Ages 8–12 · about 6 minutes

Could a Ship Become Lighter?

Could a craft make itself lighter and then move any way it likes?

Do this first · Try this at home

The same push, two masses

Start with your hands. The reading makes far more sense afterwards.

You need: Paper and a pencil

Tick each step as you do it

Draw two imaginary craft. Give them mass values 2 and 4, using invented mass units.
Give each a net force of 6 invented force units. Divide force by mass: 6 ÷ 2 = 3 and 6 ÷ 4 = 1.5.
Draw acceleration arrows in the force’s direction. Make the first arrow twice as long as the second.

0 of 3 steps done

What should I notice?

Notice: These are invented, compatible units for a calculation, not measurements of craft. With mass constant in each case, the same net force gives the smaller inertial mass twice the acceleration.

Now read on to find out why it happens.

The big idea

Weight is gravity’s pull; inertial mass determines how an object responds to a net force.

inertial mass: The measure of an object’s resistance to a change in velocity.
weight: The gravitational force pulling on an object.
net force: All the forces on an object combined, including their directions.
acceleration: A change in velocity: speeding up, slowing down or changing direction.

A rock taken from Earth to the Moon weighs less because the Moon’s gravity pulls less strongly. Its mass has not disappeared. It still takes a force to change how fast the rock moves or to turn it.

Mass has two roles. Inertial mass describes resistance to a change in velocity. Gravitational mass describes how an object responds to gravity. Measurements find these masses have matching values. Weight is the gravitational force itself, so it also depends on where the object is.

A laughing child on a beach holds a string tied to a huge grey boulder floating in the air like a balloon, with a leaping dolphin and gulls. — A floating boulder in an imagined scene. This painting is not a measurement or a demonstration of mass control.

The push and the response

For ordinary motion with constant mass, acceleration equals net force divided by inertial mass. Net force combines every push and pull, with their directions. Acceleration can mean speeding up, slowing down or turning. A turn changes velocity even if the speed stays the same.

Keep the net force fixed and halve the inertial mass: the calculated acceleration doubles. That is different from reducing weight. A weaker gravitational pull does not, by itself, make an object easier to turn with the same sideways force.

A proposed connection to the vacuum

In their 2002 paper, Puthoff, Little and Ibison discuss a model of inertia developed by Haisch, Rueda and Puthoff. In that model, interactions with fluctuating vacuum fields produce a reaction during acceleration. They do not produce a drag force on a body moving at constant velocity.

The authors ask whether changing those interactions could change effective inertial mass. Their paper explores that possibility; it does not describe a measured mass-reducing passenger craft. The idea suggests a specific test: compare an object’s response to a known force before and after changing its environment.

What would still need explaining?

A change in inertia alone does not explain how to remove air resistance, avoid forces when entering water or protect a passenger’s body during a sharp turn. Those involve interactions with fluids and forces within the craft. Weight and inertia are not a cushion around everything inside.

Momentum is mass times velocity in this ordinary-motion model. The paper keeps track of it: a craft must exchange momentum with something outside it. A useful test would measure force, acceleration and weight separately, and account for the energy supplied. Which quantity actually changed?