## Wednesday, June 16, 2010

### Where Mah Physics Peeps At

I usually talk about vector forces by pushing desks around. You know like 2 people push in the same direction, 2 people push in opposite directions, then they push on adjacent sides and the desk moves diagonally. But then we just make up forces and do practice problems.

Would it work to get 3 bathroom scales to measure the two component forces and the resultant force at the same time? Is there an easier way to do it than trying to balance a scale against a corner of a desk?

I would ask the Physics teachers at my school but I'd have to walk all the way down two hallways and I am very, very lazy.

RichTCS said...

If you had a little bit of budget, or several students/parents who love to fish, you might use three fishing scales to accomplish the same thing (and I'd think, it might be a little easier to set up than using three bathroom scales). The main difference would be that the fishing scales work in tension, vs. the bathroom scales which work in compression. But otherwise, it would allow you to do the same sort of experiment.

kalamitykat.com said...

Yeah, use the bathroom scales. If you want to get fancy and measure pull forces, go buy a few spring scales in the fishing section of your local big-box store.

Kate Nowak said...

OK! Thanks. Phew. I had just convinced myself this was the dumbest idea ever. Good idea with the tension scales.

Ricochet said...

Kate, I am going to put this in my keep file - let us know if the fish scales work.

Mat said...

My physics teacher in HS did an entire lab with us involving fishing scales, weights, metal keyrings (just the round part), and protractors.

Mr. Condescending said...

I don't know if this falls into the same category, but I know something that might be fun to get your students to figure out...

See if they can figure out how a sailboat moves forward when the wind is to it's sides or blowing toward it.

Frank Noschese said...

OPTION 1:
Paper river activity:
* long roll of WIDE paper (wrapping, etc.)
* battery powered car that moves at constant speed
* stopwatches, rulers, protractors

Have the car go downstream, upstream, and cross stream. Verify vector addition of velocities in each case. Then predict the upstream angle needed to get the car directly across the river.

OPTION 2:
You can also do something like this with spring scales (the physics class will have them):

Look at the picture on page 3. (I skip the washer and just tie 3 pieces of string together.) I have gotten very good results this way. It is helpful to put a piece of polar graph paper (ticked every one degree) under the center of the knot/washer. That way students can ensure their strings are at a fixed angle. Remember, the 3 force vectors here add to zero. You can also show that, in the zero sum case, the sum of any two forces will be equal and opposite to the third. If two of the forces are at right angles, then they are equal and opposite to the components of the third. Good luck!

Avery said...

Something to consider if you decide to use spring scales...

Once in a while I like doing labs that require rulers and not giving kids rulers (or any other measuring tool). Instead, they are forced to create their own units and write all their results in terms of this unit. If you want, you can always have them convert at the end.

I've never done this, but I think it would be interesting to also do this with the spring scales. If students were given the spring scales with the Newtons/pounds hidden, I'm curious if and how they would unitize their measurements. For example, I would assume that some would space their own units linearly which could be a great conversation starter about Hooke's Law (or a great way to test this concept).

I don't know...I'm really thinking out loud here. Am I making any sense? Your thoughts?

benblumsmith said...

There is an awesome math/science teacher in Vermont named Sarah Bertucci who uses like these circular things that have little pulleys you can slide all around the rim of the circular things so you can get the three vectors pulling from any direction, and you can attach weights to the pulleys to control how much is being pulled. I don't know what they're called or where she got them, but I do have her email address, so if you want to know more write me an email and I'll put you in touch. She's rad.

rallain said...

As mentioned, the bathroom scales might not be a good idea. Not because it is not a good idea, but because your vectors are not going to add up right. The problem with pushing a desk with three scales is that there is another force that you are not measuring - friction.

Here are some of my vector posts - maybe this will be useful:
http://scienceblogs.com/dotphysics/vector/

Also, I like this homemade spring scale:
http://scienceblogs.com/dotphysics/2010/02/diy_force_probe.php

Curmudgeon said...

BenBlumsmith:

Those are called force tables and they are, in my mind, one of the best ways to do this. Or you can go all cheap: a couple pulleys and c-clamps per desk, a ring for the intersection of the forces, heavy duty string and a collection of weights. The benefit is that you can set up the forces consistently and have a stable equilibrium state. The spring weights are good for a demonstration but it's tough to get the angles right and keep the forces even.