Unit 6 RETAKE REVIEW PACKET
Vocabulary words:
Force
Net Force
Normal Force
Weight
Balanced Forces
Unbalanced Forces
Inertia
Force pairs
Momentum
Forces
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever
there is an interaction between two objects, there is a force upon each of the objects. When the interaction
ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.
Interactions can happen either when objects are in contact with each other (for example friction) or when
objects are at a distance away from each other (for example gravity).
Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is
abbreviated by an "N." To say "10.0 N" means 10.0 Newton of force. One Newton is the amount of force
required to give a 1-kg mass an acceleration of 1 m/s/s. Therefore:
A force is a vector quantity. A vector quantity is a quantity that has both magnitude (size) and direction (left,
right, up, down). To fully describe the force acting upon an object, you must describe both the magnitude (size
or numerical value) and the direction. Thus, 10 Newton is not a full description of the force acting upon an
object. In contrast, 10 Newton, downward is a complete description of the force acting upon an object; both
the magnitude (10 Newton) and the direction (downward) are given.
Types of Forces (*not a complete list)
Fapp (applied Force): An applied force is a force that is applied to an object
by a person or another object.
Fw (weight): The force of gravity on earth is always equal to the weight of the object. The equation you would
use to find the weight of an object is Fw = mg (where m = mass and g = acceleration due to gravity, 9.8 m/s2
on Earth)
FN (normal force): The normal force is the support force exerted upon an object that is in contact with another
stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force
upon the book in order to support the weight of the book.
Ff (friction): The friction force is the force exerted by a surface as an object moves across it or makes an effort
to move across it.
Fair (air resistance): Air resistance is a specific type of friction that acts upon objects as they travel through the
air. It always acts in the opposite direction of the motion.
Multiple Forces acting on one object:
When there is more than one force acting on an object, it is necessary to look at the overall effect the forces
together have on that object. Because forces are vector quantities, they can be added and a resultant vector can be
found. That resultant vector is known as the “net force” on an object. The net force can be zero, or it can be non-zero
(like 3 Newtons to the right for example).
If the forces end up canceling each other out, they are referred to as “balanced forces” and produce a net force of zero.
It is like a stalemate in tug of war. If both sides pull with the same amount of force, then neither side is winning. The
two forces essentially cancel each other out and the rope doesn’t accelerate (this is essentially Newton’s 1st Law of
Motion).
However, if one side of the tug of war competition is stronger than the other side, then that is considered to be
“unbalanced forces.” If that is the case, the object will accelerate in the direction of the net force (this is essentially
Newton’s 2nd Law of motion).
Unbalanced forces always provide a net force. See the following examples.
PRACTICE:
1. Free-body diagrams for four situations are shown below.
For each situation, determine the net force acting upon the object.
Situation A: ____________
Situation B: _____________
Situation C: ____________
Situation D: _____________
2. Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the
magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude
of the unknown forces.
A: ________
B: ________
C: _________
D: _________
F: _________
E: _________
G: _________
H:; ________
PRACTICE: For the following scenarios, draw out the object and draw and label the forces acting on the object using
arrows. Then identify whether the forces would be considered to be “balanced” or “unbalanced.”
1.)
A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider
frictional forces. Neglect air resistance. Diagram the forces acting on the book.
2.)
A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional
forces. Neglect air resistance. Diagram the forces acting on the book.
3.)
A book is at rest on a tabletop. Diagram the forces acting on the book.
4.)
An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is
falling.
Net forces can also be in more than one direction. In the example to the right, the boat is
being pushed to the right by one person and being pushed up by another person.
Collectively the boat is being pushed up and to the right. The magnitude (size) of that force
can be found using the Pythagorean theorem (a2 + b2 = c2) and the direction would be up and
to the right.
PRACTICE:
11 N
15 N
10 N
Newton’s 1st Law of Motion
Newton’s 1st Law of motion states “An object at rest stays at rest and an object in motion stays in motion with
the same speed and in the same direction unless acted upon by an unbalanced force (or in other words, net
force).” There are two parts to this statement - one that predicts the behavior of stationary objects and the
other that predicts the behavior of moving objects. The two parts are summarized in the following diagram.
His first law can be thought of as describing the “tendency” of objects. Objects tend to "keep on doing what
they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This
tendency to resist changes in their state of motion is described as inertia.
Inertia: the resistance an object has to a change in its state of motion.
“Inertia is a property of matter” (that is for all you Bill Nye fans out there)  What that means is that all
matter has inertia. I have inertia, your book has inertia, an airplane has inertia. Anything that has mass, has
inertia. The question becomes, what determines inertia? Well, inertia depends on mass. An object with more
mass has more inertia.
PRACTICE:
1.) Which has more inertia, an elephant or an ant? ______________________
2.) Which has more inertia, an elephant standing still or a bird flying? ______________________
3.) Which has more inertia, a parked car or a moving car? __________________________
Newton’s 2nd Law of Motion
Newton's first law of motion predicts the behavior of objects for which all existing forces are balanced. The
first law - sometimes referred to as the law of inertia - states that if the forces acting upon an object are
balanced, then the acceleration of that object will be 0 m/s/s. Objects at equilibrium (the condition in which
all forces balance) will not accelerate. According to Newton, an object will only accelerate if there is a net or
unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object - changing its
speed, its direction, or both its speed and direction. This is essentially his 2nd law of motion. An object will
accelerate in the direction of the net force and the relationship between force and motion can be
mathematically shown as:
Fnet = ma
Check out the following website for help understanding the relationship between force, mass and acceleration.
http://macmillanmh.com/science/2008/student/na/scienceinmotion/Common/SIM.html?Module=../Grade4/Chapter11
-AccelerationOfDifferentMasses/
Newton’s 2nd Law often brings out a very big misconception that many people have about motion. The
question is “is a force required for an object to be moving?” The answer is NO!!!!! An object can be moving if
there are no net forces (remember Newton’s 1st law) BUT that object must be going at a constant speed in the
same direction (object in motion stays in motion).
NOT TRUE!!!!!!!!
PRACTICE:
1. Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg
object.
2. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the
mass of the encyclopedia.
3. A 50 kg object is being dragged across a room with a 30 N force. Assuming that friction is acting in the
opposite direction with 10 N of force, what is the object’s acceleration?
4. A 5 kg object is being dragged across the floor with a 15 N force. If the acceleration of the object is 1 m/s 2,
what the force due to friction (hint: find Fnet using Newton’s 2nd Law first and draw out a picture)?
5. An object is being dragged across the floor with a constant velocity. If the applied force is 20 N, what is the
force of friction?
Mass vs weight
Weight is an application of Newton’s 2nd law of motion. The equation to calculate weight comes from his
second law. Fw is the force (hence the force) due to gravity, which we call weight. So does everyone
experience the same force due to gravity? The answer is NO! We all experience the same acceleration due to
gravity (9.8 m/s2 on Earth) but the force we experience due to gravity is our weight.
Fw = mg
Mass is the amount of matter that an object has and weight is the force of gravity on that object. They are
directly related (the more mass, the more weight), but they are different because weight depends on gravity!
Check out the following tutorial on help with mass and weight.
http://www.mathsisfun.com/measure/weight-mass.html
PRACTICE:
1.) What is the mass and weight of a 10-kg object on earth?
Mass = _____________
Weight = _____________
2.) What is the mass and weight of a 10-kg object on the moon where the force of gravity 1.6 m/s2?
Mass = _____________
Weight = _____________
Newton’s 3rd Law of Motion
A force is a push or a pull upon an object that results from its interaction with another object. Forces result
from interactions! According to Newton, whenever objects A and B interact with each other, they exert forces
upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair
exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair
and a force on your body. These two forces are called action and reaction forces and are the subject of
Newton's third law of motion. Formally stated, Newton's third law is:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects.
The size of the forces on the first object equals the size of the force on the second object. The direction of the
force on the first object is opposite to the direction of the force on the second object. Forces always come in
pairs - equal and opposite action-reaction force pairs.
A variety of action-reaction force pairs are evident in nature. Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air
must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird;
the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).
For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs
make it possible for birds to fly.
Consider the motion of a car on the way to school. A car is equipped with wheels that spin. As the wheels spin,
they grip the road and push the road backwards. Since forces result from mutual interactions, the road must
also be pushing the wheels forward. The size of the force on the road equals the size of the force on the
wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the
wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Actionreaction force pairs make it possible for cars to move along a roadway surface.
PRACTICE:
1. In the top picture (below), Kent Budgett is pulling upon a rope that is attached to a wall. In the bottom
picture, the Kent is pulling upon a rope that is attached to an elephant. In each case, the force scale reads 500
Newton. Kent is pulling ...
a. with more force when the rope is attached to the wall.
b. with more force when the rope is attached to the elephant.
c. the same force in each case.
2. Consider the following examples. One of the forces in the mutual interaction is described; describe the
other force in the action-reaction force pair.
Action: Baseball pushes glove leftwards.
Reaction: ______________________________________
Action: Bowling ball pushes pin leftwards.
Reaction: _______________________________________
Momentum
The sports announcer says, "Going into the all-star break, the Chicago White Sox have the momentum." The headlines
declare "Chicago Bulls Gaining Momentum." The coach pumps up his team at half-time, saying "You have the
momentum; the critical need is that you use that momentum and bury them in this third quarter."
Momentum is a commonly used term in sports. A team that has the momentum is on the move and is going to
take some effort to stop. A team that has a lot of momentum is really on the move and is going to be hard to
stop. Momentum is a physics term; it refers to the quantity of motion that an object has. A sports team that is
on the move has the momentum. If an object is in motion (on the move) then it has momentum.
Momentum can be defined as "mass in motion." All objects have mass; so if an
object is moving, then it has momentum - it has its mass in motion. The amount of
momentum that an object has is dependent upon two variables: how much stuff is
moving and how fast the stuff is moving. Momentum depends upon the variables
mass and velocity. In terms of an equation, the momentum of an object is equal to
the mass of the object times the velocity of the object.
Momentum = mass • velocity
p=m•v
where p is momentum, m is the mass, and v is the velocity. The equation illustrates that momentum is directly
proportional to an object's mass and directly proportional to the object's velocity.
The units for momentum would be mass units times velocity units. The standard metric unit of momentum is
the kg•m/s. Momentum is a vector quantity. To fully describe the momentum of a 5-kg bowling ball moving
westward at 2 m/s, you must include information about both the magnitude
and the direction of the bowling ball. It is not enough to say that the ball has
10 kg•m/s of momentum; the momentum of the ball is not fully described
until information about its direction is given. The direction of the momentum
vector is the same as the direction of the velocity of the ball. In a previous
unit, it was said that the direction of the velocity vector is the same as the
direction that an object is moving. If the bowling ball is moving westward, then its momentum can be fully
described by saying that it is 10 kg•m/s, westward. As a vector quantity, the momentum of an object is fully
described by both magnitude and direction.
From the definition of momentum, it becomes obvious that an object has a large momentum if either its mass
or its velocity is large. Both variables are of equal importance in determining the momentum of an object.
Consider a Mack truck and a roller skate moving down the street at the same speed. The considerably greater
mass of the Mack truck gives it a considerably greater momentum. Yet if the Mack truck were at rest, then the
momentum of the least massive roller skate would be the greatest. The momentum of any object that is at
rest is 0. Objects at rest do not have momentum - they do not have any "mass in motion." Both variables mass and velocity - are important in comparing the momentum of two objects. Inertia and momentum are
often confused. An object can have inertia, but not momentum. Inertia is measured by mass only and
momentum is measure by the product of mass and velocity.
PRACTICE: Determine the momentum of a ...
a. 60-kg halfback moving eastward at 9 m/s.
b. 1000-kg car moving northward at 20 m/s.
c. 40-kg freshman moving southward at 2 m/s.
Law of Conservation of Momentum
The law of momentum conservation can be stated as follows. For a collision occurring between object 1 and
object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total
momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the
momentum gained by object 2.
The above statement tells us that the total momentum of a collection of objects (a system) is conserved - that
is, the total amount of momentum is a constant or unchanging value. For example consider the following:
A useful analogy for understanding momentum conservation involves a money transaction between two
people. Let's refer to the two people as Jack and Jill. Suppose that we were to check the pockets of Jack and Jill
before and after the money transaction in order to determine the amount of money that each possesses. Prior
to the transaction, Jack possesses $100 and Jill possesses $100. The total amount of money of the two people
before the transaction is $200. During the transaction, Jack pays Jill $50 for the given item being bought. There
is a transfer of $50 from Jack's pocket to Jill's pocket. Jack has lost $50 and Jill has gained $50. The money lost
by Jack is equal to the money gained by Jill. After the transaction, Jack now has $50 in his pocket and Jill has
$150 in her pocket. Yet, the total amount of money of the two people after the transaction is $200. The total
amount of money (Jack's money plus Jill's money) before the transaction is equal to the total amount of
money after the transaction. It could be said that the total amount of money of the system (the collection of
two people) is conserved. It is the same before as it is after the transaction.
PRACTICE:
1. Find the total momentum of the cars that are pictured below BEFORE they collide:
2. Find the total momentum of the cars that are pictured below AFTER they collide:
3. A baseball player holds a bat loosely and bunts a ball. Express your understanding of momentum
conservation by filling in the tables below.
a: ________________________
b: ________________________
c: ________________________