A Potato Trap (Gravity)

There are 4 main forces in nature.  Here they are in order of their strength:
(1)  Strong Nuclear Force
(2)  Electro-Magnetic Force
(3)  Weak Nuclear Force
(4)  Gravity

Gravity is not a force we think much about as we go through our day, but we always feel the effects of it.  When we pick up a pencil we feel it, and when we throw a ball we see its effect.  The Earth's gravity exerts its force equally on all physical bodies.  So gravity must have something to do with weight.

Gravity  -  
the attraction of the mass of the Earth, the Moon, or a planet,
for bodies at or near its surface.

Potato Trap

In this experiment we learned how to build a structure that will hold a plumb line.  We tied a "plumb bob" at the end of a string.  A plumb bob always points downward, precisely toward the center of the Earth.  The purpose of the plumb line is to create a vertical line following the direction of the gravitational force in order to determine the exact spot where the potato would land.   In this spot, we placed our wooden stake (match stick).


Use two potatoes of different sizes (one about 3 cm thick and the other 5 cm thick.  Hold the smaller potato above the "stake" and drop it.  The rounder the potato, the better your results will be.  If your potato isn't very round, hold it with the fat end down to minimize its rotation during the fall.

Place a mark on the matchstick to record the depth of penetration.

Now drop the larger potato and determine the stake's depth of penetration.

Is there a difference?


The Earth's gravitational force acts more weakly on objects that are "light" and more strongly on objects that are "heavy".    Thus, the strength with which the Earth attracts a body depends on its mass. 

Mass is not the same as weight.    Weight is the force with which a body is attracted toward the earth or another celestial body, whereas mass is a measure of the property of a body  -  a measure of the amount of material it contains.   So, we may weigh a lot less on the Moon than on Earth because the Moon's gravity is much weaker than that of the Earth.  However our mass remains the same, whether we are on the Moon or on the Earth.

Note:  Even though it's easy to build a contraption similar to this one from things around the house and do the experiment, for some of us busy homeschool moms it's just easier when it all comes in a kit.  The kids received this kit at Christmas time and it's been a life saver, making science fun and hands-on!

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