A success for the Newton's Second Law experiment

 

In this experiment I take a 900 gram dynamics trolley and have it pulled by weights on a string through two light gates so that the acceleration can be measured. The first time I ever did this, I made the mistake of just adding weights to the hanger on the end of the string using weights from a stack on the table. That did make the pulling force go up but it also increased the total mass being accelerated, because these hanging masses are accelerated just the same as the trolley. So now I use a total of 2000 grams. I start with 1000 grams on top of the 900 gram trolley, with 100 grams hanging from the string. I then transfer across 100 grams at a time so that the pulling force increases by 1N every time but the mass stays constant.

The force is actually supposed to be the resultant force. This would need to include friction, meaning that the actual force wouldn't be the 1N, 2N etc on the string but would be less than that. You get rid of friction not by eliminating it but by compensating for it. You do this my propping the book up ever so slightly so that a trolley without string would roll down at constant speed.

The reward was a decent straight line of best fit through the origin. It doesn't happen very often!