BHS -> Staff -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page
Demonstrate the difference between mass and weight.
Heavy wrench (or other massive object), string, sturdy support
Note: You may have to use some trial and error to find the right strength of string for your mass...
Pulling quickly breaks the bottom string because the inertia (mass) of the wrench causes it to resist sudden accelerations.
Pulling slowly breaks the top string because it must support both the weight of the wrench and to added force.