acceleration due to gravity.
W = mg is a different form of Newton’s Second Law, F = ma.
The value of g in the formula is _________ m/s2.
If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to
gravity is 9.8 m/s2?
On the moon, the acceleration due to gravity is only about one sixth that on Earth.
• The astronaut _______________ only about one sixth as much on the moon as on Earth.
• The __________________ of the astronaut is the same on the moon and on Earth.
Lesson 12.3 Newton’s Third Law of Motion and Momentum
When this bumper car collides with another car, _______ forces are exerted. Each car in the collision
exerts a force on the other.
Newton’s Third Law
What is Newton’s third law of motion?
According to Newton’s third law of motion, whenever on object exerts a force on a second object, the
second object exerts and _________________ and ___________________ force on the first person.
Action and Reaction Forces
• The force your bumper car exerts on the other car is the _____________________ force.
• The force the other car exerts on your car is the _________________________ force.
• These two forces are equal in ________________ and opposite in direction.
Suppose you press your hand against a wall.
• Your ____________ exerts an action force on the wall.
• The wall exerts an equal and opposite reaction force ___________________ your hand.
Action-Reaction Forces and Motion
A swimmer pushing against the water is an ______________ force.
The reaction force acting on the swimmer causes _____________________ through the water.
Action-reaction forces propel the swimmer through the water. The swimmer pushes against the water,
and the water pushes the swimmer.
Action-Reaction Forces Do Not Cancel
For the swimmer, why do the action and reaction forces not cancel each other and produce a net force
of zero?
The action and reaction forces do not act on the ______________ object.
Momentum
What is needed for an object to have a large momentum?
_____________________ is the product of an object’s mass and its velocity.
An object has a __________________ momentum if the product of its mass and velocity is large.
An object with large momentum is harder to ________________ than an object with small momentum.
The momentum for any object at rest is _____________________.
Mass is measured in __________________________.
__________________________ is measured in meters per second.
Momentum is measured kilogram-meters per second.
Which has more momentum, a 0.046-kilogram golf ball with a speed of 60.0 meters per second, or a 7.0kilogram bowling ball with a speed of 6.0 meters per second?
The bowling ball has considerably ______________ momentum than the golfball.
Conservation of Momentum
How is momentum conserved?
A ____________________ system means other objects and forces cannot enter or leave a system.
In a closed system, the ____________ of momentum of one object equals the ____________ in
momentum of another object – momentum is conserved.
Objects within a closed system can exert ____________________ on one another.
According to the law of ________________________ of momentum, if no net force acts on a system,
then the total momentum of the system does not change.
In each collision, the total momentum of the train cars does not __________________—momentum is
conserved.
A class studied the speed and momentum of a 0.25-kilogram ball dropped from a bridge. The graph
shows the momentum of the ball from the time it was dropped until the time it hit the river flowing
below the bridge.
1.
Applying Concepts At what time did the ball have zero momentum? Describe this point in the
ball’s motion.
2. Using Graphs At what time did the ball have the greatest momentum? What was the peak
momentum value?
3. Calculating What is the ball’s speed after 1.25 seconds?
(Hint: Use the graph and the momentum formula.)
Lesson 12.4 Universal Forces
An artist’s depiction of a planet’s surface shows a world very different from Earth.
Certain universal forces are present.
Observations of planets, stars, and galaxies strongly suggest four universal forces exist throughout the
universe.
• electromagnetic
• _____________ nuclear
• weak nuclear
• _________________________
Universal forces act over a ___________________________ between particles of matter.
• The particles need not be in _____________________.
• Force is affected by the distance between particles.
Electromagnetic Forces
What force can attract and repel?
Electric force and ________________________ force are the only forces that can both attract and repel.
Electric and magnetic force are two different aspects of the electromagnetic force.
Electromagnetic force is associated with _____________________ particles.
Electric Forces
Electric forces act between charged objects or particles.
• Objects with opposite charges _____________________ one another.
• Objects with _______________ charges repel one another.
Clothes often acquire electric charges in the dryer. Clothes with opposite charges tend to cling together.
Magnetic Forces
Magnetic forces act on
• certain __________________,
• the poles of magnets, and
• ________________ charges.
Magnets have ______________ poles—north and south.
• Two poles that are ___________________ attract each other.
• Two poles that are alike _______________________ each other.
A magnetic force of attraction holds the two train cars together.
Nuclear Forces
What force holds the nucleus together?
Two forces, the strong ____________________ 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
___________________ in the nucleus.
The weak nuclear force is involved in certain types of ____________________ processes.
Strong Nuclear Force
The strong nuclear force is a powerful force of ______________________ that acts only on the
neutrons and protons in the nucleus.
• It acts over ____________ distances—approximately the diameter of a proton (10–15 m).
• It is __________ times stronger than the electric force of repulsion at these distances.
Weak Nuclear Force
The weak nuclear force is an attractive force that acts only over a _________________ range.
The weak nuclear force acts at about _________ meters, less than the range of the strong nuclear force.
Gravitational Forces
What is Newton’s law of universal gravitation?
Newton’s law of _______________________ gravitation states that every object in the universe attracts
every other object.
Gravitational force is an attractive force that acts between any two _______________________.
Gravitational force depends upon mass and ________________________.
Gravity Acts Over Long Distances
The gravitational force between two objects is ______________________l to their masses.
Gravitational force ________________________ with the square of the distance between the objects.
Gravity is the _______________________ universal force, but it is the most effective force over long
distances.
The Earth, Moon, and Tides
The moon’s inertia acts to move it ________________ from Earth.
Earth’s gravitational attraction keeps the moon in a nearly circular _______________ around Earth.
A _________________________ force is a center-directed force that continuously changes the direction
of an object to make it move in a circle.
The moon’s inertia and the gravitational pull of Earth result in a nearly ____________________ orbit.
The gravitational pull from the moon produces two bulges in Earth’s _________________–one on the
side of Earth closest to the moon, the other on the side farthest from the moon.
As Earth rotates ______________ per day beneath these two bulges, there are two high and two low
tides per day on Earth.
Satellites in Orbit
An artificial satellite needs only its inertia and the centripetal force provided by gravity to maintain its
____________________.
Satellites in a low orbit are slowed by ______________________with Earth’s atmosphere and eventually
reenter Earth’s atmosphere.
Uses of Satellites
Hundreds of artificial satellites orbit for many functions:
• monitoring Earth’s ____________________
• creating detailed _______________ maps of Earth’s surface
• using telescopes to study space
• studying Earth’s ____________________
• receiving and transmitting radio and microwave signals
Satellites are used to receive and transmit electromagnetic waves over ________________ distances.