Overview of Force
by Ron Kurtus (revised 30 September 2014)
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.
The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object
Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in fluids.
In simple terms, a force is a push, a pull, or a drag on an object. There are three main types of force:
An applied force is an interaction of one object on another that causes the second object to change its velocity.
A resistive force passively resists motion and works in a direction opposite to that motion.
An inertial force resists a change in velocity. It is equal to and in an opposite direction of the other two forces.
There is no such thing as a unidirectional force or a force that acts on only one object. There must always be two objects involved, acting on each other. One object acts on the other, while the second resists the action of the first.
Questions you may have include:
- What are examples of applied forces?
- What are resistive forces?
- How is force affected by mass?
This lesson will answer those questions. Useful tool: Units Conversion
An applied force is an interaction that causes the second object to change its velocity.
The force required to overcome the inertia of an object is according to the equation:
F = ma
- F is the force
- m is the mass of the object
- a is the acceleration caused by the force
Types of applied force
There are several types of applied force:
The most common form of force is a push through physical contact. For example, you can push on a door to open it. An object can also collide with another object, exerting a force and causing the second object to accelerate. This is another type of push and can be called an impulse force, since the time interval is very short.
You can pull on an object to change its velocity. Gravitation, magnetism, and static electricity are some of the pulling forces that act at a distance with no physical contact required to move objects.
Finally, if two objects or materials are in contact, one can drag the other along by friction or other means.
A resistive force passively inhibits or resists the motion of an object. It is a form of push-back. It is considered passive, since it only responds to actions on the object. Friction and fluid resistance are the major resistive forces.
When an object is being pushed along the surface of another object or material, the resistive force of friction pushes back on the first object to resist its motion.
Fluid resistance pushes back on the moving object, which is basically trying to plow through the fluid. It also included friction on the surface of the object.
Air resistance and water resistance are common forms of fluid resistance.
An inertial force works against a change in velocity, caused by an applied force, as well as a resistive force.
Against applied force
According to Newton's Third Law of Motion or the Action-Reaction Law:
Whenever one body exerts force upon a second body, the second body exerts an equal and opposite force upon the first body.
This is often stated as: "For every action there is an equal and opposite reaction."
When you push on an object, an equal inertial force pushes back. This is the resistance to acceleration.
Likewise, when swinging an object on a rope around you in a circle, you pull on the rope to change the direction of motion. In turn, you can feel a pull on the rope.
Against resistive force
When a resistive force like friction, slows down the motion of an object, the inertial force will push in the opposite direction and tend to keep the object moving.
The main type of force is an applied force, which is an interaction of one object on another that causes the second object to change its velocity. Other types of forces include a resistive force that passively resists motion and an inertial force that resists a change in velocity.
There must always be two objects involved in a force, acting on each other.
Become a positive force in your community.
Resources and references
Forces - Physics Hyperbook
Force - Wikipedia
Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) $25.25 - Understanding gravitational, electrical and magnetic force
The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines
Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)
Questions and comments
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Characteristics of Force