The Pantry Lift: Planning Tests of Net Force and Mass

Kitchen Notes: Teaching the Pantry Lift to Start Gently

1Kaveh Rostami lives with his grandmother in an old house where the kitchen sits one floor above the basement pantry. Long ago, an owner built a small lift between the two rooms: a wooden box that hangs from a cord running up over a pulley. For years, Grandma Dolores cranked jars of tomato sauce, cans of soup and bags of rice up by hand. This fall, a family friend who is an electrician mounted a small winch motor at the top so nobody has to crank. On the first trial, the box jerked upward so quickly that a stack of cans toppled over inside it.

2Kaveh, an eighth grader, decided to plan an investigation before anyone used the motor again. Grandma Dolores asked for just one thing: the lift must never tip over her jars again. His question: how do the net force on the box and its total mass affect how quickly its speed changes?

3The empty box has a mass of 2 kg. Kaveh added cans of soup, each with a mass of 0.5 kg checked on the kitchen scale, to change the total mass. A force sensor clipped into the cord reads the upward pull in newtons (N), and the motor can hold that pull steady. Earth pulls down on the box with about 10 N for each kilogram of total mass. The pulley turns so easily that Kaveh treated its friction as zero, so the net force on the box is the upward pull minus Earth's downward pull. A motion sensor on the pantry floor measured the box's speed 1.0 s after it started from rest.

4For safety, every run started with the box resting on a folded towel just above the pantry floor, and nobody stood under the box. Kaveh ran three trials for each setting and averaged the speeds.

5Kaveh's first plan for the mass test was to set the motor to a 22 N pull for every load. After his first calculation, he changed the plan and set a different pull for each load (runs 3, 4 and 5). In run 7 he set the pull to 80 N, and the 8 kg box stayed where it was without rising.