Robert Basile, age 10, of Newark, N.J., for his question:

How do they measure gravity?

When we listened to astronauts orbiting the earth, we heard the word gravity used as a unit of measurement. At one point, the astronaut traveled under the stress of five gravities, or 5 g. For most of the flight, he traveled at zero gravity, or 0 g. We know that gravity gives weight to an object. But the measurement of gravity from the speed of a space ship seems a strange idea.

In order to measure something, we must have a standard unit. The ton is a measuring unit for weight, the mile is a unit for measuring a unit of distance. The measuring unit for the force of gravity is taken from the amount of gravity built into our own planet. One gravity, written as 1 g, is the pulling power exerted at the surface of the earth.

This force gives weight to objects on the face of the earth. A planet with more mass, more material packed into a certain space, exerts stronger gravity. A person who weighs 100 pounds on earth would weigh 264 pounds on Jupiter. Since the earth's gravity is 1 g, Jupiter's gravity is 2.64 g. This unit of gravity gives us the weight of an object on the surface of other planets.

But gravity is a very complex force. It reaches out and exerts its pulling power above the surface of a planet. When you drop a stone, the force of the earth's gravity pulls it down to the ground. This is the action of the force of gravity on a falling body   and a falling object is a moving object. This aspect of gravity takes us into the realm of speed, a special kind of speed called acceleration.

When a space. ship is launched into orbit, it must fight against the earth's gravity which is trying to pull it down at a rate of 1 g. This is somewhat like trying to run up an escalator stairway which is moving down, The ship needs speed energy to escape. It must accelerate faster than 1 g. If it adds 64 feet to its speed every second, it is accelerating at 2g. If it adds 160 feet to its speed every second, the ship is accelerating at 5 g.

We can measure gravity from the weight of an object on the surface of a planet. We can also measure it from the special speeding up speed called acceleration. In both cases, we take our measuring unit of gravity from our home planet. We can use this unit to estimate the weight of objects on other worlds. We can also use it to figure the complex problems of launching space ships and orbiting satellites.