Hello all, I'm using a Lego Mindstorms NXT kit for a physics project of mine. The project is relatively simple. I'm basically proving F=m/a. About the project: I'm using various weighted pingpong balls to roll down a track of a known, fixed distance. d is known, t will be measured, and F will be calculated. I'm using the various mindstorms components to 'kick' the ball and measure the time it takes for the ball to pass from one light sensor to another. The problem: I was able to program the motor to rotate an arm 360* (providing the kicking motion) pretty easily. However, **I need to program the timer to start as soon as the ball passes one light sensor and stop when it passes the other,recording the time it takes** I can not, for the life of me, figure this out. I know the light sensors can essentially function as motion sensors using reflected light from the built-in LEDs. If it helps, the pingpong balls are white (though they can be painted if necessary). I was wondering if someone could please, please help me out with a program that can preform this function.

the car seems to be fine it has some mods done to it like a turbo xs Front Mount Intercooler Kit (WS-FMIC), a down pipe but I cant see what it is. Seems to have an exhaust but with a stock muffler not too loud. Now the bad It has a lot of gages but they dont work. A double din but it doesnt work either. Now i drove it but its on a small parking lot and cant really go too fast only used first and second. Now when accelerating hard on first and it gets to around 5000 rpm it jerks and its like it restricts the power like if it has a traction control but it doesnt. it also smells like fuel like if theres a fuel leak and the engine light is on. That seems to be the only bad things ive seen on the car. Now im no mechanic thats why im asking for advise if it would be too expensive to fix these problems or too mayor. I would be modifying this car more if i get it anyways so I will be putting more money into it since i will be using it for autocross and track days. any advice would be appreciated and recomendations like i said im no mechanic but i think it has some small problems but for the price and compared to KBB it seems really good let me know thank you