Once you have constructed your track, use the MASS slider to set the mass of the roller coaster car. The GRAVITY slider sets the acceleration due to gravity (it's like changing the planet on which you are building your roller coaster). These values, together with the height of your starting point, determine the initial potential energy of the roller coaster car. Use the INITIAL X VELOCITY slider to set the initial horizontal velocity of the car if you are beginning with a straight section of a track. If you start with a vertical section of track, your velocity is always zero. The velocity and the mass will set the initial kinetic energy of the car.