 Now that you've calculated the mass, we'll find the mass on this scale and compare values. It's more than 500, I'll put that on 500 grams, and this one is on 40, and that's just about right. So we're going to call the mass of the truck 0.540 kilograms. Now you shouldn't expect to get exactly that value because there are some sources of error in this experiment. The strings may not have been exactly perpendicular and parallel to the plane. That's probably the biggest source of error. Another one is that the pulleys, even though that they move pretty freely, they do have some friction. And that friction actually helps out the weights hanging over the end so they don't have to be as much in order to support the truck. Now if you were off by more than about say 20 grams between the value on the mass and the value you calculated, then you may have made a mistake in your calculation, so go back and take another look at those.