Force

orces are pushes or pulls that can cause objects to accelerate or change shape. Aircraft fly because they produce forces that overcome grav­ity and allow them to rise into the air. They move through the air because of the forces produced by their engines. Spacecraft, which are not traveling in air once they leave Earth’s atmosphere, use the force of gravity to move in a circle.

Basics of Force

In everyday language, acceleration means speeding up. To a scientist or engineer, however, acceleration can mean speeding up, slowing down, or changing direction.

If a force acts on an object that is free to move, it makes the object accelerate. The larger the force, the greater the acceleration. Acceleration also depends on mass. A small mass accelerates faster than a big mass pushed with the same force.

Every force has size and direction. Quantities such as these are called vec­tors. Forces acting in the same direction combine to produce an even larger force. When forces act in opposite direc­tions, they produce a force equal to the difference between them. If the forces acting on something exactly balance each other, there is no overall force, and the forces are said to be in equilibrium.

Some forces, such as friction, act when objects touch each other. These are contact forces. Other forces, such as gravity and magnetic forces, work at a distance. The objects that experience these forces need not touch each other. These are noncontact or distance forces.