Download Motions and Forces

FORCES
We have been studying two types of motion, uniform motion and non uniform motion.

Uniform motion is constant velocity motion - motion in a straight line with constant speed.
Uniform motion includes constant zero velocity, i.e. sitting still, zero speed.

Non uniform motion includes all other types of motion, all of which involve changing motion,
like speeding up, slowing down and changing direction.
Our naïve idea is that motion, any motion, is directly related to force. Our naïve idea leads us to
assume that uniform motion occurs with the application of force, but we are going to come to
understand that this is a misconception coming from our naïve idea. We shall find that non-uniform
motion, i.e. changing motion and not motion itself is directly related to force(s). But how are they
related? First, we need to investigate forces to get a clearer idea of what they are, how they combine
and then to relate them to changing motion.
We call events or processes like pulling or pushing on an object applying a force on the object.
There are a number of different ways for a body to experience forces.
Bump hips with a friend or groupmate. You will feel a force in a direction toward you and your
groupmate will feel a force toward her. By measuring the size of these forces we would find that
they are equal in size; they are clearly oppositely directed. Forces always occur between bodies
and occur in pairs, for example you are pushing on your groupmate and s/he is pushing on you.
We cannot have one force without the other. This law of physics is stated as: In an interaction
between body A and body B, body A exerts a force on body B and body B exerts an equal and
opposite force on body A. The equal and opposite forces of an interaction never act on the same
body, because an interaction is between bodies. You have weight because you are attracted to the
earth by a gravitational force between you (your mass) and the earth (the earth’s mass). The
earth is attracted to your body with the same magnitude force; the earth is so massive that we
cannot measure the effect of this force, while our mass is smaller and we can observe the effect.
In any interaction between bodies, there are pairs of forces, equal and opposite
forces acting on the two bodies interacting. When body A exerts a force on
body B, body B exerts an equal and opposite force on body A. The equal and
opposite forces of an interaction NEVER act on the same body, because an
interaction is between bodies.

Have your instructors show a video of person shooting a gun (Becky can we include the URL
here?)
Discuss with your group members how this rule is exemplified in the video of the person
shooting a gun.
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We will study the characteristics of forces using small group experiments. Each two person group needs the
following equipment:

two spring scales for measuring forces

one wooden block with connecters

one small low friction car
a) You will be given a wooden block with hook eyes. Predict what will happen if you pull with
a spring scale on the wooden block with a very, very small pull. The force will be applied
continuously. Predictions are important so do them every time they are called for; you will be
reminded by the bold letters.
Spring scale pull
b)
Get a spring scale and attach a spring scale on one side of the block and check your prediction.
Check with your instructor about how to “zero” the gauge.
c)
Predict what constant applied pull or force will be required for moving the block at constant
speed.
d)
Use the spring scale to measure the constant, continuously applied, pull required to move the
block at constant speed. Record the value of constant, continuously applied, pull required to get
uniform motion.
e)
Predict what will happen if you apply a constant, continuously applied, large pull to the block,
i.e. a pull larger than the one applied for getting constant speed.
f)
Try to check your prediction. You may not be able to successfully complete this experiment, but
try your best. The force, as measured by the spring scale, must be kept continuously applied and
constant as the block moves, and the experiment ends when the force is not constant. This is
similar to our experiment with the skateboard; we stopped the experiment when the pusher could
no longer keep up with the speeding-up skateboard. I suggest doing this pulled block experiment
along as large a length as possible having one person move the block and keeping the force
constant while the other group members observe and record the motion.
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g)
Next, we are going to predict and observe what happens when we pull on the block with two
spring scales EACH pulling with the value required for getting uniform motion. Predict what
will happen if you use two spring scales hooked to the block connector and apply TWO forces to
the block in the same direction. Each spring scale pulls with the force used for one spring scale
when you had uniform motion. Do you think you will you have uniform motion in this case?
Spring scale pull
Spring scale pull
h)
Check your prediction. When we have more than one force continuously acting on an object in
the same direction do the forces seem to add, subtract, or do something else?
i)
Predict what will happen if you use two spring scales and apply two equal forces to the block
initially at rest but apply them in opposite directions.
Spring scale pull
Spring scale pull
j)
Check your prediction. When we have two forces continuously acting on an object in opposite
directions do the forces seem to be additive, subtractive, or do something else?
k)
If we have two equal forces applied to an object in opposite directions, do the forces seem
balanced or unbalanced? If the forces are balanced what is the net effect?
l)
Predict what will happen if the same force used in the uniform motion of the block experiment
is continuously applied to a small low friction car using a spring scale in the same way as for the
block.
m)
Check your prediction with an experiment.
n)
Predict what will happen to the motion of a low friction car sitting at rest if you apply equal
forces to the low friction car, with one force in one direction and the other force in the opposite
direction.
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o)
Observe the motion of the resting car with equal forces. Do two forces applied to the same body
seem to act in an additive, subtractive, or some other way? Are the forces balanced or unbalanced? If
the forces are balanced what is the net effect?
p)
Predict what will happened to the motion of a low friction car while it is moving at uniform
speed if equal and opposite forces are continuously applied to the car while it is moving. To
imagine this experiment, after the car is moving, picture two identical fans attached to the car
each facing in opposite directions or imagine two small equal rocket engines, each facing in
opposite directions. The identical fans or identical little rocket engines are turned on
simultaneously. Now back to reality. We will do the experiment with two spring scales:
THIS EXPERIMENT IS DIFFICULT. Check your prediction by having one of the group members
move the car at constant speed in a straight line while other group members read the force
gauges. Are the forces essentially equal in size or different? THIS EXPERIMENT IS DIFFICULT.
Other group members observe spring scale pulls
Spring scale pull
Spring scale pull
One person moves the car with constant speed
holding the spring scales

Note that the experiment is occurring while the car is moving at constant speed, not
during the times when the car is speeding up or slowing down.

Check with your instructor about how to “zero” the force gauges.
CHECK#1: Write a summary of the important ideas you have developed and learned in the
exercises and experiments in this section. Also, write concise rules which will describe your
conclusions about the relationship between motion and force; you will have one rule involving
balanced forces and another rule about unbalanced forces. If you have balanced forces there is no
net force and no total force since the forces balance or cancel each other out. They balance to
give a net or total force of zero. The term net-force is a use of the word “net” similar to the use
in net pay, total pay minus deductions, or net earnings, total income minus expenses. If you
have unbalanced forces you must have a (non-zero) net force since unbalanced forces cannot
balance out. Note that one force acting cannot be balanced out, leading to unbalanced forces
and a net force that is not zero.
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HOW CAN WE UNDERSTAND THE FORCES ON THE LOW FRICTION CAR AND
THE FORCES ON THE BLOCK?
a) We just did an experiment with equal but opposite forces applied to a low friction car and observed
that it moved with uniform motion. Earlier we pulled with one force on a block that had friction and
it moved with uniform motion. The motion was observed to be uniform in both cases. If the motion
is uniform we know that we must have balanced forces. The balancing of the horizontal forces is
directly observable in the case of the two springs pulling equally and oppositely on the low-frictioncar. In the case of the block, we only directly see the pulling force. Have a discussion about what the
force in the opposite direction that leads to balancing for the block might be.
b) Friction can be described as a force. Does it act like a pushing or pulling? If friction is a force does it
usually act opposite to the motion, impeding it or in the same direction as motion pushing it along
even more? Do you think that friction could be the unseen force that balances the pulling single
force with the block?

Draw and label a picture showing the forces acting on the car and the block. This will involve
arrows representing the forces; the direction of the arrow is in the direction of the force and the
length of the arrow is scaled to the size of the force. Show the forces acting on 1) the car with two
spring scales pulling and 2) the block with a pulling spring and friction.

Are the forces balanced or unbalanced? Since we know uniform motion requires balanced forces,
investigate how the picture of the forces could be made to show the balancing of the forces.
CHECK #2: Write a summary of the important ideas about forces and non-uniform motion you
have developed from your diagrams for the uniform motion block and uniform motion car.
HOW CAN WE UNDERSTAND THE FORCES ON THE PERSON SITTING ON THE
UNIFORMLY MOVING BURLAP BAG THAT WAS PUSHED?
a. Remember back to our first meeting at which time we did a number of experiments on motion,
including pushing continuously someone on a skateboard, pushing and releasing a Kick Dis,
pushing continuously someone on a cloth/burlap bag, and a kicked soccer ball.
b. Let's focus on the experiment where someone was pushed on the cloth/burlap bag. Make a
diagram showing all the forces acting on the pushed person on the cloth/burlap bag when she is
moving at constant velocity, i.e. constant speed in a straight line. Carefully make the diagram
and explain your results.
c. Can you think of a special situation where the person on the cloth could move at constant
velocity with no one pushing her? How? What special conditions? Make a diagram showing all
the forces acting on her for this situation, i.e. the person on the cloth is moving with constant
speed in a straight line.
CHECK #3: Write a summary of the important ideas you have developed about the person moving on the
cloth with constant speed and fully explain your diagrams, ideas and conclusions.
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APPLYING OUR IDEAS ABOUT FORCES AND MOTION.
Some of our experiments are described below in the table.
Complete the table using your ideas about whether the motion is changing or the motion is unchanging,
whether the forces are balanced or the forces are unbalanced and whether the experiment is consistent
with your rule or rules(developed in Check #1). Note that when the forces are balanced there is just as
much force one way as in the opposite way. Balanced is another way of saying the forces subtract and
net force is zero. If there is an unbalanced force, the forces may add or they do not totally subtract or
there may be only one force so that there is a net force, i.e. the net force is not zero.
Object
State of motion
Is motion
continually
changing, i.e.
nonuniform?
Applied forces,
i.e. Pushes or
pulls?
Size of Friction
Yes or no?
Yes or no?
KD
At rest
No
Essentially
Zero
KD
Speeding up
Yes
Essentially
Zero
KD
Moving
uniformly
No
Essentially
Zero
Person
on cloth
with
small
pushing
force
At rest
Yes
Large
Person
on cloth
Moving
uniformly
Yes
Large
Person
on cloth
Speeding up
Yes, and more
than what is
needed for
uniform motion
Large
Person
on SB
Speeding up
Yes
Essentially
Zero
Person
on SB
Moving
essentially
uniformly, after
release
No
Essentially
Zero
Balanced or
unbalanced?
Consistent
or
inconsistent
with your
rules and
checks?
CHECK#4: With an instructor, discuss your table entries and whether there is consistency with your
rules or rules.
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WHAT HAPPENS WHEN I DROP AN OBJECT AND WHY?
Gather the following equipment.

Dense, unbreakable object like steel ball about 1 or 2 cm in diameter

Mr. Coffee type coffee filter that is fresh from the box and not flattened.

Globe optional

Videos of falling objects (Becky could we include the URL?)
The following set of experiments will involve objects in "free fall" and "non free fall" motion.
a) Get a steel ball 1 or 2 cm in diameter. Hold it. When you are holding the ball still what kind of
motion does it have? Describe all the forces that are being applied to the object. Does the ball have
weight? Is weight a force? Can your hand apply a force? Considering all the forces applied to the ball,
are the forces balanced, that is, equal forces in opposite directions so that the total or net force is zero?
Or, are the forces unbalanced so that there is a total or net force?
b) Now, release the ball. What is its direction of motion after release? What is its speed just before you
release and its initial speed just after you release it? As time evolves and the ball goes down to the
floor, does it speed up, slow down or move with uniform motion? In addition to observing the ball all
the way down, draw upon your previous experiences. Does the fact that you would rather jump from
the first floor of a building to the ground than jump from a higher floor to the ground have something to
do with speed? We will project a video shows the slowed motion of a falling object, allowing the
increasing speed to be made evident.
c) Using your response to (b) part above, draw a conclusion about a single force or net force applied to
the ball while it is falling down faster and faster. If you conclude that there is a force in the downward
direction, is this a regularly occurring force or a rare force? Do we have a name for this downward
directed force? What is the direction of the force relative to the surface of the earth? Thinking of the
earth as a sphere or globe what is the direction of this force?
d) If we did this experiment someplace on the side of the earth opposite from Lexington, relative to the
earth would the force be in the same direction? In an absolute sense are the forces in c) and d) in the
same direction or opposite directions. BECKY SHEET. Make a drawing or get a globe and imagine
the direction of the gravitational forces or weight for points on various positions on the surface of the
earth. Make a side view drawing of the earth showing the direction of the gravitational force for a
number of spots on the surface of the earth. BECKY SHEET. Develop a way of efficiently describing
the direction of the gravitational force or weight of an object near the surface of the earth.
e) Does the moon experience force? What force is it? In terms of the center of the moon and the center
of the earth, what is the direction of the force?
e) Get a paper Mr. Coffee type coffee filter. Hold it. When you are holding the filter still what kind of
motion does it have? Describe the forces that being applied to the object. Are they balanced so that the
net force is zero or are they unbalanced?
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f) When a coffee filter is released and falls down, it behaves a lot like a parachute. Now, release the
filter so that the open part faces upward. What is its direction of motion after release? What is its
speed just before you release and its initial speed just after you release it? After a little time has passed
and the filter goes down, does it appear to be speeding up, slowing down or moving with uniform
motion? Draw upon your previous experiences and decide if it is a regular occurrence or a rare
occurrence for objects such as these coffee filters and parachutes to fall down in this way.
g) Using your response to (f) part above, draw a conclusion about a single force or net force applied to
the filter while it is falling down. What can you conclude about the forces and the net force acting on
the filter for the time period starting a second or two after it is released until it gets to the floor?
CHECK#5: What force is always acting on objects near the surface of the earth? Describe the direction
of this force. In all cases, is this the only force applied to an object near the surface of the earth or
sometimes are there other forces? List some of these forces. Write a summary of what you learned in
this section about falling objects.
**********************************************************************************
Objects accelerate toward the Earth because of the gravitational force of attraction between the object and the
Earth. We call the downward force of gravity that acts on an object the weight of the object. When this is the
only force that acts on an object -- that is, when nothing is holding it and when air resistance and similar
frictional forces are negligible – the object falls and speeds up due to the force of gravity and we say that the
object is in a state of "free fall."
Objects falling without air resistance (or other forces) are one thing but what of the practical cases of objects
falling in the air? After an object is released, in the presence of air resistance the total force that acts on an
object is the difference between the weight (the downward force of gravity that acts on an object) and the
upward force of air resistance. The force of air resistance depends primarily on two things: the size of the
falling object and the amount (density) of air the object passes by or through. This makes the air resistance
dependent upon the size and the speed of the falling object. The air resistance is greater for greater speed. In
the case of a parachute, the falling object increases in speed until the size of air resistance force in the upward
direction comes to equal the downward force of gravity leading to balanced forces and uniform motion.

The preceding discussion related to "free fall" and "non free fall" motion was adapted from
"Conceptual Physics" by Paul Hewitt.
WHAT ARE SOME FORCES BESIDES PUSHES AND PULL?
a) Through our observations we have learned that pushes and pulls are forces and that unbalanced forces
lead to changing motion. Whenever a body is changing motion, i.e. experiencing non-uniform
motion, there must be unbalanced forces applied to that body.
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b) Whenever a body experiences uniform motion, i.e. motion with constant speed in a straight line, i.e.
constant velocity, we know the forces are balanced. Balanced forces can arise from the total absence
of forces. Balanced forces can also arise from the application to a body of two different forces, equal
in size and opposite in direction.
c) By observing the uniform motion of the block we were able to develop the idea of friction as a force.
When we pull the application of the force is obvious and directly visible to us. However friction is not
directly visible or is invisible. We cannot directly observe the friction force, but we can indirectly
observe since it must be present to give the balanced forces required for the observed uniform motion.
d) We used the fact that a small ball near the surface of the earth experience speeding up motion
downward to reinforce the idea of the gravitational force.
e) So we have pushes and pulls, friction and gravitational forces among the forces that we experience.
Think of other forces that you have experienced either directly or indirectly as above. Is each of the
forces visible or invisible?
We will have a whole class discussion of this issue to share ideas and confirm scientific language and
accuracy.
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