# Relationship between velocity of wheel and car

1. Oct 5, 2012

### dan38

So I've designed a car which is powered by a falling mass.
Using Energy Questions, i.e. mgh = 0.5*I*w2
I've worked out the angular velocity of the wheel; so my question is whether this represents this velocity of the whole car or do I need to multiply this value by 4?

2. Oct 6, 2012

### tiny-tim

hi dan38!

(try using the X2 button just above the Reply box )
your RHS needs to be the total energy of the car, ie 4 times the rotational kinetic energy of each wheel, plus the ordinary translational kinetic energy of the car-plus-wheels

3. Oct 6, 2012

### dan38

ohh i see, so basically we have mass attached to a string connected to a rotating skewer with two wheels on either side.
so my workings would be:
mgh = 0.5*m*v2 + 4*0.5*I*w2

Where v = Speed of Car and m = Mass of Car
w = Rotational Speed of Wheels

And by figuring out the speed of the string, I can use the relationship v = w*r to find "w" of the wheel?
:D