A soccer ball hangs at the top of its flight for an instant. Does it experience a force at that instant?
Mission planners talk about 'the gravity at' a place: at Mars's surface, at a point along an orbit. They name no particular spacecraft. How can a strength of gravity belong to a place?
You have seen that a field fills the space around a mass, and that any mass inside it feels a force. What is missing is a way to state the field's strength at a point.
The field's strength at a point is read from the force on each kilogram placed there.
At Mars's surface, the field exerts a force of 3.7 newtons (N) on each kilogram.
The gravitational force per unit of mass at a point is called the gravitational field strength, g. g is measured in newtons per kilogram, written N/kg.
So 'g = 3.7 N/kg at Mars's surface' says: the field there exerts a force of 3.7 N on each kilogram.
In general, field strength at a point is the force per unit of the property the field acts on. For the gravitational field, that property is mass. That is why the unit is newtons per kilogram.
At Venus's surface, g is 8.9 N/kg. The field there pulls…
You met the symbol g earlier in the course, in the motion topic, as the free-fall acceleration, 9.8 m/s². This g is the field strength in newtons per kilogram: the same 9.8, because N/kg and m/s² are the same unit read two ways.
Why the free-fall reading and the field reading had to agree comes later in this topic.
On Titan, the field exerts a force of 1.4 N on each kilogram. What is the gravitational field strength on Titan?
Write down the fact:
g is the force on each kilogram at the point.
On Titan, that force is 1.4 N.
g = 1.4 N/kg on Titan.
A point along a transfer orbit has g = 0.2 N/kg. What does that say about the field there?
Write down the fact:
g states the force on each kilogram.
Here g is 0.2 N/kg.
The field at that point exerts a force of 0.2 N on each kilogram.
Type the name of this quantity: the gravitational force per unit of mass at a point.
A chart lists 6.2 N/kg at a point above Saturn's clouds. What does the entry state?
Earlier in the course, g = 9.8 m/s² was the free-fall acceleration. In this topic, g = 9.8 N/kg is the field strength. How are the two related?
A 1 kg camera and a 5 kg battery sit side by side on Mars. Which statement about g at their spot is correct?