# The Four Forces

## Four Forces Affect Things That Fly:

1. Weight is the force of gravity. It acts in a downward direction—toward the center of the Earth.
2. Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure.
3. Thrust is the force that propels a flying machine in the direction of motion. Engines produce thrust.
4. Drag is the force that acts opposite to the direction of motion. Drag is caused by friction and differences in air pressure.

## All Four Forces Act on an Airplane

When an airplane is flying straight and level at a constant speed, the lift it produces balances its weight, and the thrust it produces balances its drag. However, this balance of forces changes as the airplane rises and descends, as it speeds up and slows down, and as it turns.

## Only Two Forces Affect a Spacecraft in Space

A spacecraft has weight, even in orbit, and uses thrust to reach space and to maneuver. But lift and drag—both created by movement through air—are absent in the near vacuum of space.

Q:

## What is a force?

A:

A force is basically a push or a pull that causes an object to undergo a change in speed, a change in direction, or a change in shape. A force has both magnitude (size) and direction.

## Did You Know?

An airplane in flight is always in the middle of a tug-of-war with the four forces. For an airplane to takeoff, thrust must be greater than drag and lift must be greater than weight. To maintain level flight, lift must equal weight and thrust must equal drag. For landing, thrust must be less than drag, and lift must be less than weight.

## Pop Quiz

Which of these forces is a field force?

A) Lift
B) Weight
C) Drag
D) Thrust

Lift, Drag and Thrust are all “mechanical forces.”  A mechanical force is generated by the interaction between an object and a fluid (liquid or gas).  Weight is a “field force.”  A field force does not have to be in physical contact with the object to have an effect on the object.