Download Lab #5 – Newton`s First Law

Lab 2: Newton’s First Law
Photo by Keith Larrett
http://www.webshots.com/photos/skydiving1.html
Introduction:
A common misperception is that astronauts are not subject to
the force of gravity during space flight. In reality, for missions
orbiting the earth, astronauts are subject to a force of gravity
that is only slightly less than what they would experience on the earth’s surface. Why, then, do they appear
to be “weightless”? The answer is simple. While in orbit around the earth, they experience free fall motion
in the same way that skydivers do. (There is simply no air pushing against them.) Free fall motion provides
the sensation called “weightlessness”. Both orbiting astronauts and skydivers undergo free fall motion.
There is a difference, however. The difference between skydivers and astronauts can be illustrated by
looking at the forces acting upon them. An astronaut orbiting the earth is subject to only one force, the
force of gravitational attraction to the earth. A skydiver, on the other hand, is subject to two forces:
gravitational attraction to the earth and air resistance. Different net forces cause different types of motion.
Since an astronaut is subject to only one force, the net force cannot be equal to zero. Newton’s Second
Law states that this type of situation causes accelerated motion. In this case, the astronaut is accelerating
towards the center of the earth with centripetal acceleration. The result is circular, or orbital, motion.
Because a skydiver is subject to two forces, the net force may or may not be zero. The value of the net
force depends upon the magnitude and direction of each of these forces. Newton’s First Law, the Law of
Inertia, states that a net force of zero causes an object to be at rest or in motion with constant velocity. The
purpose of this lab activity is to investigate each of these cases.
Air resistance is not a constant force. As you fall through air (or water or oil) a viscous force, air resistance,
is generated as you push the molecules out of the way. This force increases with your velocity, the density
of the medium and your cross-sectional area. A skydiver, after jumping out of a plane, eventually comes to
be in equilibrium when air resistance balances the force of gravity pulling downwards. At this point, the net
force is zero and the velocity becomes constant. This constant velocity is known as terminal velocity. Since
our insurance was cancelled after the last class "jump", you will explore this phenomenon using coffee
filters.
What does it mean for the Net Force on an object to equal zero? Many students think that it means there
are no forces acting on the object. This is incorrect. The term Net Force is used to symbolize the sum of all
of the individual forces.
n
Fnet   Fi
i 1
Since force is a vector quantity, both magnitude and direction must be considered when forces are added.
Equilibrium results when these forces effectively “cancel each other out”. In other words:
Fnet  0
What happens to the motion of an object if the Net Force on it equals zero? Many students think that the
object must be at rest, but this is only true if the object was at rest originally. If the object was in motion
Lab – Newton’s 1st Law
originally, then the object remains at constant speed in one direction for as long as the forces are in
equilibrium.
Newton’s First Law of Motion, although simple to describe, is not intuitive. Understanding it and applying
it to different situations can be difficult. Remember that Newton’s Laws are the foundation of dynamics,
the study of why objects move the way that they do. Relationships between force and motion are one of the
central themes of Mechanics.
Lab – Newton’s 1st Law
Newton’s First Law
Goals:
• Verify that Newton’s First Law applies to objects at rest.
• Verify that air resistance causes an object to reach a state of equilibrium.
• Investigate the relationship between air resistance and velocity.
Equipment List:
Force table w/ Force sensor & 3 pulleys
Data Studio™
Excel™
Hanging masses
Motion Detector
Coffee Filters
Scale Balance
Activity 1: Newton’s First Law for an Object at Rest
Calibration of the Force Probe
Prior to beginning this activity it will be necessary to calibrate the force sensor. This can be accomplished
in just a few easy steps once the force probe has been installed in Data Studio. Make sure that the Low
Sensitivity (1x) window is selected in the drop down menu. This may NOT be the only time that you may
need to calibrate the force sensor during this lab. As the lab progresses you will need to periodically re-zero
the probe by pushing the TARE button on the side of the probe.
1.
Set up Data Studio™ to read the data collected by the force probe located on the force table. While
nothing is pulling on the force sensor, press the TARE button to zero it. Do not graph the data.
Instead, set up a Digits screen.
2.
Place three different masses on the other strings. Move the pulleys around the force table until the ring
is held in equilibrium over the very center of the table.
3.
Make a sketch of your force table. Label each mass and record the angle positions of each of the
strings.
4.
Calculate the magnitude of the force (in Newtons) exerted on the ring by each of the hanging masses.
Add this information to your sketch.
Lab – Newton’s 1st Law
5.
Determine the magnitude and direction of the resultant of the three force vectors created by the
hanging masses. By both a graphical method and an algebraic method. Clearly explain in each case
your method of vector addition.
6.
Compare your CALCULATED resultant to the force exerted on the ring by the force probe. What do
you notice?
7.
Is it reasonable to conclude that Newton’s First Law has been verified for an object (the ring) at rest?
Explain.
Activity 2: Newton’s First Law for an Object Moving with Constant Velocity
1.
Set up Data Studio™ to read the data collected by the ultrasonic motion detector. Create graphs of
Position vs. Time and Velocity vs. Time so that the data will be displayed on the screen as it is
obtained.
2.
Place the motion detector carefully on the floor, facing upward. Set the beam so that it takes in a wide
view. Practice holding a coffee-filter high above the detector so that, when released from rest, it falls
and rests upon the detector (or close to it).
3.
Record the motion of the falling coffee-filter. Be sure to obtain a set of data in which the sensor
picked up the entire motion of the filter, starting prior to its release. Was terminal velocity
(equilibrium) reached? How can you tell? How much time did it take the filter to reach terminal
velocity? Import the graphs of one trial to your template.
4.
Determine the value of the terminal velocity of the coffee-filter. Explain how you did this.
5.
Measure and record the mass of your coffee-filter using a scale provided.
6.
Draw a free-body (force) diagram of the coffee-filter while it was experiencing terminal velocity.
7.
Use Newton’s First Law to determine the magnitude of the force of air resistance acting on the coffeefilter while it was experiencing terminal velocity.
Activity 3: The Nature of Air Resistance
1.
Consider the coffee-filter data obtained between the time that it was released and the time that it
reached terminal velocity. What evidence is there of acceleration? Does the data suggest that the
acceleration of the filter is constant, increasing, or decreasing? Explain.
2.
Draw a free-body (force) diagram for the filter for each of the following: 1) the moment of release
from rest; 2) before reaching terminal velocity; 3) after obtaining terminal velocity. In your sketches
estimate the magnitude of the various forces by showing force vectors of different length at each
of the times.
3.
According to theory, what is causing the force of air resistance on the coffee-filter to change? Is the
magnitude of the force of air resistance increasing or decreasing during this time interval (from the
moment of release until the terminal velocity has been reached)? Why?
4.
Describe how the magnitude and direction of the Net Force on the coffee-filter is changing over this
time period (from the moment of release until the terminal velocity has been reached).
Explain why it is inappropriate to apply Newton’s First Law to the coffee-filter during this time period.
5.
Lab – Newton’s 1st Law