Introductory Laboratory Category

2002 AAPT Apparatus Competition, Boise State University

Name: Robert A. Morse

Address: St. Albans School

Washington, DC 20016

Phone: 202-537-6452

Fax: 202-537-5613

E-mail: Robert_Morse@cathedral.org

Apparatus Title: “Push-me Pull-you” fan cart controller

Abstract (40-50 words)

A push button controller lets students apply forces from opposing home-made or commercial fan units to a low-friction cart and observe the motion with none, one or both forces acting. Students keep the cart moving back and forth, and fill in worksheets showing force, acceleration and velocity vectors.

Sketch of the apparatus:

Description

The addition of a closed-circuit type 1/8 inch phone jack (Radio Shack 274-248A) to home-made fan units such as those described by R.A. Morse (The Physics Teacher, 31, 7, 1993, p436-438, detailed plans on the web at http://Physics.Dickenson.edu/Workshop_Physics/Instructor_Resources/Equipment/Apparatus_Plans/Cart_Fan_2) allows two such fan units to be mounted in opposition on a PASCO low friction cart and controlled by a simple two button control box. New part of this apparatus is the controller box and set up.

Parts Lists:

Two fan units (see web site above for construction details)

Pasco low friction cart

Pasco track or homemade track for cart

2 two conductor 1/8 inch phone jack closed circuit type (RS 274-248A)

2 SPST momentary contact pushbutton switch (RS 275-0646)

1 12 ft mini plug to lug cable (RS 4202454)

1 two conductor mini plug (RS 274-0286)

1 plastic project box 3x2x1 (RS 2701801)

Construction

Drill holes in the homemade fan units to mount the closed circuit phone jacks. Break the circuit connection for the positive motor lead and connect the wires to the closed circuit jack. (Note: there are three connectors on the jack. One lead goes to the solder lug which connects the inner conductor on the plug. The other lead connects to the remaining two lugs.) Drill holes in the top of the project box and mount the push button switches. Cut the 12 foot cable in half. One end has a plug already wired- solder the two leads from this piece to one switch. Solder a plug on one end of the other piece and the leads from the other end to the second switch. cut the edge of the box so that the leads will be lightly clamped when the top is in place. Label the buttons and plugs.

Operation

Secure the fan units to the cart so they oppose each other. With the plugs inserted in the jacks on the fan units, the buttons will allow the fans to be turned on.

Start with the cart at one end of the track. Use the buttons to control the cart’s motion so that the cart moves back and forth on the track without hitting the ends. (Support the wires so they do not drag on the cart.)

Use the worksheet to indicate which fan(s) are on, the direction and magnitude of the forces, the acceleration and the velocity of the cart at the ends of the track, and the left, right and middle parts of the track.

Compare this with the motion you would expect if the cart had springs attached to each end of the track. Get it moving and then see what happens when both fans are turned on at the same time. What is the net (combined) force in this situation? What is the acceleration in this situation? What happens to the velocity in this situation?