EngArc - L - Force

Force

Quick

A force is a push or pull. It is a vector quantity. English units are pound-force, lb_f. Metric units are newton, N. Force is equal to mass multiplied by acceleration.

Details

A force is that which causes a change in the motion of a body that is free to move. Gravity is a force. The concept of force allows for a quantitative description of the interaction between two bodies or between a body and its environment. One that pushes on a car that is stuck in the snow, is exerting a force on the car. A locomotive exerts a force on the train cars it is pulling or pushing. A steel cable exerts a force on the beam it hoists at a construction site.

Contact forces include pushes or pulls exerted by a hand, the force of a rope pulling on a block to which it is tied, or the friction force that the ground exerts on a ball player sliding into home. There are also long-range forces, for example such as what are experienced between a pair of magnets. Gravity, too, is a long-range force — the sun exerts a gravitational pull on the earth, even over a distance of 150 million kilometers, that keeps the earth in orbit. The force of gravitational attraction that the earth exerts on your body is called your weight.

Force is a vector quantity; a body can be pushed or pulled in different directions. Thus to describe a force, the direction in which it acts as well as its magnitude need to be described. In this case, the magnitude describes "how much" or "how hard" the force pushes or pulls. The SI unit of the magnitude of force is the newton.

When two forces

₁ and

₂ act at the same time at point A of the body shown, the effect on the body's motion is the same as the effect of a single force

equal to the vector sum of the original forces:

₁ +

₂. More generally, the effect of any number of forces applied at a point on a body is the same as the effect of a single force equal to the vector sum of the forces. This important principle goes by the name superposition of forces.

The fact that forces may combine according to vector addition allows a force to be replaced by its component vectors, as with displacements. For example, in the following figure, force

acts on a body at point O.

Fig1.

The component vectors of

in the directions O_x and O_y are

_x and

_y. When

_x and

_y are applied simultaneously, as in the next figure, the effect is exactly the same as the effect of the original force

Fig2.

Any force can be replaced by its component vectors, acting at the same point.

It is frequently more convenient to describe a force

in terms of its x- and y-components F_x and F_y rather than by its component vectors. For the case of the above two pictures, both F_x and F_y are positive; for other orientations of the force

, either F_x or F_y can be negative or zero.

There is no law that says our coordinate axes have to be vertical and horizontal.

Typically, a force is represented as either a push or a pull. A push is associated with compression and a push is associated with tension.

For a static body, the sum of all forces is equal to zero.

Sometimes a force is referred to as the load. Or when loading is mentioned, it is typically referring to force.

A force has both magnitude and direction; hence, it is a vector quantity. The magnitude is a scalar quantity that defines the strength of the force. The direction is represented by an arrow pointing in the direction that the force is acting.

There are three ways in which a load can be applied to a machine member. These are static, impact, and variable.

Fig3. Static load.	Fig4. Impact load.	Fig5. Variable load.
Static loads are steady, constant or are applied slowly. The load does not change very much in value. Examples include the weight of a fluid in a storage tank and the dead weight of a structure upon it supports.	Impact loads are applied suddenly, usually at high velocity. There is, frequently, actual impact (a blow) on the machine member by another machine member or some external body. Impact loads are common to such machines as pile drivers and punch presses, to name a few.	Variable loads are applied in various ways, but in each case the value of force is variable. In some cases the load is constantly varying, as in the connecting rods in an engine. An extreve condition is typified by a rotating shaft which experiences a complete reversal of load on each cycle. If fibers along the top of a shaft are stressed in compression, those along the bottom are stressed in tension. At any point on the shaft, each revolution produces a change from tension to compression.

Over an extended period of time a member can withstand much less stress under severe variable load conditions. As a measure of the maximum unit stress that a material can withstand indefinitely under variable loading, its endurance limit is often established by testing. For this reason, some forms of variable loads are commonly referred to as fatigue loads.