- Computers are needed for lab this week
- Determine the initial velocity of the launcher so that you can shoot the projectile into a trash can
- The launchers used have been hand-chosen for their success rate (and I tell the students as much). Here's my table of calibration data for all projectile launchers (Select the lab being used: Launchers in Bewkes 204 – Launchers in Bewkes 231). Average velocities will range from about 6.3 to 7.1 m/s; anything significantly higher or lower means students used the wrong range setting (something other than medium) or made a measurement/calculation error
- The key to success here is careful measurements and calculations, as well as double-checking everything! Mistakes frequently are made due to errors that are easy to catch, especially starting the tape measure at the 10 cm mark and forgetting to make appropriate adjustments
- I spend a lot of time watching over them, asking prodding questions and giving them tips so they think about a better way to do something carefully. Getting them to succeed in the trash can shot is the goal here
- I also try to check their spreadsheet work before they take a trash can shot. I've had students forget to enter the height of the ball (duh) and not use the trash can icon to determine its placement (double-duh)
- During the pre-lab, I walk them through the steps of deriving an expression for vox. Note that weak students will numerically calculate the time for the projectile to hit the floor and use a rounded value to calculate the velocity!
- Also during pre-lab, show students how to use the launcher:
- Loosen/tighten both screws on the launcher to adjust angle, which is checked with built-in angle gauge. The angle should be checked (but not necessarily adjusted!) before each shot, and especially before trash can shot
- With launcher horizontal (at 0°) show how to mark the start position of ball on floor (drop a plumb bob along the cross marking the center of the ball image printed on right side of launcher. If the launcher is mounted along the center of a bench, note that the front of the launcher barrel is parallel to the front edge of bench; mark the front edge of launcher on floor, and measure the set back to the center of the ball image (~1 cm)
- The initial height of the projectile is measured to the bottom of the image of the projectile on the side of the launcher (since it's the bottom of the projectile that hits the floor)
- Show them how to load the ball with the ramrod: the plunger will go back 2 clicks so that the yellow plunger appears near "medium range". Short range can be used, but it's not very dramatic; long range cannot be used, since an angle greater than ~10° will hit the opposite wall. Remind students not to look in the loaded barrel, or stick their fingers inside!
- If they jam the projectile back to the long range setting, it can be safely released by placing the ramrod inside the launcher on the projectile, and gently releasing the trigger while securely holding the ramrod
- Show students the proper procedure for pulling the trigger: gently pull the rope straight up. Don't jerk it an an angle; a student in the past bent the launch trigger doing so. Also, pulling at an angle will cause the launcher to fire inconsistently
- After students load the projectile into the launcher, they should listen carefully to make sure the ball does not fall out of its 'cup'. It will make a loud 'plink' if it does and not fire correctly. The ball can be placed back in its cup by gently pushing with the ramrod
- If the ball refuses to stay in the launcher cup before firing, Jeff can fix it. The fix depends on which model launcher is used:
- Launchers A - L: These are older launchers, and replacing the blue ring that surrounds the launcher cup fixes the problem
- Launchers M & N: these have a different style cup, fixed as follows:
- With the launcher triggered and ball removed, stick your fingers in the ball cup to depress it slightly
- Rotate the cup in the barrel 10° clockwise. All should be good now
- Tell students to fire the ball a couple of times to figure out where it hits the floor, then place the catch box on the floor at this position and try a couple more times. The ball should land in the center of the catch box. Then remove the catch box and tape the target sheet (with carbon paper underneath) directly to the floor. Don't 'weld' the target sheet to the floor with tape (especially the carbon paper!). The projectile should land near the center of the target sheet
- Remind students to mark the front edge of the target sheet so that they measure from the correct side of the paper. Weak students will still skip this step!
- Time of flight: attach the double-photogate to the front of the launcher; the first gate should be no more than a couple of millimeters from the front edge of the launcher. Load the projectile, reset the timer, fire. Students should measure a time ~0.015x sec, and will be very consistent. Check connections if the time is drastically different.
- A few students will be able to figure out how to attach the photogate bracket to the launcher, but you'll likely have to show most of them how it is connected
- Students should have less than a 5% difference between their average velocities before proceeding to trash can shot. This doesn't guarantee success, but catches their larger errors
- Students can keep measurements in cm (therefore, velocity in cm/s), but the spreadsheet will only accept velocities in m/s
- Trash can shot:
- Two trash cans are in the room. Need more? Grab them from your office. Remove the trash bag from the can first!
- Velocities from trajectory data (not photogate data), initial height of projectile and chosen angle are entered into spreadsheet. Resulting graph shows where the projectile hits the floor; a scale trash can appears in the lower-left corner of the graph, and should be dragged to the appropriate location before printing
- The angle chosen should be some increment of 5° between 20° and 50°. These angle positions are easier to set and read on the launcher angle gauge
- What angle gives the longest distance (to the floor)? It's not 45°, since the ball does not land at the same height as it is launched. Trying different values in the spreadsheet shows that the farthest distance occurs at 42°; this is due to the steeper angle that the approaching ball makes with the floor (at least for the height of our benches)
- Take care not to change the shape of the can while dragging!
- Holding the <Shift> key while dragging will keep the can on the ground
- New feature (Fall 2023): After an absence of at least a decade, the current version of Excel now allows you to select the trash can and use the arrow keys to move it with precision. Hallelujah!
- Have students hold a meter stick along the top edge of the launcher so that their partner can move the trash can left or right. Since this can be difficult to do when launchers are mounted in the center of a bench, we allow left-right movement as needed after a shot, without penalty. However, once the can is in position, it cannot be moved closer or further from the launcher
- Common mistakes:
- Frequently, if the distance from the launcher to the edge of the target sheet (dA) is something like 3.02 m, students will record this as 3.12 or 3.20 m!
- Similarly, they will make a mistake with the height: 1.08 m instead of 1.18 m!
- Not knowing where '0' is on the tape measure (tell them to hold a meter stick alongside of the tape)
- Incorrect measurement of distance to trash can, or launcher angle (shot can be long or short)
- Forgetting that they started tape measure at 10 cm mark (shot is long)
- Used photogate velocities on spreadsheet (shot can be long or short)
- Entering height, velocity or angle values incorrectly into spreadsheet. That's why I have a table of their input appear on the graph
- Rounding velocities to two sig figs (since g = 9.8 m/s2!); velocities should be calculated to four sig figs (0.001 m/s)
- Dragging the scale trash can image on the spreadsheet to the wrong location. The weakest students will place the can image so that the trajectories hit the bottom of the can; this means that the ball must enter through the side of the can!
- Measured values:
- The distance between the photogates is 10.0 cm
- The height of the launcher from a standard lab bench or sink is around 1.2 m. It will be lower for the table in the hallway, about 1.0 m
- The dimensions of the garbage can are 38.0 cm high, 34.3 cm long, 23.5 cm wide (width and length are measured inside the lip of the can; length is 37.0 cm to outer edge of lip)
- A small juice can is 9.8 cm high, circular diameter of 5.5 cm
- The minimum angle of launchers is 30o, and the maximum angle is 50o; they must use the medium range setting
to fit in the room!
- Lab room ceiling height is ~2.6 m; hallway ceiling height is ~2.45 m
- Two launchers can only be set to a maximum angle of 45o: Launcher C (which has the highest vox, and is near the side bench in Bewkes 204 or the printer in Bewkes 231) and whatever launcher is in the hallway (H or I). A higher angle will hit the light fixture or ceiling! Signs will be placed next to each of these launchers
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