Definition of Force in Physics

An Interaction That Causes a Change in an Object's Motion

Newton's cradle

Force is a quantitative description of an interaction that causes a change in an object's motion. An object may speed up, slow down, or change direction in response to a force. Put another way, force is any action that tends to maintain or alter the motion of a body or to distort it. Objects are pushed or pulled by forces acting on them.

Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Other forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.

Key Takeaways: Key Terms

  • Force: A description of an interaction that causes a change in an object's motion. It can also be represented by the symbol F.
  • The Newton: The unit of force within the International system of units (SI). It can also be represented by the symbol N.
  • Contact forces: Forces which take place when objects touch each other. Contact forces can be classified according to six types: tensional, spring, normal reaction, friction, air friction, and weight.
  • Noncontact forces: Forces that take place when two objects do not touch. These forces can be classified according to three types: gravitational, electrical, and magnetic.

Units of Force

Force is a vector; it has both direction and magnitude. The SI unit for force is the newton (N). One newton of force is equal to 1 kg * m/s2 (where the "*" symbol stands for "times").

Force is proportional to acceleration, which is defined as the rate of change of velocity. In calculus terms, force is the derivative of momentum with respect to time.

Contact vs. Noncontact Force

There are two types of forces in the universe: contact and noncontact. Contact forces, as the name implies, take place when objects touch each other, such as kicking a ball: One object (your foot) touches the other object (the ball). Noncontact forces are those where objects do not touch each other.

Contact forces can be classified according to six different types:

  • Tensional: such as a string being pulled tight
  • Spring: such as the force exerted when you compress two ends of a spring
  • Normal reaction: where one body provides a reaction to a force exerted upon it, such as a ball bouncing on a blacktop
  • Friction: the force exerted when an object moves across another, such as a ball rolling over a blacktop
  • Air friction: the friction that occurs when an object, such as a ball, moves through the air
  • Weight: where a body is pulled toward the center of the Earth due to gravity

Noncontact forces can be classified according to three types:

  • Gravitational: which is due to the gravitational attraction between two bodies
  • Electrical: which is due to the electrical charges present in two bodies
  • Magnetic: which occurs due to the magnetic properties of two bodies, such as the opposite poles of two magnets being attracted to each other

Force and Newton's Laws of Motion

The concept of force was originally defined by Sir Isaac Newton in his three laws of motion. He explained gravity as an attractive force between bodies that possessed mass. However, gravity within Einstein's general relativity doesn't require force.

Newton's First Law of Motion says that an object will continue to move at a constant velocity unless it is acted upon by an external force. Objects in motion remain in motion until a force acts on them. This is inertia. They won't speed up, slow down, or change direction until something acts on them. For example, if you slide a hockey puck, it will eventually stop because of friction on the ice.

Newton's Second Law of Motion says that force is directly proportional to acceleration (the rate of change of momentum) for a constant mass. Meanwhile, acceleration is inversely proportional to mass. For example, when you throw a ball thrown onto the ground, it exerts a downward force; the ground, in response, exerts an upward force causing the ball to bounce. This law is useful for measuring forces. If you know two of the factors, you can calculate the third. You also know that if an object is accelerating, there must be a force acting on it. 

Newton's Third Law of Motion relates to interactions between two objects. It says that for every action there is an equal and opposite reaction. When a force is applied to one object, it has the same effect on the object that produced the force but in the opposite direction. For example, if you jump off a small boat into the water, the force you use to jump forward into the water will also push the boat backward. The action and reaction forces happen at the same time.

Fundamental Forces

There are four fundamental forces that govern the interactions of physical systems. Scientists continue to pursue a unified theory of these forces:

1. Gravitation: the force that acts between masses. All particles experience the force of gravity. If you hold a ball up in the air, for example, the mass of the Earth allows the ball to fall due to the force of gravity. Or if a baby bird crawls out of its nest, the gravity from the Earth will pull it to the ground. While the graviton has been proposed as the particle mediating gravity, it has not yet been observed.

2. Electromagnetic: the force that acts between electrical charges. The mediating particle is the photon. For example, a loudspeaker uses the electromagnetic force to propagate the sound, and a bank's door locking system uses electromagnetic forces to help shut the vault doors tightly. Power circuits in medical instruments like magnetic resonance imaging use electromagnetic forces, as do the magnetic rapid transit systems in Japan and China—called "maglev" for magnetic levitation.

3. Strong nuclear: the force that holds the nucleus of the atom together, mediated by gluons acting on quarks, antiquarks, and the gluons themselves. (A gluon is a messenger particle that binds quarks within the protons and neutrons. Quarks are fundamental particles that combine to form protons and neutrons, while antiquarks are identical to quarks in mass but opposite in electric and magnetic properties.)

4. Weak nuclear: the force that is mediated by exchanging W and Z bosons and is seen in beta decay of neutrons in the nucleus. (A boson is a type of particle that obeys the rules of Bose-Einstein statistics.) At very high temperatures, the weak force and the electromagnetic force are indistinguishable.