# COMMUNITY : FORUMS : SCIENCE

## Mousetrap car Pulley help.

Hi! ive been working on a mousetrap car for my physics class and i was looking into the idea of pulleys to get more power to pull on the axle to rotate it.
Ive noticed that alot of the Pulleys I see all rotate on an axle which I can not acheive due to the small space on the car.

So my thought would be to anchor these small wooden pucks (small like 1 cm diameter) and have notches in them so that the fishing line wouldnt slip. (picture)

Can I get some help on wether or not this would work?
If it works can I get some input on how i could make it better? ex. different pulley designs that dont require moving parts.

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kelseymh7 years ago

A simple line of pulleys like that won't change the magnitude of the force, only its direction. The force here is determined by the tension of the rope (or fishing line, or dental floss, or whatever).

If you want a force multipler, you need to use a block and tackle (look it up on Wikipedia for pictures). Wind the rope between two pulleys (or sets of pulleys), and attach a separate line to the axle onto the pulley closest to it. You get one component of tension for each segment of line between the set of pulleys, so the net force pulling on the axle is multiplied.

Of course, nothing is free: work (energy) is force times distance. Since you're getting more force out the far end, you have to pull the rope on the outside N times farther than with the simple system above (where N is the number of rope segments in your tackle), to make the axle turn the same amount.

7 years ago
Plus, if those pucks are fixed, the extra friction will cut the distance the car will travel.
7 years ago
Though they'll definitely sap away power the big problem for most alway seems to be gearing it down so the wheels don't spin away the majority of the spring movement, so it might end in a better distance than before, though in an inefficient way. Personally I preferred the competitions run over the UK for tech and design GCSE students. All given identical sets of motors, gears, batteries and some construction materials. Clever gearing and not trying to jam every single component on to the car won us that.
noobiefied (author)  killerjackalope7 years ago
Im sorry, What do you mean by gearing?
7 years ago
In a mousetrap car pulleys would be the best way to gear it down or up. Say the trap pulls a string wrapped directly to the axle, that would be ungeared in this example.
noobiefied (author)  killerjackalope7 years ago
Hmm This is still slightly confusing but, gearing is good for the car right?
7 years ago
"Gearing" is being used as a general term for "changing the angular speed."

Let's use a bicycle as a simple example. The gear attached to the pedals has, in general, a different number of teeth (and a different diameter) than the gear attached to the back wheel. Consequently, one full rotation of the pedals does not correspond to one full rotation of the wheel.

Similarly in a car transmission, or inside a watch, or whatever. By connecting differently sized gears you can convert a slow rotation to a fast rotation, or vice versa.
7 years ago
Just adding to that, a big pulley is like more teeth on a gear cog than a smaller one.
noobiefied (author)  killerjackalope7 years ago
Hmm so you would want more "gearing" on a project like this right? so more transfer of a slow motion into a faster one?
7 years ago
You need to figure out the best way to turn that trap swinging down in to as much wheel turning as possible from a practical standpoint, the problem is that the trap comes down very fast and results in the wheels spinning, you need to engineer a way to balance between that and it being too hard for the trap to come down.
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