Abrief outline for a horizontal projection lab

A teacher was asking about projectile motion labs in a Facebook teaching group; I thought I’d outline how I might do this. The teacher’s class is a traditional math-based class, looking for student-centered learning.

Introductory demonstration

1. Roll a ball off a table. Ask students to sketch what the path looks like after it leaves the table.
   • Hopefully they will draw some sort of a curved path.
2. Ask what details about the motion we could measure.
   • Generate distance the ball traveled from the table; how much time it was in the air; how far down it fell, mass of the ball, and speed of the ball.

Developing the experiment

1. Ask how we could change the situation so the ball would travel further from the table.
   • They might generate: make it go faster, raise the table, and maybe use a different ball.
   • Explore how these would affect the distance.
   • If they have previously learned that all object fall with the same acceleration, ask them if the mass of the ball should change the distance (hopefully they will answer that it won’t).
2. Ask them which variable would be easier to change: The speed of the ball or the height of the table. Unless you have tables that rise up easily, you can settle on speed being an easy ‘independent variable.’ Start a discussion of how you might change the speed of the ball in a controlled way.
   • They may come up with some creative ways, but I’d guide them towards rolling the ball down a ramp from different heights.
3. Next we need to decide how to measure the speed of the ball.
   • If they have used motion sensors, they might suggest this. Ask them how they would position the motion sensor so it could measure the speed of the ball without getting in the way or being angled from the ‘straight forward’ motion of the ball.
   • If they have used photogates, they might suggest using those. Using just one just before it leaves the table would introduce significant uncertainty: How certain can they be that the photogate is on the exact center of the ball???? If you want to use photo gates, one near the bottom of the ramp and one right before it leaves the table might work (recording the time between the two positions).
   • A ‘low tech’ method is to mark off a distance along the table from the edge to close to the ramp. Demonstrate this. You might see the ball bounce a few times, so remind the students that they should try for a position when it is not bouncing.
     • What assumptions are we making here? The table should be level, but is it? Ask for other assumptions students might think of.
   • They can now calculate the speed of the ball.
4. How many trials, etc.
   • Discuss how many trials they should make for each speed (I usually suggest three)
   • How many different speeds do they want to record (the length of your tables and how many times your ball bounces before it just rolls across the table may help you decide how many is practical).
   • Ask them ‘launching’ it with a zero speed is possible (i.e. they should realize they can just drop the ball off the edge!)
5. Now I think the class is ready. A few other tips
   • You can use a plumb bob (any object hanging from a string tied a the launch point) if the edge of the table doesn’t have a straight wall down to the floor).
   • Data, data, and more data: Suggest that they measure the height of their launch position, as well as the mass of the ball. If you’ve taught energy, they can look at the height of the ball on the ramp (gravitational energy) and compare this to the speed (kinetic energy)–but caution that some of the energy is now stored as rotational energy of the ball–might be a good way to bring this out if they haven’t already studied rotational energy!
   • They can also time the ball’s projectile motion, although getting accurate times for this may be difficult…but worth a discussion of uncertainty in measurements.

Data analysis

1. Students can now plot speed and x-distance on a graph; they should find that the slope is the time the ball is in the air (delta x / speed -> m/(m/s) -> the time units go to the top and the distance units cancel). Time is a constant!

OK, that’s all I have now; I will come back on the weekend and add some images and maybe catch a few typos or grammatical problems.