Newton’s 1st and 2nd laws
p.1/6
PRELAB: NEWTON’S FIRST AND SECOND LAWS
1. A cart is speeding up as it moves to the right, as in Activity 1-1. Predict the direction of the
combined (net) force. Explain your reasoning.
2. Sketch Prediction 2-1 on the axes below.
Force (N)
+1
0
velocity (m/s)
-1
3.
+1
0
-1
time (s)
How does the net force on the cart (after release) in Activity 2-1 compare to the net force on
the cart (after release) in Activity 2-2?
4. Compare to the direction of net force in Activity 3-1 to the direction of the net force in
activity 1-1. Explain.
5. Sketch Prediction 3-1 on the axes below.
Force (N)
+1
0
velocity (m/s)
-1
+1
0
-1
time (s)
Created 8/2003 by DSA for P3
Adapted from RTP and ILD
Newton’s 1st and 2nd laws
p.2/6
NEWTON’S FIRST AND SECOND LAWS
Topic: Forces and acceleration in 1D
Overview: In this lab, you will explore the relationship between force and acceleration. You will use the
motion detector and a force probe to generate force-time and velocity-time graphs.
Writing it up: Throughout this handout, you will be asked to answer questions, sketch graphs and do
calculations. Write these things in your lab notebook as you go through the experiment. Label each
answer/graph/calculation so that you (and your lab TA) can find things quickly. After lab, write a short
(<300 words) conclusion of the experiment that summarizes what you did and the major findings of the
experiment.
Investigation 0: The force probe
What does the force probe measure? In this investigation, you will get a feel for force by applying force to
a force probe.
Activity 0-1: What the force probe measures
1. Plug in the force probe and start the software. Set up to graph force vs. time. Start graphing.
Pull and push on the hook of the force probe.
Q0-1: Describe the difference between a graph made by pulling the hook and one made by
pushing.
Pulling on the hook cause the software to register a negative force. Pushing causes
the software to register a positive force. The software can be configured to reverse
the sign as needed.
Q0-2: Describe the difference between a graph made pulling the hook weakly and one made by
pulling more strongly.
The points for a strong pull will be farther away from the x-axis than a weak pull.
Q0-3: Does the force probe read zero when there is nothing pushing on it? Should it?
The force probe I used did read zero when nothing was pushing or pulling on it. If
the net force on the hook is zero then the force probe should register that.
2. Start graphing and press the “zero” button on the probe.
Q0-4: What is the purpose of the “zero” button?
With nothing pushing or pulling on the hook, if the force probe is registering a nonzero value then the zero button will reset/calibrate the force probe so that it reads
zero when the net force is zero.
Created 8/2003 by DSA for P3
Adapted from RTP and ILD
Newton’s 1st and 2nd laws
p.3/6
Investigation 1: Combining forces
In this investigation, you will use the force sensor and a motion sensor to examine the motion of a cart
being pulled by a constant applied force. In the first activity, the frictional force is very small. In the
second activity, you will increase the frictional force and examine what happens.
Activity 1-1: Cart with small frictional force pulled by a constant force
1. Place the cart on track and use the cart to make sure the track is level. If the cart tends to move in one
direction or the other when released from rest, you may need to adjust the height of one end of the
track.
2. Plug in the motion detector. Set up to plot force-time and velocity-time graphs.
3. Attach the force sensor to the cart with the screw. Set up the ramp, pulley, cart, string, hanging mass*
and motion detector as shown above. Make sure that the string is horizontal. (You may have to adjust
the pulley slightly). Record the value for the hanging mass. Begin taking data, and after you hear the
clicks of the motion detector, release the cart. You may need to do this a few times before you get a
good set of data.
*Comment: Choose the amount of hanging mass to use. The amount should be large enough that
friction is negligible, but small enough that you can observe the motion of the cart. The cart should
take at least 1 sec to move across the track once it is released.
4. Sketch the force-time graph in your notebook. Use the software’s statistics feature to calculate the
average force applied to the sensor by the string. (Pay attention to the portion of the graph when the
cart was being pulled by the string).
Q1-1: In this activity, is the force recorded by the probe equal to the net force? Explain.
5. Sketch the velocity-time graph in your notes. Use the software’s fit feature to determine the
acceleration of the cart.
Activity 1-2: Cart with large frictional force pulled by a constant force
1. Screw the friction pad into the bottom of the cart. Adjust the height of the friction so that the cart can
roll, but slows down to a stop within a few centimeters when given a push. Set up the cart, string and
hanging mass as you did in Activity 1-1. (Use the same hanging mass). Again, place the cart near the
motion detector, begin taking data, and after you hear the clicks of the motion detector, release the
cart.
2. Sketch the velocity-time and force-time graphs in your notebook. Calculate the average force applied
by the string and the average acceleration of the cart from the graphs.
Q1-2: In this activity, is the force recorded by the probe equal to the net force? Explain, using a
force diagram.
Q1-3: Compare the force applied to the cart in this activity with the applied force in Activity 11. Are they the same? Should they be?
Q1-4: Compare the cart’s acceleration in this activity with the cart’s acceleration in Activity 11. Are they the same? Should they be?
3. Remove the friction pad.
Created 8/2003 by DSA for P3
Adapted from RTP and ILD
Newton’s 1st and 2nd laws
p.4/6
Investigation 2: No net force (combined force = zero)
In Investigation 1, you saw that an object acted on by a constant (but non-zero) force accelerates. In this
Investigation, you will look at two situations where the net force is essentially zero.
Activity 2-1: Cart pulled and released
Prediction 2-1: Suppose you give the cart a quick pull to start it moving away from the motion detector
(by pulling on the force probe hook) and release it. What will the force-time and velocity-time graphs
look like?
1. Test your predictions. The cart’s friction should be as small as possible. Be sure that the cable for the
force probe won’t interfere with the motion of the cart. Record the force-time and velocity-time
graphs in your notes. Indicate the moment the cart was released on each graph.
Q2-1: Do the velocity and force graphs agree with your predictions? If not, how do they differ?
What happened to the force of the pull after you released the cart? What happened to the
acceleration after you released the cart?
Activity 2-2: Dueling fan units
1. Set up the cart with two fan units and the ramp as shown above. The fans should push in opposite
directions. Make sure that if the fan blade is facing the motion detector the blade does not extend
beyond the end of the cart.
Equipment note: The cart is just barely long enough for the two fan accessories. In order to get them
both on the cart, you will need to use a little ingenuity. If your fans have protective covers, you can do the
following: Place one fan on the side of the cart that faces the sensor. Be sure it is flush and does
not hang over the edge. Turn on the fan. Turn on the other fan unit. Carefully place the second
fan up against the first fan as shown in the illustration. The two on/off switches should nuzzle up
against one another.
Prediction 2-2: Suppose the two fans are pushing on the cart with the same magnitude force. You give
the cart a push to start it moving. What do you predict the acceleration of the cart will be after the cart
starts moving? Given the prediction for the acceleration, how do you predict the velocity will change?
2. Turn on both fan units. If the cart tends to move in one direction or the other, you may need to make
to exchange batteries between the two fan units to make sure the cart remains at rest when both fans
are turned on.
Created 8/2003 by DSA for P3
Adapted from RTP and ILD
Newton’s 1st and 2nd laws
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Q2-2: When both fans are turned on, and the cart has no tendency to move, what is the
combined (net) force exerted by both fan units on the cart? How do you know?
3. Test prediction 2-2. Vary the size and direction of your initial push.
Q2-3: Compare the graphs with larger pushes to those with smaller pushes. Are there any
differences in the velocity graphs? Is there any difference in the acceleration?
Q2-4: Compare the graphs with push toward the right with those with push to the left. Are there
any differences in the velocity graphs? Is there any difference in the acceleration?
Investigation 3: At the turning point
Suppose you use the same setup as in Investigation 1, but now the cart starts at the end of the ramp
furthest from the detector. Now when you give the cart a push toward the motion detector and release it, it
will slow down, reverse direction and move back toward the motion detector.
Activity 3-1: Slow down and speed up (negligible friction)
Prediction 3-1: Sketch prediction graphs for velocity, acceleration and force probe reading after the cart
leaves your hand and before you stop it. Indicate the moment the cart reverses direction on each graph.
1. Set up the equipment as in Investigation 1. Use the same hanging mass you used in Investigation 1.
2. Test your predictions. Stop the cart with your hand before it reaches the motion detector.
Q3-1: Compare your observations with your predictions. Describe the force and acceleration at
the moment when the cart reverses direction.
Q3-2: Based on your knowledge of acceleration and force, explain why the force and
acceleration have the signs they have at the moment the cart reverses direction.
Activity 3-2: Slow down and speed up (non-negligible friction)
If you use the same setup as Activity 3-1, but use a much smaller hanging mass, the effect of friction will
become more apparent.
Prediction 3-2: Sketch prediction graphs for velocity, acceleration and force probe reading after the cart
leaves your hand and before you stop it. Indicate the moment the cart reverses direction on each graph.
1. Set up the equipment as in Investigation 1. Choose a much smaller hanging mass than you used in
Activity 3-1. The mass should be just large enough that the cart will be able to reverse direction.
2. Test your predictions. Stop the cart with your hand before it reaches the motion detector.
Q3-1: Compare your observations with your predictions. Does the acceleration remain constant
after the cart leaves your hand? Does the force applied by the string remain constant?
Q3-2: Does the net force change when the cart reverses direction? Explain how you know.
Q3-3: Based on your knowledge of the way that forces combine, explain why the acceleration
of the cart might change when the cart reverses direction. Would expect the magnitude of
Created 8/2003 by DSA for P3
Adapted from RTP and ILD
Newton’s 1st and 2nd laws
p.6/6
the acceleration to be larger when the cart is moving toward the detector or away from
the detector? Explain. (Diagrams might help.)
Created 8/2003 by DSA for P3
Adapted from RTP and ILD