# Homework Help: Gravity Car

1. Aug 28, 2009

### hazman900

For my physics EEI i have contructed a gravity car that works by a mass connected by a string and run through pully's to the drive axel. When i drop the mass it pulls the string, spins the wheels and the car propels forward.
The car itself weighs approximately 400g
The wheels radius on both front and back is 2.25cm
The Drive wheels radius is 0.2cm
The drop mass falls about 16cm and lands back on the car.

I am just a little confused what to investigate in my EEI. I would like to be able to predict it's distance for any given mass on a constant surface and also calculate the co-efficent of friction for the surface using the car. I have plotted displacment/time graphs to find average velocity of the car but thats about it. I'm just not sure where to go from here.

I am also trying to investigate torque and angular momentum but i'm finding it difficult.

I'm just looking for advice with where to go, and what to do with it.

2. Aug 29, 2009

### RTW69

Go online and research mouse trap cars. They are similar but get their energy from a spring on the mousetrap rather than a falling weight. There is already quit a bit of analysis already done on these cars and can help with torque questions etc. The potential energy released in the falling weight will be translated into the kinetic energy that moves the car. There will be losses not just to rolling friction but to friction in the axles and pulley. The energy in the falling weight minus the energy lost to friction = energy that moves the car.

3. Sep 1, 2009

### hazman900

Thanks that was really helpfull...
Could i also possibly use F= ma for the weight falling minus F = ma of the car to find force lost to friction?

4. Sep 1, 2009

### hazman900

As this would then help me find the co-efficent of friction