Archive for the ‘Physical Science’ Category
I did this engineering challenge last year, but it didn’t go the way I wanted it to. I provided the kids with a stick, about 25″ long as the basis of a reach device. My mistake was allowing them to split the stick in half. As a result, they all made a big scissor and then made some small modification to make it grip.
This year, I have added a whole new level to the challenge. The stick can not be cut, only drilled for brass fasteners. They are given a budget and each item has a cost. I made sure they can only purchase a single stick with their budget. Purchasing two of them will put them immediately over budget and cost them lots of points.
The challenge came from pbs.org/designsquad, they have a few things there worth looking at if you are into engineering challenges. I downloaded the information a while ago. Here is the link for the activity: http://pbskids.org/designsquad/parentseducators/resources/helping_hand.html
Here is how I modified the activity for use in my classroom: Helping Hand Challenge. If you use this, let me know what you add or change. I will be using this tomorrow. The scoring rubric is at the bottom.
Update: I’m still angry over how some of the kids approached this. A couple of the really lazy ones did nothing but complain for two days. At the last second, they purchased a foot of tape, wrapped it around the stick and then tried picking up the objects with the sticky tape. Naturally, I changed the rules so that can’t happen next year. Tape got very expensive. I also changed the grading to downplay the points earned for moving the object and increasing points for the design. Essentially, I changed the grading to reflect effort and creativity. With the change in focus, I will need to watch for students copying ideas.
My father just sent me this TED Talk. He doesn’t read my blog and didn’t know about the other TED Talks I posted. This one is a little different, Ramesh Raskar from MIT has developed a camera that can slow motion down to the point of being able to see a pulse of light travel. You just have to see it to believe it.
And in case you aren’t seeing the embedded video:
I never loved my pulley lab. I was never pleased with the learning, the kids seem to focus on trying to set up the pulleys and not on what is happening.
Two days ago I handed them this revised instruction sheet: Pulley Lab Rev D – Discovery Lab. The instructions are simple, “Your job is to come up with a set of rules that explains what is happening with the pulleys, ropes, and weights.”
Besides the ring stand, support, pulleys, weights, and string, I gave each group a Vernier and force meter. They set to work trying to figure out what is going on.
After they finished with the first setup, I asked them to tell me what happened. The weight was the same, so what was the purpose? Finally one of them said it changed the lifting up to pulling down. So a single pulley can be used to change directions. I gave them a few applications, like pulling something up into a tree or the mast of a ship.
Next they started on the other pictures. They noticed the force changed. I mentioned that there is a cost to the reduced force, what is the cost? Their response was less work. No, work is conserved. Keep going.
When they got through picture 3, I explained that they were experiencing Mechanical Advantage. If you hang by one arm, you hold all of your weight. Add the second arm and you are splitting the weight. Add a third arm… you get the picture.
As they got through the rest of the diagrams, two challenges remained: 1) what is the cost, and 2) figure out how to rig this up to get a mechanical advantage of 5. I gave them a hint – picture 2. They worked for a while without success. End of the first day.
I came into class and was pleasantly surprised to see all of the groups were already set up and working. They were twisting the ropes all over the place. I gave them 15 minutes to play. They still didn’t understand the cost, so I drew the solution for the MA of 5. We put a bunch of weights on the pulleys, almost 10 lbs, and I had them all take a turn lifting. I wanted them to experience the mechanical advantage. Then we measured how far the weights moved and how much string was pulled to make this happen.
That was all it took, they got it. In a perfect system, work is conserved. This led to a discussion of efficiency and how a lever also provides a mechanical advantage. It was a good day.
Note: Here is the solution to the challenge at the end of the lab: MA 5 Solution
When we did the egg drop challenge a couple of weeks ago, I asked the students to write about their design and the concepts involved in safely landing the egg in their structure.
For them, they had fun and were rewarded for their hard work with no lab report, just a dialog of what they built, why they built it, and the concepts we’ve been studying. I wanted them to talk about forces, gravity, momentum, impulse, collisions, and any other concept we’ve studied in order to explain the physics behind the effort to save the egg.
I’m thinking the egg got off easy. I had to read phrases like “depending upon how fast you dropped the egg,” and “the impact of momentum, ” and best (worst) or all, “the egg has many things to be concerned about it not to break”
Other than labs, I haven’t given a writing assignment before and I now think it needs to be a regular event. Clearly the students can not talk about the concepts. Although we spend weeks problem solving, discussing, and working in the lab, they can’t put the concept into an intelligent sentence. How did this happen? I feel like I’ve failed.
I like saving money for my school. Nothing against Carolina Biological, we’ve been really pleased with their equipment and service, but sometimes their stuff is just way overpriced. Their Hooke’s Law Device is $35 each. I made a similar set of devices for a couple of dollars using what we already had in the lab and classroom.
My device is made with a ring stand and small rubber bands to hold a ruler in place. I used a pendulum clamp as the top support, but any clamp will do. To connect everything, I used a bit of chain that comes with a shop fluorescent lamp and opened up the links. These are in an S shape and twisted to be offset by 90 degrees. You could just as easily purchased a package of S hooks at Home Depot for a couple of dollars. At the top, I hang one side on the clamp and hook the spring over the other opening. Same on the bottom, the hook provides a place to hang a weight.
We use hanging weight sets, I just didn’t have them hand for the picture, this is one of my 500g medicine bottle weights. The springs come from Harbor Freight. They have a box set of 200 springs for about $5. Lots of springs to play with, many look alike but have very different spring constants.
My “indicators” were fabricated on the 3D printer, but you can just as easily make the same thing with two Popsicle sticks and some glue. Drill the holes or it will split.
That’s really the whole thing. I found that if I spun the whole setup one rotation CCW, the spring would try to rotate clockwise, holding the indicator against the ruler and making it much easier to read.
I’m planning on adding a scatter chart to this lab. The students will enter the points in Excel to create the chart and then plot a trendline. We can then use the first order trendline to determine an unknown weight based on the distance the spring stretched. This setup was inexpensive and effective.
A couple of weeks ago we did a lab straight from the textbook. (Here is the Lab Instructions, typed up and put it into my words.) I’d been looking for a good Conservation of Energy Lab. I wanted to use the Vernier devices, but there wasn’t anything in their book that I liked. Rather than make something up from scratch, I decided to work directly from the Holt Physics textbook.
The lab had two parts; the first was to calculate the spring force constant using a Hooke’s Law device. I didn’t have that device, so I created my own by first designing a simple indicator on Solidworks (3D CAD Software) and then printing eight of them in the 3D printer. That worked out great. (I will try to remember to post my Hooke’s device design, you can build them for about $0.25 each.) Unfortunately, it was the only part of the lab that worked out at all.
We were able to use a ring stand, ruler, and indicator to successfully calculate the spring force constant. The second part of the lab was supposed to demonstrate conservation of mechanical energy by bouncing the weight and measuring the high and low point. Quite honestly, it just didn’t make sense. At first it did, but the more I thought about it, the less sense it made.
To begin with, it was nearly impossible to measure the bottom and top of the bounce with any accuracy. It was pure guesswork and the kids were really struggling.
This is a great bunch of kids, I warned them ahead of time that this was the first time using this lab, there might be some hiccups. They were understanding and really tried to make this work, but they were totally frustrated. I told them I would grade the lab on their effort, spring constant results, qualitative analysis, and attempt at explaining the results.
Clearly, I need a much better lab for next year. I was originally thinking of calculating the spring force constant, then determining the weight of an unknown object based on distance the spring stretches. That makes sense for the chapter on oscillations, but not for Conservation of Mechanical Energy. I don’t have an air track, but I’m really good at McGuyvering solutions, as you all well know.
Please throw your awesome labs my way, I need help.
Teachers – how are you handling significant figures? I’m a bit at odds with my textbook and I’m wondering what the rest of the world is doing. I’ve discussed my issues with our chemistry teacher, he tends to agree with me, but it’s just the two of us. Let me explain.
I teach from Holt Physics. The book treats sig figs mostly okay. When they provide numbers for problems, they are always precise. Usually the numbers are in scientific notation, so you know where you stand with your given information. The book does state that 1500 could be 2 or 3 or 4 significant figures because we don’t know about the two trailing zeroes. I tell the students to err towards caution in those cases and treat that number as though there are 4 sig figs. The book correctly states that the answers are rounded to the least significant number of figures. You all know what I mean.
Here is where we part ways. I teach my students to carry an extra place while doing calculations. For example, if I’m dividing 35 by 62, my working answer is 0.565. If this is my answer to the problem, I would round this to 0.57. If I’m using this number in another calculation, I would use all three digits. The textbook rounds this here along the way even when it is used later. I’ve even seen problems where they have rounded more than once in the same problem. (There is no way I can remember the actual problem right now.) The results are often an error of about 10% difference between my answer key and my calculations.
While we are at this, I have a question I’m stuck on. Suppose you read a meter stick and you get a reading of 8.65 cm. That is three significant figures. Now you move a little ways up the ruler and read 22.40 cm. The accuracy of the ruler hasn’t changed, but I’m now working with 4 significant figure versus 3 before. I wouldn’t round the second number, it is as accurate as the device, but the first number isn’t 8.600. How do you account for this when you are dealing with the significant figures of a problem?