Crates are stacked onto one of two parked trailers, the gap unchanged. The pull between the trailers…
Astronomers keep finding 'super-Earths': planets with several times Earth's mass squeezed into roughly Earth's size. What would standing on one feel like?
This lesson predicts a planet's surface field strength when only its mass differs.
Surface g is the force the planet's field exerts on each kilogram sitting at its surface.
That force grows with the planet's mass: twice the mass, twice the force on the same kilogram at the same distance.
Hold the radius equal, and the distance from the center is the same in both cases. Only the mass is left to act.
A planet with twice Earth's mass and Earth's radius has twice Earth's surface g. A planet with three times Earth's mass and Earth's radius exerts three times the force on each surface kilogram.
Planet X has three times Earth's mass and the same radius. Calculate its surface g.
Write down the values in the question:
Earth's surface g = 9.8 N/kg
mass factor = 3, radius equal
Write down the rule:
with radius held equal, surface g grows in proportion to the mass
Substitute in the values:
g = 3 × (9.8)
Simplify:
g = 29.4 N/kg
Planet Y has twice Earth's mass and Earth's radius. How does its surface g compare with Earth's?
Esme says a planet with half Earth's mass and Earth's radius has Earth's surface g. Is she correct?
The radius condition matters. If the radius differs too, the inverse-square distance effect changes g as well. This lesson's rule covers the mass effect alone: mass up, surface g up, in direct proportion.
Lena says a planet with four times Earth's mass and Earth's radius has four times Earth's surface g. Is she correct?
Planet Z has twice Earth's mass and Earth's radius. Earth's surface g is 9.8 N/kg. Enter Planet Z's surface g, in N/kg.
Planet W has Earth's radius, and its surface g is 4.9 N/kg. How does its mass compare with Earth's?
Which statement about a planet's surface g is correct?