The force may be thought of as an influence which tends to change the motion of an object. Forces are inherently vector quantities, requiring vector addition to combine them.
The SI unit for force is the newton [N], which is defined by Newton = kg·m/s2 as may be seen from Newton’s second law. In mechanics, forces are seen as the causes of linear motion, whereas the causes of rotational motion are called torques. The action of forces in causing motion is described by Newton’s Laws under ordinary conditions, although there are notable exceptions.
Types of force
Conservative and non-conservative forces
A good way to think of conservative forces is to consider what happens on a round trip. A conservative force is a force with the property that the total work is done in moving a particle between two points is independent of the taken path.
If a particle travels in a closed loop, and the kinetic energy is the same after a round trip, the force is a conservative force, or at least is acting as a conservative force. Informally, a conservative force can be thought of as a force that conserves mechanical energy.
A conservative force is dependent only on the position of the object. If a force is conservative, it is possible to assign a numerical value for the potential at any point. When an object moves from one location to another, the force changes the potential energy of the object by an amount that does not depend on the path taken. If the force is not conservative, then defining a scalar potential is not possible, because taking different paths would lead to conflicting potential differences between the start and end points.
Consider gravity; you throw a ball straight up, and it leaves your hand with a certain amount of kinetic energy. At the top of its path, it has no kinetic energy, but it has a potential energy equal to the kinetic energy it had when it left your hand. When you catch it again it will have the same kinetic energy as it had when it left your hand. All along the path, the sum of the kinetic and potential energy is a constant, and the kinetic energy at the end, when the ball is back at its starting point, is the same as the kinetic energy at the start, so gravity is a conservative force.
Kinetic friction, on the other hand, is a non-conservative force, because it acts to reduce the mechanical energy in a system. Note that non-conservative forces do not always reduce the mechanical energy; a non-conservative force changes the mechanical energy, so a force that increases the total mechanical energy, like the force provided by a motor or engine, is also a non-conservative force.
For macroscopic systems, the non-conservative approximation is far easier to deal with than millions of degrees of freedom. Examples of non-conservative forces are friction and non-elastic material stress.
Other examples of conservative forces are: force in elastic spring, the electrostatic force between two electric charges, and magnetic force between two magnetic poles. The last two forces are called central forces as they act along the line joining the centers of two charged/magnetized bodies. Thus, all central forces are conservative forces.
Other examples of non-conservative forces are: frictional forces, viscous forces, induction forces, air resistance force, tension in a string, normal force, propulsion force of the rocket, propulsion force of the boat.
Forces that act on all objects
- Weight (W, Fg) – The force of gravity acting on an object due to its mass. An object’s weight is directed down, toward the center of the gravitating body; like the Earth or moon, for example.
Forces associated with solids
- Normal (N, Fn) – The force between two solids in contact that prevents them from occupying the same space. The normal force is directed perpendicular to the surface. A “normal” in mathematics is a line perpendicular to a planar curve or surface; thus the name “normal force”.
- Friction (f, Ff) – The force between solids in contact that resists their sliding across one another. Friction is directed opposite the direction of relative motion or the intended direction of motion of either of the surfaces.
- Tension (T, Ft) – The force exerted by an object being pulled upon from opposite ends like a string, rope, cable, chain, etc. Tension is directed along the axis of the object. (Although normally associated with solids, liquids and gases can also be said exert tension in some circumstances.)
- Elasticity (Fe, Fs) – The force exerted by an object under deformation (typically tension or compression) that will return to its original shape when released like a spring or rubber band. Elasticity, like tension, is directed along an axis (although there are exceptions to this rule).
Forces associated with fluids (include liquids and gases)
- Buoyancy (B, Fb) – The force exerted on an object immersed in a fluid. Buoyancy is usually directed up (although there are exceptions to this rule).
- Drag (R, D, Fd) – The force that resists the motion of an object through a fluid. Drag is directed opposite the direction of motion of the object relative to the fluid.
- Lift (L, Fℓ) – The force that a moving fluid exerts as it flows around an object; typically a wing or wing-like structure, but also golf balls and baseballs. Lift is generally directed perpendicular to the direction of fluid flow (although there are exceptions to this rule).
- Thrust (T, Ft) – The force that a fluid exerts when expelled by a propeller, turbine, rocket, squid, clam, etc. Thrust is directed opposite the direction the fluid is expelled.
Forces associated with physical phenomena
- Electrostatic Force (FE) – The attraction or repulsion between charged bodies. Experienced in everyday life through static cling and in school as the explanation behind much of elementary chemistry.
- Magnetic Force (FB) – The attraction or repulsion between charged bodies in motion. Experienced in everyday life through magnets and in school as the explanation behind why a compass needle points north.
The fundamental forces, also known as fundamental interactions, are a type of interaction between physical particles that do not appear to be reducible to more basic interactions. Four fundamental forces or fundamental interactions have been identified:
- gravitational interaction (gravity): the interaction between objects due to their mass. Weight is a synonym for the force of gravity;
- electromagnetic interaction (electromagnetism): the interaction between objects due to their charge. All the forces discussed above are electromagnetic in origin except weight;
- the weak nuclear interaction: the interaction between subatomic particles with “flavor” (an abstract quantity that has nothing to do with human taste). This force, which is many times weaker than the strong nuclear interaction, is involved in certain forms of radioactive decay;
- strong nuclear interaction: the interaction between subatomic particles with “color” (an abstract quantity that has nothing to do with human vision). This is the force that holds protons and neutrons together in the nucleus and holds quarks together in the protons and neutrons. It cannot be felt outside of the nucleus.
These are apparent forces that object experience in an accelerating coordinate system like an accelerating car, airplane, spaceship, elevator, or amusement park ride. Fictitious forces do not arise from an external object like genuine forces do, but rather as a consequence of trying to keep up with an accelerating environment.
- Centrifugal force – The force experienced by all objects in a rotating coordinate system that seems to pull them away from the center of rotation.
- Coriolis force – The force experienced by moving objects in a rotating coordinate system that seems to deflect them at right angles to their direction of motion.
- “G-Force” – Not really a force (or even a fictitious force) but rather an apparent gravity-like sensation experienced by objects in an accelerating coordinate system.