PreLab for Introduction to Forces Lab

Use the accepted value of the acceleration due to gravity on Earth (9.8 m/s/s) to fill in the 2nd column of the table below:

Mass (kg) / Weight (N)
.100
.200
.300
.400

INTRODUCTION TO FORCES

Introduction – Five Defining Characteristics of Force

  1. A force is a push or a pull on an object
  2. A force is a vector
  3. A force requires an agent
  4. With the exception of a few special noncontact forces (e.g. gravity) the agent must be in contact with the object for the force to exist
  5. If an agent exerts a force on an object, the object exerts the same magnitude force in the opposite direction on the agent (Newton’s 3rd Law).

PURPOSE: To investigate several types of forces; to identify some of their characteristics of each type of force; to be able to identify the force.

APPARATUS: Force probe, LabPro interface, computer, Logger Pro software, elastic bands, meter stick, string, slotted masses, 50-g mass holder,

PART I: GRAVITATIONAL FORCE - WEIGHT

OBJECT: To study the gravitational force

INTRODUCTION: The gravitational force is a long range force that exists between two objects that have mass. The earth exerts a gravitational force on every object on or near its surface. This gravitational force exerted on some object on or near its surface is the product of the object's mass and the acceleration due to gravity.

FG = mg

In some texts, this is called the weight force (or simply the weight). The SI units of the weight force are Newtons (kg x meter/s/s).

PART II – THE FORCE PROBE

The force probe measures the force interaction between it and any object pushing or pulling upon it. This is the same as when you put an object on a scale. Like a scale, the force probe cannot “know” the force with which the earth attracts the object. It can only measure how much of a push or pull it experiences. Newton’s Third Law (#5, above ) indicates that the object being measured will experience the same magnitude push or pull from the force probe.

PROCEDURE: Attach the force probe with a hook on the end to the support stand with the hook pointing down. Plug the force probe into CH 1 of the LabPro. Use the 10 N setting on the top of the probe. Select “Experiments” under Logger Pro 3. Open “Additional Physics”, “RealTime Physics”, “Mechanics”, “L03A1-2a – Measuring Force”. Zero the probe by clicking on Zero button.

Note: Zeroing the force probe eliminates any reading due to the weight of the probe. Any small mechanical change in the force probe -- including changing its orientation -- will change the zero reading. Therefore, zero the probe frequently -- before taking measurements -- with nothing attached to it, always holding it in the orientation in which it will be used.

Which way is up?

Click on Collect to graph a force. While graphing, push up on the hook gently and then pull down gently.

ctivity,

a. Describe the results of pushing and pulling on the hook. Does the graph show a “push” differently from a “pull”? If so, how? Is that different than our usual convention? If so, how?

b. What does the force probe read with no force applied______?

If this value is greater than +/- 0.1N, (and you didn’t forget to zero) call an instructor.

QUESTIONS:

1. Draw a free body diagram of a mass hanging from the hook of a force probe.

2. What kind of contact force is the force probe exerting on the mass?

  1. What kind of noncontact force is the Earth exerting on the mass?
  1. Which of these forces do you think that the probe actually graphs?

PART II: TENSION FORCES

OBJECTIVE: To answer the question: If the force probe can’t measure the weight force, can it still be used to “weigh” something?

PROCEDURE: We are going to start by taking a mass and hanging it from the force probe.

1. Set the force axis to from 0 to 5 N. Change the data-collection period to 80 seconds (click on the clock icon in the tool-bar).

2. Click on START. Carefully add 100 grams to the hook about every 10 seconds (add 50g the first time to account for the massholder). Minimize oscillation of the masses. After the data run is finished, select Examine from the Analyze Menu. Highlight the best region for each mass and read the mean values of force using the Statistics feature under the Analyze menu. Record your data in the following table and print your graph with the display boxes visible.

3. Copy your values from the table on the previous page into the 2nd column of the table below.

4. Record your force probe readings for each mass in the 3rd column.

5. Find the percent difference between the force probe measurement and the weight force (|Theoretical – Measured|/Theoretical) x 100%.

Mass (kg) / Theoretical (prelab) Weight mg, (N) / Force Probe measurement
(N) / %Difference
.100
.200
.300
.400

Questions

1. Explain how the force probe and hanging scales “measure” the weight of an object. What force is the probe measuring?

2. If you pull down on the mass hanging from the probe, or partially support it, does the force probe reading change? Does this mean the weight changed? Explain.

Prediction # 1: If you hang a 100 g mass from a light string attached to the hook of the force probe, will the force measured by the probe be greater than, less than or equal to the force measured with the mass attached directly to

the hook?

Try it. Remember to zero the probe with the hook pointing vertically downward and with nothing hanging from it. Hang the 100 g mass directly from the hook first and then hang the same mass from the hook at the end of the string.

4. Do the results agree with your predictions? When a force is exerted on one end of a string, how large a force is felt at the other end ofthe string? Does the string “absorb” any of the force?

Prediction # 2: If you hang the 100 g mass from an elastic band (of very small mass) instead of a string, will the force measured by the probe be greater than, less than or equal to the force measured with the mass attached directly to

the hook?

Try it, but zero the force probe first. Hang the 100 g mass at the end of the band from the hook. Be sure that the mass does not oscillate at the end of the band. Begin to graph the force.

Did the measured force agree with your prediction? Based on your observations, how much of the force at one end of a string or elastic band is transmitted to the other end? Does the rubber band “absorb” some of the force?

PART III: NORMAL FORCES

OBJECT: To study the normal force

PROCEDURE: Remove the force probe from the support stand. Remove the set screw and put it in the force probe box. Replace the hook with the rubber stopper. Set the force probe on the desk and zero. Set the 200 gram mass on the rubber stopper.

The force probe as scale, again

QUESTIONS:

1. Draw a free body diagram of a mass sitting on the rubber stopper of a force probe.

2. What kind of contact force is the force probe exerting on the mass?

3. What kind of noncontact force is the Earth exerting on the mass?

4. Which of these forces do you think that the probe actually measures?

1. What is the value (magnitude and sign) read by the probe?______

2. How does that compare with:

  1. the magnitude of the weight force (mg

b. the magnitude of the hanging weight shown by the probe in the previous part

3. Why is the value negative?

NEWTON’S THIRD LAW: A PREVIEW

PROCEDURE: Here, we are going to use two force probes balanced on top of each other. The bottom force probe (#1) will be pointing up and the top force probe (#2) will be pointing down. The program has “reversed direction” for Probe #2, so that a push (up)will display as positive and a pull(down) is negative. 1. For this situation, draw 2 separate free body diagrams; one of the top force probe and one of the bottom probe. Don’t forget the gravitational force. Draw a dotted line between the two diagrams to link the force on the top probe due to the bottom probe with the force on the bottom probe due to the top probe.

a. What kind of force is the top probe exerting on the bottom probe? In what direction?

b. What kind of force is the bottom probe exerting on the top probe? In what direction?

Prediction #3 , Will the magnitude of the force measured by the surface (bottom) probe be greater than, less than or equal to the magnitude of the force measured by the object (top) probe?

2. Try it. First, attach two force probes (with stoppers replacing the hooks) to the LabPro interface by plugging the bottom probe into CH1 and the top probe into CH2. Go under "File" menu, and "Open". Select L07A2-1 (Tug of War).

3. Balance the two probes on their respective stoppers. Assign one group member to catch the top probe if it topples off!

4. Be sure to zero both probes before taking data.

5. Start collecting data. Push gently on the object probe a few times to see what happens when the force is increased. Print your graph.

a. Did the measured forces agree with your prediction? Explain.

For this activity,

  1. Which of the defining characteristics of a force (in the introduction) does this part of the experiment illustrate?

PART IV: FREE BODY DIAGRAMS

  1. Identify all the forces acting on the object in the sketches below.
  2. In the second box, construct a free body diagram with appropriate axes.
  3. Label each force arrow in the sketch with the appropriate variable. Assume no air resistance unless told otherwise.

Sketches /
Freebody Diagram
/ Sketches / Freebody Diagram
  1. Static traffic light
/
  1. Static traffic light

  1. truck, no friction
/ 4. Man on scale

5.Truck, wheels locked, sliding frictionless
/ 6. Truck, wheels locked, sliding with friction

7.
Ball falling, no air resistance
/
  1. Truck, slowing down due to friction

9. Ball tied to a rope and pulled straight up
/ 10. Ball tied to a rope and pulled straight down

1