Finding the Normal Force on an Incline

When we discuss normal forces, I drum into the students’ heads that the force is perpendicular to the surface.  They get that eventually.  Where they get into trouble is on an incline when the angled component of the weight of an object F(i)=mg*cos(Θ).  I called this force F(i) meaning the force exerted on the incline (and the (i) is really a subscript, but I can’t make that work in this blog).

This is sometimes the normal force, but not always.  If there are any other forces, like F(x), in the angled Y-axis, then the normal force is not the same as F(i).

Here is what I tell them:  imagine there is a scale under the object in question, what would it read?  If it is only the object and no other forces, then F(n) is F(i).  From the drawing above, it’s fairly clear that there are two forces down that combine to create our normal force up.  The scale under the block would read the value of F(i) + F(x), so that is our F(n).

Building Model Rockets

I started building model rockets with my students this year and I’m glad I did. Most of my students have never built or launched rockets before. A few did in eighth grade, I think maybe two or three did with their parents, but out of the 100 or so seniors that I teach, that’s was it. About 80% of my students are college bound and only a couple of them are going into science or engineering, so connecting a subject like physics to something hands-on is critical to their understanding of the material. Not that I think most of them understand it, but let me delude myself please.

The school purchased one rocket for every two students. I know in some area schools, the students are required to purchase the materials. I know that most of mine could, but quite a few can’t. So the school paid for them. It took about three days to build and paint the rockets. They build, I paint. I knew I had to when one of my more trusted students came in with a rocket dripping paint. “Several light coats are better than one heavy coat.” Didn’t matter how many times I said that, apparently it didn’t stick.

The next nice day we all trudged out to the field, took lots of pictures posing with our rockets, then we launched them one at a time. I tried to explain how high and how fast they go, but until they saw it they just didn’t get it. A few dramatic failures are good. We had one tail fin fall off because it wasn’t glued on well. The rocket looped just barely over our heads. A few had the nose cone too tight.  The ejector charge couldn’t pop off the nose cone, they come down fast and tend to stick in the mud. We even had one actually explode. I’ve never had that happen, I think it was an engine failure and not the work of the student. All these events add to the teachability of the lesson.  We learn from our failures.

As a follow up homework assignment, they each had to write an article telling about the project, the launch, and explaining the theory to someone who hasn’t had physics. I chose two of the articles and a couple of photos and submitted it all to the school newspaper for publication.

Some thoughts:

• Each group got a single A engine with the rocket. If they wanted to launch again, they had to purchase an engine for $2. I had some B and C engines, but our field isn’t very big and we lost anything launched with C’s.
• I wanted the kids to purchase the rockets for $2, but only a few did. I would either get them to purchase them up front with their own money or just give it to them. The teams would have to decide ahead of time which of the two gets to keep the rocket.
• I bought a mix of Viking and Wizard rockets. Both are good, they use streamers for recovery rather than parachutes. A parachute in a 10 mph wind will drift twice as far as it is high. So if it goes up 500ft, it will drift 1000ft.
• Walmart is the cheapest place to find engines. A three-pack is under $5.

Hoverpuck – Must Have

Have you seen one of these? It’s a small, battery powered hovercar in the shape of a large hockey puck. It’s only about six or eight inches across. When I’m teaching Newton’s 1st law, I take these out (I have three of them) and I launch them around the room. I let the kids play, kicking it around. Usually we take it into the hallway and see if we can make it all the way to the office from my room. After I get them back in their seat and on task, we review the concept of a inertia and the need for a force to change the puck’s directions.

I haven’t done this yet, but I’d like to show some video of a hovercraft and how difficult it is to make them turn.  Actually, I’d love to build one someday.   Something to keep in mind for a future lesson.

NOTE: The hoverpuck I purchased from the supply company uses a rechargeable battery.  I don’t know how well it’s going to work next year.  I just saw them at a local store called “$5 and Below” and it uses regular AA batteries.  I think over the long haul, that is a better idea.  I don’t expect the rechargeables to be working in a year or two.

The Apple and the Banana

These were new to me, I saw Lou Bloomberg do them at an Educator’s Day up at Penn State. This is for the teacher to do since it involves a sharp knife.

Place an apple on the edge of the table and use a long knife and a quick flick of the wrist to slice through the middle of the apple. The apple should stay in place because of its inertia.

The second part of the demo was even better. Lou held the knife in the air and threw the banana sideways at it. The banana traveled in an arc cut itself in two on the blade and kept on the same trajectory. Although expected, it was odd to see both halves continued to travel next to each other.

NOTE: Lou must have practiced this a million times. It sounds easy and he made it look easy, but damn, it ain’t easy. You might want to buy some bananas ahead of time and practice throwing a banana over your head.

Pulling out the Tablecloth

I hope everyone that teaches inertia does this demonstration. I let the kids do it also. I have a couple dos and don’ts for making this a success.

• Pull the tablecloth out with a quick, single motion.  If you pull slowly, friction will start the dishes moving towards you.
• Choke up on the tablecloth. You want to have your full arms length to pull it. If the cloth is slack, you might only pull the tablecloth half way out.
• Friction is your enemy. A smooth, silky tablecloth works better than a rough, burlap bag.
• Pull quickly and pull slightly down. If you pull upward, you are likely to launch the dishes into the air.
• Make sure the tablecloth doesn’t have a hem. The edge of the plate and cup will snag on these and go flying.
• I tried this the first time with a dollar store tablecloth. The plate didn’t move, but the cloth ripped in half. Just a heads up.
• I’ve never had a failure at this demonstration.  The closest I’ve come is when one of the kids does the trick and starts to pull up.  I usually stand close by when it’s their turn.

I always tell the kids to make sure you go home and try this with mom’s good china.

Matchstick Rockets

This is a favorite of the students. After studying Newton’s Laws of motion, we spend a day making rockets from paper match sticks. The process is simple, tear out a paper match, cover the top with a small piece of tightly wrapped aluminum foil, heat the match with a lighter and watch it launch.

Students will typically make at least three of these in a class period. Most go no more than an inch or two, many doing that backwards. Last year one match went 26 feet. This year, I believe the record was 18 feet. Credit goes to Richard White for sending me this last year.

The directions call for making the exhaust tube with a paperclip. That hasn’t worked well for us, the hole is too big. I’ve seen other directions that use a pin. I haven’t tried it with a pin, but I’d like to. In class, we simply wrap the tip tight and count on the gases finding their own exit.

matchrockets

Note: This is an outside activity unless you have a really well ventilated lab, which I don’t. Also, spend $5 on a BernzOmatic lighter. You can get it from Walmart. Aim for a day with very little wind otherwise you will have a hard time keeping the rockets on their launcher and even harder time getting them lit.