For an investigation, I have built a mousetrap car. The two variables in this investigation is the mass of the mousetrap car and the distance it travels. When there is no added mass on the cart, it does not travel far due to massive wheel slippage. As I increased the mass it carried, it travelled further up to a certain point (around 700grams). From that point onwards, adding extra mass decreased the distance it travelled. The resulting graph of mass v distance gave a parabola shape for the initial masses (a sharp increase with the initial masses) and then a linear decrease past the optimal mass. After doing some research I have found that there is an amount of usable traction before the wheels start to slip. This is given by the equation: usable traction = μ (coefficient of static friction) x Fn (Normal force). I have been unable to explain the shape of this graph. Theoretically, if the usable traction is a linear line (as there are no exponentials in the equation), then the initial distance increase as mass increase should be linear, but it is not. Is there some other variable I am missing? Why would the distance increase at such a large rate initially, but then decrease after the optimal mass?