Why Won't My Mousetrap Car Move?

[theOkilla]

Hey guys! I'm new to Physics Forum and I have a question. I am building a mousetrap car for my 8th grade science class and have run into a problem. I have built a pretty sophisticated pulley system using a cassette tape. The lever arm and string turn the cassette tape which turns the drive axle. I am using two records as my back wheels, and I fear that they are too heavy. Whenever I arm the car and set it down, nothing happens. It does spin very quickly if I give it a little spin.

I will post some pictures of what it looks like.

 

[berkeman]

Welcome to the PF. What are your bearing surfaces for the axles/wheels? Wood-on-wood? Or something that gives lower friction.

You need to find where the friction problems are, and figure out how to minimize friction at those bearing surfaces...

 

[theOkilla]

I have bearings like on skateboard wheels for that and my axles are wood. I can get the wheels to spin very quickly while holding it, but when I set it down it won't spin.

Should I try and grease up the bearings?

Thanks

 

[berkeman]

I have bearings like on skateboard wheels for that and my axles are wood. I can get the wheels to spin very quickly while holding it, but when I set it down it won't spin.

Should I try and grease up the bearings?

Thanks

Do the wheels spin easily when you load each on individually when spinning it with your finger? If individual loading is okay, look to see what is diffferent when you set the car down. Like, do the wood axles bend under load, making them bind a bit on the bearings?

Or, if you just sit the car on the ground and rock it gently back and forth with your finger, does it move easily? If it does, the problem may be in your string/pulley system. What are you using for bearings there? How does it feel to your fingers as you apply load to the string?

 

[SteamKing]

Using CDs for the drive wheels may not be the best choice to convert the torque input to the driven axle into a "driving force". What I'm trying to say is that there only a very small contact area between the edge of the CD and the surface on which the car rests. In addition, the edge of the CD is very smooth. In a real car, wide tires give better traction and the material of the tire is formulated to have a high coefficient of friction. Road surfaces are also designed so that sufficient friction is available so that the rotation of the tire is converted into thrust to drive the car. In a car where the coefficient of friction is very low, application of torque to the driving wheel causes the tire to spin without producing any driving thrust. This is why driving on wet or icy roads is potentially dangerous: the vehicle wants to slide and the driver loses control.

 

[theOkilla]

The wheels don't spin that easily when I use my finger. My parents keep telling me that I should take the records off and use CD's on all 4 wheels. What do you think?

And thanks for your help so far.

 

[theOkilla]

Using CDs for the drive wheels may not be the best choice to convert the torque input to the driven axle into a "driving force". What I'm trying to say is that there only a very small contact area between the edge of the CD and the surface on which the car rests. In addition, the edge of the CD is very smooth. In a real car, wide tires give better traction and the material of the tire is formulated to have a high coefficient of friction. Road surfaces are also designed so that sufficient friction is available so that the rotation of the tire is converted into thrust to drive the car. In a car where the coefficient of friction is very low, application of torque to the driving wheel causes the tire to spin without producing any driving thrust. This is why driving on wet or icy roads is potentially dangerous: the vehicle wants to slide and the driver loses control.

Oh great idea. Thanks! I'm going to try and put some rubber or something around the wheels for some extra traction. I will start by putting rubber bands around the CDs

 

[SteamKing]

From the photos posted I couldn't clearly see it, but it appears you are using vinyl records for the driven wheels. What I said in my previous post would also apply to the vinyl records. The larger diameter vinyl records could give you a higher top speed, but they do take a lot of energy to turn. If speed is not important for your demonstration, then substituting CDs for the vinyl at the driven wheels would reduce the amount of energy it takes to get your car rolling with the mousetrap mechanism.