The g-force of an object is its acceleration relative to free fall. On earth the force is 1g, or 9.8 meters per second squared (m/s2), but astronauts experience unusually high and low g-forces. g-force can also be seen on rollercoasters. When the coaster goes down the drop, you are pushed back into your seat because of g-force.
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G-Force Facts for Kids. Kiddle Encyclopedia.