Name: Dale Freeland
Institution: Portage
Central High School
Address: 8135
South Westnedge
Portage,
MI 49002
E-mail: dfreeland@portageps.org dale.freeland@wmich.edu
Apparatus Title:
Projectile Launcher
Abstract:
The crossbow is used to launch a suction cupped dart during projectile motion and energy conservation portions of the physics class. The dart speed (about 14.5 meters per second) allows the instructor to make projectile motion problems tailored for the lecture hall. Verification of student calculations is done by placing launcher and target at distances specified in student problems, adjusting the elevation (or depression) angle and launching the dart at the target. Students “see” that their calculations are verified in the lecture classroom when the dart strikes the target. Students may also compute the work done on the crossbow while preparing to launch a dart and compare that work done to the kinetic energy of the dart as it leaves the crossbow.
Equipment ($38.10) (All materials from Home Depot unless noted below)
- Supa Sukka© Crossbow ($22) purchased in sporting goods at Meijers store
- Oak veneer plywood base (3/4 x 11 x 33) ($4)
- Nine - 5/16 dowel pins 1 ½ inches long ($2)
- ¼ x 1 lag screws ($1)
- bolts, nuts, fender washers, screws ($2)
- support block (1 3/8 x 2 3/8 x 6 3/8) ($.50)
- plastic container with lid ($1) from Dollar Store
- Masonite for protractor support ($.50)
- copy protractor ($.10) (done at school)
- iron-on oak veneer edging ($3) from wood working supply store
- wood finish ($2)
Description
The crossbow device is used during projectile motion and energy conservation studies. Students are aware that horizontal and vertical velocity are independent of each other when they first see the crossbow and they are asked to find the speed of the suction cupped dart that is launched from it. The launch device is placed on the projection stand in the back of the lecture hall and set to launch the dart horizontally. A laser light seen on the projection screen shows the height of the launcher. Students are encouraged to watch for the dart’s position as it strikes the screen. A poster board 2-meter strip shows that the dart falls 1.8 meters as it travels the 8.8-meter horizontal distance to the screen. Since students know that horizontal velocity is independent of vertical velocity, the time of fall may be computed. The horizontal distance of 8.8 meters and fall time of .61 seconds yields a speed of 14.5 meters per second for the dart.
After computing the speed of the dart as described above and listed as #1 below, other projectile motion problems may be given students. Suggestions for projectile motion problems are listed below (1, 2, 3, 5, & 6). The generated problems may be related to these areas. Problems 1 & 3 deals with uncertainty calculations while problems #7 and #8 deal with energy conservation.
1. Find the speed of the projectile. What uncertainty is in this computation?
2. Find elevation angle that gives maximum range.
3. Find the elevation angle so that the dart strikes a target located on horizontal plane of launcher and 5 meters from the launcher.
4. With the uncertainty of the dart speed as computed in #1 above, how large should the bull’s eye be made so that the launched dart will strike the bull’s eye of the target?
5. Find elevation angle so that the dart strikes a target 2.5 meters above and 8 meters horizontally away from the launch position. (These distances correspond to the crossbow being placed on the lecture hall demonstration table and the target being located on the projection stand in back of lecture hall.)
6. Find elevation angle so that the dart strikes a target 3 meters below and 8.5 meters horizontally away from the launch position. (These distances correspond to the crossbow being placed on the lecture hall projection stand in rear of room and the target being located on demonstration counter in front part of lecture hall.)
7. Using 20 Newton scale and meter stick, find the energy stored in the stretched crossbow.
8. Find the kinetic energy of the launched projectile. Compute the portion of the stored energy (from #7) that is transferred to kinetic energy of projectile.