What Causes Motion? The Meaning of Force

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4.2What Causes Motion? The Meaning of Force


In these revision notes for What Causes Motion? The Meaning of Force, we cover the following key points:

  • What is/are the factor/s affecting the motion?
  • What effects can a force cause on the object it acts?
  • What kind of quantity is force?
  • What is the unit of force?
  • How can we classify the forces?
  • How can we express a force in components?
  • How to find the resultant force?

What Causes Motion? The Meaning of Force Revision Notes

In Physics, the factors that cause an object move or change its motion are known as forces. By definition, "a force is any interaction that, when unopposed, will change the motion of an object." In simpler words, a force can be described as a push or a pull.

Basically, a force can cause one of the following effects on objects:

  • Changing the velocity of an object. This includes starting a motion, stopping a motion, speeding up or slowing down a moving object.
  • Changing the moving direction of an object. This includes deviating, rotating, swinging.
  • Changing the shape of an object. This includes squeezing, bending, stretching, compressing, twisting, breaking an object in pieces or welding a broken object.

However, a force cannot change the mass of an object. Mass is a physical quantity related to the amount of matter contained in an object, so a force can neither create nor destroy the matter.

Force is a vector quantity. This means it involves the direction. We know the information regarding a vector quantity in physics must contain four elements: direction, magnitude, unit and application point, in order to be considered as complete. If one of them is not given, this creates serious problems in studying any process in which the force in question is involved.

The unit of force is Newton (in short N). Newton is a derived SI quantity because when splitting it into fundamental SI units, we obtain

1[N] = 1[(kg × m)/s2 ]

Forces are divided in two main categories regarding the way on how they act in objects. They are:

  • Contact forces. These kinds of forces cause their effect on an object only when they touch it.
  • Field (distant) forces. These kinds of forces are produced by objects, which are able to extend their effect in the space around them (known as "fields") and as a result, all the other objects which enter inside the range of these fields, are affected by their pushing or pulling effect. Therefore, the force is produced indirectly through the field, not through the object itself. Gravitational force is such an example (it is produced by the gravitational field).

When two or more forces act on the same object, an overall effect is produced. This effect is known as resultant or net force. Symbolically, we write FR or Fnet to express this overall effect that is nothing more but the sum of all forces acting on the same object.

The resultant of two forces acting in the opposite direction represents their numerical difference (this derives from the fact that subtraction is the opposite operation of addition).

When two or more forces acting at the same object are neither in the same direction nor in the opposite, it is better to express them in components. Then, after calculating the resultant force in each direction, the Pythagorean Theorem is used to calculate the magnitude of the resultant force.

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