# Scientists Say: Force

A force is a push or pull on an object that changes its motion

#### Force (noun, “FORHS”)

A force is an interaction that can change an object’s motion. Forces can make objects speed up or slow down. They can also make objects change their direction. Such a change in motion is known as acceleration. When a force acts on an object, that force is equal to the object’s mass multiplied by its acceleration. You might have seen this written as F = ma. Because force = mass × acceleration, a bigger force causes a bigger change in an object’s motion. It also takes more force to change the motion of a more massive object.

All the pushes and pulls we experience in daily life arise from four fundamental forces. These forces affect all objects in the universe. The first is gravity. This attractive force holds Earth in orbit around the sun and pulls you toward the ground.

The second force is electromagnetism. That’s a combination of the electric force and the magnetic force. The electric force causes electrons to swarm around protons in the cores, or nuclei of atoms. Electric forces between the electrons of different atoms are at the root of many pushes and pulls we feel in daily life. The friction that drags your bike tires to a stop, for instance. Or the forces you and your bike seat exert on each other as you sit atop it. As for magnetic forces, one important example is the Earth’s magnetic field warding off harmful radiation from the sun.

The third force, called the strong force, holds protons and neutrons together inside atomic nuclei. The final force is known as the weak force. This force governs particle interactions that cause radioactive decay.

#### In a sentence

Forces overcome objects’ inertia to change their motion.