Kinematics describes how bodies move; dynamics asks why. The answer lies in forces: whenever a body accelerates, changes direction, or deforms, there is a force responsible. Before studying how forces produce motion, however, it is worth putting them in order. All the forces we shall meet in mechanics can be grouped into two large families, according to the way they act: those that require physical contact between bodies, and those that act even at a distance, through empty space.
Principle — Action-at-a-distance forces and contact forces
Action-at-a-distance forces — act without the bodies touching:
- Weight force (gravitational)
- Electrostatic force
- Magnetic force
Contact forces — require physical contact between the bodies:
- Constraint reaction (normal force)
- Tension in a rope
- Elastic force (spring)
- Friction force
- Archimedes’ thrust (buoyancy)
Action-at-a-distance forces are the ones that seem most mysterious to us: the Earth attracts the Moon, one magnet repels another, two electric charges push each other apart, with nothing visible connecting them. They are mediated by a field (gravitational, electric, magnetic) that fills space and transmits the action from one body to another. Contact forces, on the other hand, come into play only when two surfaces touch, a rope is taut, or a spring is compressed: they are the forces of everyday experience, the ones we feel when we push, pull, or rest an object on something.
The map below brings together the two families and anticipates a further distinction, which we will return to later: some of these forces store energy and others do not.
Classification of forces. The forces enclosed within the dashed gold outline (weight, elastic, electrostatic) admit an associated potential energy.
Note
At a fundamental level, all contact forces are electromagnetic in nature: when you push a table, it is the electrons in your fingers repelling the electrons in the table. The distinction between “contact” and “at a distance” is therefore macroscopic and practical, not fundamental. The only forces that are truly at a distance are gravity and the electromagnetic forces.
Links
Topics: Dynamics Concepts: Weight force · Normal force · Tension · Elastic force and Hooke’s law · Static friction
Related exercises: Worked exercise — two boxes with a spring and a pulley · Problem — Spring with two hanging masses · Spring on an inclined plane