Click here to purchase the entire book in PDF format. The kilogram Pick up a block of lead - you'll notice that it's heavy. (if you don't have a block of lead lying around, print this book and pick it up instead... It's also heavy.) Take the same block of lead to the moon and lift it up - you'll notice that it's lighter than it was back home. Now take the same block of lead to Jupiter and it will be so heavy, you can't lift it at all... So, in the presence of different gravitational forces, the same block of lead will weigh different amounts. However, in all cases the same block of lead will have the same mass. Let's try another experiment to illustrate the difference. Take the same block of lead and put it on perfectly smooth ice that is a perfectly horizontal surface - let's make it perfect ice that has absolutely no friction. Now push the block hard enough so that, in 1 second, it reaches 1 m/s, skidding along the ice. While you do this, pay attention to how hard you had to push. Now take the same block to the moon, place it on a similar ice surface and push it so that reaches 1 m/s in 1 second. Do the same thing again on Jupiter. In all three cases, you will push equally hard. So, on the moon, the block weighs less than it does on Earth, where it weighs less than on Jupiter. However, if you're just trying to move it on a frictionless surface, you have to push equally hard in all three places to get the same acceleration. This is because the block's mass, which determines its inertia (or its ``desire'' to keep doing what it's doing - be it staying still or moving in a given direction) is the same regardless of gravitational pull, so the block will be equally easy to move sideways in all three places. However, the block's mass and the gravitational field it's in determine its weight, and therefore how hard it is to lift.