Jun. 03, 2011

Science Dad on Levers (and Bikes)

by Vince Harriman

Click to enlarge images

Beckett on his old bike

Beckett got a new bike last week, a 'real' bike with gears and brakes. We've all been out several times already, including one near disaster in which Beckett crashed into me on a local bike trail. He was trying to learn how to use his gears, and I found it harder than I thought it would be to explain how they work. So, time for a science project to figure it all out! I decided to introduce him to a couple 'simple machines' to explain it all.

Traditionally, there are six 'simple machines': the lever, the incline, the wheel, the screw, the wedge, and the pulley. The primary function of each simple machine is to use mechanical advantage to change the way work or force is applied. The simplest machine, the incline, allows an object to be moved vertically at the same time it is moved laterally. It is often easier to drag or push something up an incline than to lift it straight up. A bicycle incorporates three of the simple machines into a single machine that lets us move -- and if you ride a bike up a hill, you are using four simple machines!

Beckett and I started by looking at how levers function. We found a nice straight wooden beam and Beckett dug a box of blocks out of his closet that we thought would be perfect for this project. We very carefully measured the beam and marked the center line. We then balanced the beam on a wooded triangle and placed two 'equal' blocks on either end and...they didn't balance. We tried several more, before we learned an important lesson about science: don't assume that two things are equal. Even though the blocks were identical in size and shape, they varied greatly in weight. Since we did our project on weight, mass, and density, we knew that the blocks must have different densities. So we looked around for some items that were more equal in weight and found a change cup full of quarters. We balanced six quarters on each side, then started experimenting. Beckett managed to balance two quarters against six quarters by changing the fulcrum point of the lever. A lever with the fulcrum in the middle, a weight on one end, and an opposing force on the other end is the simplest.

Different weights 'balancing' at different distances

Balancing items is a great way to see gravity acting on a lever, but since I wanted Beckett to apply the lever concept to a bike and bike gears, we then had Beckett act as the opposing force trying to lift the weight on one end of the lever. We set up a lever with two blocks on one end and the fulcrum roughly in the middle. It looked like this:

It wasn't too hard for Beckett to move the two blocks with one finger. Next, we moved the fulcrum to one quarter the distance from the end of the beam and tried again. It looked like this:

Beckett could still move the blocks easily with one finger. We cut the distance in half again and I had Beckett see what he could do. It looked like this:

Finally, we cut the distance in half again. I think the look on Beckett's face says it all and tells you how hard it is to lift to small blocks. (You can also probably see that Beckett lost two teeth last week!):

During our experiments with levers, I wanted Beckett to notice that when we shortened the distance between our force and the fulcrum, we increased our workload; when we did the opposite, that is, when we increased the distance between force and fulcrum we decreased our workload. On a bike, the first lever that allows us to decrease the work load is the crank arm. The crank arm translates the force of our legs into a rotational force that propels the bike forward. The longer the crank arm, the lower the work load.

Next we talked about wheels -- how rolling something is easier than dragging or lifting something. Remember our experiment on work? Beckett's bike has two wheels and allows him to move around quite quickly.

Finally, we set up an experiment to measure the work of the gears (pulleys, effectively) on his bike. We dug through the Lego bin and found a couple of wheel mechanisms that could be attached to an axle, then attached a length of ribbon to them with a crank. It looked like this:

With a crank arm fixed to the axle, I had Beckett turn the crank two full turns then measure the amount of ribbon pulled up. He then fixed a ribbon to the smaller pulley and again turned the crank two complete turns and measured the amount of ribbon. The ribbons looked like this:

'Seeing' the different amount of work performed by each pulley was a lot more meaningful than measuring the difference. The same amount of effort (two full crank turns) accomplished more work (pulled up more ribbon) with the larger pulley than with the smaller pulley. On a bike, the cogs translate the torsional force applied by the crank arms into the work that moves the bike. The smaller the cog, the harder the work.

Now there are a few more complicated things about bikes. Balance is one of them -- remember our experiment? There was also a great video pick of the week recently on Science Friday about bikes balancing that you can watch here.

I think I know what we will be doing this weekend! So get out your bike, put on a helmet, and find a good hill. Let us know how it goes!

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.
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