Ignoring Friction - Not This Time!

Background Information:

    This lab allows students to discover what factors effect the amount of frictional force of a block as it is pulled across a surface. Hopefully students will discover that the only factors affecting friction is the amount of mass and the nature of the surface. The surface area and the speed the block is pulled should not matter.

Pre-Lab

    Ask the students what factors affect the amount of friction. List these on the board. Some possible responses are mass, surface area, nature of the surfaces (rough vs. smooth) and speed of movement. Demonstrate how the data can be collected with the computer. Tell them to design and perform an experiment that will test each of the factors they listed.

Procedures:

  • Graph the frictional force of the block under as many different conditions as possible.

Data / Observation:

  • Students should record the data determined from the graph and should record the conditions for each run.

Calculations:

    No calculations for this lab.

Conclusions:

  • Students should be draw some conclusions about what factors effect frictional force and how these factors effect frictional force.
  • It would be nice for the students to discover the actual factors and how they effect frictional forces. However it is more important for their conclusions to be supported by their observations.

Questions:

  • Discuss the factors effecting frictional forces
  • Discuss questions raised by the students.
  • Assign students to find answers to questions raised.
  • Extensions / Post-lab:
  • Discuss the difference between static and sliding friction.

Notes:

  • If no trucks are available the experiment can be done by pulling the force sensor by hand. The truck just gives a more consistent pull and is more fun
  • Inexpensive slow moving toy trucks work best. The speed of these trucks can be varied by replacing batteries with a metal "shunt". Just replace a battery with metal of similar size to complete the circuit. A short wire with alligator clips can also be used to complete the circuit. This enables you to run a four battery truck with 4, 3 or 2 batteries and thus obtain 3 different speeds.
  • The wooden blocks included in the PASCO dynamics sets are perfect for the activity. Otherwise blocks can be made by gluing felt to one wide and one narrow side of 5 inch pieces of 2 X 4's. This gives a large and small area of a rough (felt) surface and a smooth (bare wood) surface.
  • There are no unusual safety hazards for this lab.

Student Instructions for ULI

The Big Question:

    What determines the amount of frictional force?

Introduction:

    In a perfect physics world there would be no forces such as air resistance or friction to mess up or calculations or waste our energy. You could get in your car and accelerate to 55 mph and coast to your destination. You would only have to use enough gas to get your car rolling. However, as you know we live in a real world and we have to learn to deal with those pesky frictional forces. We cannot always ignore friction. If you slide heavy box across the floor, you have to overcome the frictional forces resisting your efforts. These frictional forces depend on different factors. In this lab you explore frictional forces and the factors affecting them.

Equipment / Materials:

  • Computer with ULI
  • MacMotion software
  • Force sensor
  • One or two masses of equal mass (250g or 500g)
  • String
  • Battery operated truck
  • Wooden block with felt or sandpaper on two sides

Computer Set Up

  • Connect the ULI to the computer (use the modem port) and connect the ULI to a power supply.
  • Connect the Force Sensor to DIN 1 of the ULI.
  • Turn on the ULI and the computer.
  • Configure the computer to collect Force data only. For Macintosh open MacMotion and select "Force Data Only" under the "Collect" menu.
  • Calibrate the force sensor. A stored calibration will work because we will be comparing amounts of frictional force. For Macintosh select "Calibrate" under the collect menu and follow the on screen instructions. You will need a 500 gram (4.9 N) mass for the calibration.

Computer Use

  • Click the "Start" button. The computer will begin collecting data.
  • Start the truck to produce the graph.

Set-Up:

    Set up the truck and the block as shown in figure 1.

Procedures:

  • Make as many graphs of the frictional forces as possible under different conditions.
  • Describe your procedures on the student lab sheet.

Data / Observation:

  • For each run describe the conditions (For example: large side with felt, with one 250 g mass added and pulling with a truck with 2 batteries.
  • For each run record the amount of frictional force.


Calculations:

    There are no calculations for this lab.

Conclusions:

    State your conclusions about what factors effect frictional force and how these factors effect frictional force. Be sure to support your conclusions with your observations.

Questions:

    Record any questions that arose in the lab. Include any answers to the questions you may have found.

Student Instructions for LabPro

The Big Question:

    What determines the amount of frictional force?

Introduction:

    In a perfect physics world there would be no forces such as air resistance or friction to mess up or calculations or waste our energy. You could get in your car and accelerate to 55 mph and coast to your destination. You would only have to use enough gas to get your car rolling. However, as you know we live in a real world and we have to learn to deal with those pesky frictional forces. We cannot always ignore friction. If you slide heavy box across the floor, you have to overcome the frictional forces resisting your efforts. These frictional forces depend on different factors. In this lab you explore frictional forces and the factors affecting them.

Equipment / Materials:

  • Computer
  • LabPro
  • LoggerPro software
  • Force sensor
  • One or two masses of equal mass (250g or 500g)
  • String
  • Battery operated truck
  • Wooden block with felt or sandpaper on two sides

Computer Set Up

  • Connect the LabPro to the computer and to a power supply.
  • Connect the Force Sensor to the LabPro in channel 1. Turn on the interface and the computer.
  • Under the Experiment menu choose Show Sensors and select the student force sensor in channel 1.
  • Calibrate the force sensor. A stored calibration will work because we will be comparing amounts of force. Under the Experiment menu choose Calibrate and the force sensor and follow the on screen instructions. You will need a 500 gram (4.9 N) mass for the calibration.

Computer Use

  • Click the Collect button. The computer will begin collecting data.
  • Start the truck to produce the graph.

Set-Up:

    Set up the truck and the block as shown in figure 1.

Procedures:

  • Make as many graphs of the frictional forces as possible under different conditions.
  • Describe your procedures on the student lab sheet.

Data / Observation:

  • For each run describe the conditions (For example: large side with felt, with one 250 g mass added and pulling with a truck with 2 batteries.
  • For each run record the amount of frictional force.


Calculations:

    There are no calculations for this lab.

Conclusions:

    State your conclusions about what factors effect frictional force and how these factors effect frictional force. Be sure to support your conclusions with your observations.

Questions:

    Record any questions that arose in the lab. Include any answers to the questions you may have found.

 

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