Hi kimnamhyun,
Welcome to the forum -- it's great that you have an interesting idea for a project that comes from your own experience!
You will probably find our science project guide helpful, particularly (right now) the parts dealing with the question, variables, and hypothesis:
https://www.sciencebuddies.org/science- ... ndex.shtml
https://www.sciencebuddies.org/science- ... tion.shtml
https://www.sciencebuddies.org/science- ... test.shtml
https://www.sciencebuddies.org/science- ... bles.shtml
https://www.sciencebuddies.org/science- ... esis.shtml
A lot of questions take the form "The effect of [independent variable] on [dependent variable]." Right now, your question could probably be phrased something like "The effect of load weight on the motion of a wagon pulled by a bike." It would be good to use a more concrete variable than "motion", though -- and this is where your initial observations will come in handy. How did the wagon circle the bike differently? And how might you describe this as precisely as possible, ideally with a number? If you think a diagram would help, it's possible to attach files such as images to a post.
If you give us a description of how the motion changed with weight (and what path you were taking with your bike -- circle or straight line, accelerating or constant speed, etc), we can probably point you toward some principles of physics that you can read and think about to better understand your observation. Then you can turn your question into a hypothesis -- a testable prediction of the relationship between the independent and dependent variables. After this, you will want to plan and repeat your experiment. When you read about constant variables, think about whether you need to do anything differently. You might also have an idea for testing something slightly different to learn something beyond what you've already seen. A "dry run" like you've already performed is very common in experiments -- it helps us to work out any problems and design the experiment to reveal the most interesting results -- so you have taken an excellent strategy so far.
A quick note about safety: you might have noticed that when you stop, the wagon doesn't stop immediately (unless you have brakes attached to it somehow). Especially since you're going downhill and experimenting with different weights, try increasing things like speed and weight slowly to make sure that the wagon won't run into you and damage your bicycle, the wagon, or especially
you.
I bet that when you display your project, a lot of people will be interested: bicycle commuting is becoming more attractive with the current economic situation (I just learned to ride one last summer!), and especially if people completely get rid of their cars, they occasionally need to tow things behind their bikes. It might be interesting for you to look at carts, etc, designed to attach to bikes, and see if their designs make sense in light of your observations. For example:
http://www.xtracycle.com/
Amanda