Physical Science 1421
Department of Physics and Geology
Force
Background
A force is a vector quantity capable of producing motion or a change in motion. In the SI unit
system, the unit of force is the Newton (N). The properties and effect of forces in the physical world is
described by Newton’s three laws of motion which first appeared in 1687 in Isaac Newton’s book, the
Principia.
st
1 law of Motion (Law of Inertia)
An object moving uniformly in a straight line will continue to move with constant velocity
unless a net force acts on it. An object at rest remains at rest as long as no net force acts on
it.
nd
2 law of Motion (Law of Mass and Acceleration)
The acceleration of an object is proportional to the net force acting on it and is inversely
proportional to the mass of the object.
Acceleration =
Net Force
Mass
3rd law of Motion (Law of Action and Reaction)
For every force that acts on an object, there is a reaction force that is equal in magnitude
and opposite in direction.
Although there are many types of forces in the Universe, they are all based on four fundamental
forces. The strong and weak forces only act at very short distances and are responsible for holding nuclei
together. The electromagnetic force acts between electric charges and the gravitational force acts
between masses. Some other examples of forces are: resistive force acting between two objects in
contact with one another is called the frictional force; and the force acting on strings called the tension.
Section 1: Weight
Equipment Needed
Force Sensor
Balance
Qty
1
1
Equipment Needed
Mass and Hanger Set
In this section use the Force Sensor to measure the weight of a hanging mass.
Qty
1
Part 1: Background
Weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, and
weight is a force that results from the action of gravity on matter. In many everyday situations the word
"weight" continues to be used when strictly speaking "mass" is meant. For example, we say that an
object "weighs one kilogram", even though the kilogram is actually a unit of mass.
The SI unit of mass is the kilogram [kg] and the SI unit of weight is the Newton [N]. Weight is
defined as mass times the gravitational acceleration.
Weight = mass x acceleration due to gravity, where g = 9.8 m/s2
The weight of an object changes depending on where it is measured but the mass does not change.
For instance, the weight of an object will be different on the moon because the Moon’s acceleration due
to gravity is less than that of the Earth’s. Weight is commonly measured using one of two methods. A
spring scale or hydraulic or pneumatic scale measures weight force directly. If the intention is to
measure mass rather than weight, then this force must be converted to mass. As explained above, this
calculation depends on the strength of gravity.
SAFETY REMINDER
•
Follow the directions for using the equipment.
Part 2: Lab Activity
PART I: Computer Setup
1.
Connect the DIN plug of the Force Sensor to Analog Channel A.
2.
Open the document titled Force1.DS
PART II: Equipment Setup
1.
Hold the Force Sensor vertically so its hook end and mass hanger is down as shown.
PART III: Data Recording
1.
Start data recording. Click on the ‘Start’ button (
) to start recording data.
The ‘Start’ button changes to a ‘Stop’ button ( ). The Force will appear in the scale
display.
2.
Press the tare button on Force Sensor to zero the Force Sensor.
3.
Add 20 grams of mass to the hanger. Make sure that the mass and the mass hanger are not
swinging.
4.
Read the Force value by clicking on the ‘Stop’ button (
p. 2
) in DataStudio, ending data recording.
5.
Record the force value in your data table in the lab report.
6.
Calculate the weight of the mass using W = mg and record the weight in the data table in your lab
report. (Force is the experimental value and weight is the accepted value.)
Record your results in the Lab Report section.
Section 2: Hooke’s Law
Equipment Needed
Force Sensor
Spring
Qty
1
1
Equipment Needed
Meter stick
Table Clamp
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1
Part 1: Background
Hooke's law is named after the physicist Robert Hooke. In physics, Hooke's law of elasticity is
an approximation that states that the amount by which a material body is deformed is linearly related to
the force causing the deformation.
For systems that obey Hooke's law, a spring for instance, the extension produced is directly proportional
to the load and is described in the following equation:
Where x is the distance by which the spring is stretched (usually in meters), F is the force exerted to
stretch the spring (usually in Newton), and k is the spring constant or force constant of the spring. The
spring constant has units of force per unit length (usually in Newton/meter).
When Hook’s law holds true, the graph of the Force (in the vertical axis) and the distance
through which the spring is displaced (on the horizontal axis) shows a linear slope. This slope is equal to
the spring constant:
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If we pull the spring too much or past its elastic limit, the spring may snap, buckle, or permanently be
deformed.
SAFETY REMINDER
•
Follow the directions for using the equipment.
Part 2: Lab Activity
PART I: Computer Setup
1.
Connect the DIN plug of the Force Sensor to Analog Channel A.
2.
Open the document titled Force2.DS
PART II: Equipment Setup
1. Slip the post of the Universal Table Clamp through one of the end-loops of the spring.
2. Screw down the clamp to the edge of a table. (See photo)
3. Line up the 0 cm mark of the Meter stick to the end of the spring. (See
photo.) Note: You may choose to define the "end of the spring" as either
the end of the end-hoop or the end of the coils. However, you must make
all measurements from this position once it has been determined.
4. "Zero" the Force Sensor by holding it horizontally on the table and pressing the "Tare" button.
5. Attach the hook of the Force Sensor to the other end-loop of the spring.
PART III: Data Recording
1.
Press the ‘Start’ button (
) in DataStudio to start recording data.
2.
Pull the Force Sensor until the spring is stretched 5 cm.
3.
Press the ‘Keep’ button (
4.
When prompted, enter the amount the spring has been stretched then click OK.
5.
Repeat Steps 2-4 for 10 cm, 15 cm, 20 cm, 25 cm and 30 cm. Do not overstretch the spring.
6.
Press the ‘Stop’ button (
) in DataStudio to save the current measurement.
) in DataStudio.
Analyzing the Data
1. From the "Fit" (
) menu button select "Linear Fit."
2. Record the slope and vertical intercept in the lab report section and print the graph
p. 4
Record your results in the Lab Report section.
Section 3: Friction
Equipment Needed
Friction Tray
500g Mass Bars
Qty
1
4
Equipment Needed
Force Sensor
String
Qty
1
20 cm
Part 1: Background
Friction is the force that opposes motion or the tendency toward motion of two surfaces in
contact. Friction between solid objects and fluids (gases or liquids) is called drag. The direction of the
frictional force is always opposite to the direction of the motion.
The coefficient of friction is the ratio of the force of friction between two bodies to the force
pressing the two bodies together. The coefficient of friction depends on the materials used -- for
example, ice on steel has a low coefficient of friction (the two materials slide past each other easily),
while rubber on pavement has a high coefficient of friction (the materials do not slide past each other
easily).
There are two types of friction: The kinetic friction and the static friction.
Static friction exists between two objects that are not moving relative to each other (like a book
on a desk). When a force is applied to an object resting on a surface, it will not move until the force
applied to it is greater than the maximum force due to static friction.
The coefficient of static friction, denoted as s is simply the ratio between the maximum static
frictional force (Fs) and the normal force (FN):
s 
Fs
FN
Kinetic friction exists between two surfaces that are in relative motion with respect to each other. To
keep an object moving at a constant velocity, a force must be applied to the object equal to the kinetic
frictional force. Hence, the coefficient of kinetic friction (k) is the ratio between the kinetic frictional
force (Fk) and the normal force (FN):
k 
Fk
FN
The static friction is in most cases higher than the kinetic friction.
For this lab we are going to find the FN by multiplying the mass of the object and acceleration due to
gravity ( FN = m X g )
SAFETY REMINDER
•
p. 5
Follow the directions for using the equipment.
Part 2: Lab Activity
PART I: Computer Setup
1.
Connect the DIN plug of the Force Sensor to Analog Channel A.
2.
Open the document titled Force3.DS
PART II: Equipment Setup
1. Cut about 20 cm of string.
2. Make a loop on each side of the string
and connect the Friction Tray to the Force Sensor as shown in the figure.
PART III: Data Recording
1. With no tension on the string, press the "tare" or "zero" button on the force sensor.
2. Place one mass in the Friction Tray.
3. Press the ‘Start’ button (
) in DataStudio to start recording data. DataStudio will
automatically stop collecting data after 5.0 seconds.
4. Place the Friction Tray on a surface and hold the Force Sensor. With the force sensor tied to the tray,
slowly pull the Friction Tray horizontally, from rest, across the lab table until it reaches a constant
velocity. Continue pulling at a constant velocity until the computer stops recording.
5. Measure the weight of the Friction Tray with the bar mass.
Weight of the Friction Tray is the normal force (FN).
6. Use the cursor to highlight the region that corresponds to
the maximum static friction. The legend box gives the
value of the maximum Static Friction. Enter this value into
the corresponding STATIC data table. Also, enter the
value of the Friction Tray's normal force.
7. Use the cursor to highlight the region where the velocity is
constant. The legend box gives the MEAN or average
Kinetic Friction. Enter this value into the corresponding
KINETIC data table. Also, enter the value of the Friction
Tray's normal force.
8. Repeat steps 3-7 with two masses on the Friction Tray.
9. Repeat steps 3-7 with three masses on the Friction Tray.
10. Repeat steps 3-7 with four masses on the Friction Tray.
p. 6
Analyzing the Data
1. Click on the graph labeled "Friction vs. Normal."
2. In the legend box
‘Fit’ (
intercept.
, highlight the Data that corresponds to the static frictional force. From the
) menu at the top of the graph, select "Linear Fit". Record the slope and vertical
3. From the legend box, highlight the Data that corresponds to the kinetic frictional force. From the
‘Fit’ menu at the top of the graph, select "Linear Fit". Record the slope and vertical intercept.
Record your results in the Lab Report section.
p. 7