The concept of force is so fundamental to physics that it is hard to develop a definition. At the same time, it is abstract and invisible. Whereas with motion, the change of position of an object can be observed, force cannot. There are no direct ways to see force. Only the effects of force can be observed.
To further complicate matters, every effect of force can be connected to one of four fundamental forces. The complexities around this topic abound. At an introductory level, the following ideas are important to remember:
There are (as a general model) four fundamental forces.
The four fundamental forces are:
Strong (also called Strong Nuclear)
Weak (also called Weak Nuclear)
All observed effects of forces can be explained in relationship to one of the fundamental forces.
To illustrate the above, consider this: if you put a book or a laptop on your hand, you can feel (observe) the weight as it presses down. Weight is the force that creates the perception (pressing down) you feel. However, weight is not a fundamental force. It is (as are all forces) caused by one of the fundamental forces, in this case, gravitation force.
The key concepts above are the groundwork for a vast amount of additional information. However, at an introductory level, the few points above are sufficient to move into additional explorations.
Returning to the most general level, force can be defined as a push or pull that acts on an object.
The effect of a force is to create the potential for an object (even an object as small as an electron or sub-atomic particle or even a photon of light) to change direction or magnitude or motion. This leads to a more complicated definition that can be explored in detail here: https://en.wikipedia.org/wiki/Force.
In physics, a force is any interaction that, when unopposed, will change the motion of an object. In other words, a force can cause an object with mass to change its velocity, i.e., to accelerate.
Because the force of gravity is everywhere, it becomes a very convenient source for examples. Looking at the wikipedia definition, there are a few key points to think about.
The laptop on your desk is exposed to gravity. Therefore, gravity creates the potential for it to move downward. If unopposed, the laptop will accelerate downward. The desk on which it sits opposes that force for reasons related to the forces that hold the molecules together. If the desk did not oppose the force of gravity, the laptop would move.
A better example would be a pencil. Pushing a pencil (with the force of a finger) until the desk was no longer under it would cause the pencil to fall (that is to say accelerate downward).
It is relatively easy to observe the effects of forces. Leaves move when the wind blows because of the force of the moving air. A spring (such as in a grocery store scale) stretches as the result of the gravity pulling on the things being weighed.
As easy as it is to observe, measuring force becomes abstract. The units on force come from the definition. Force is the potential to move something, thus it is related to acceleration, so the unit for acceleration gets mixed into the unit for force. Also, the object getting moved has mass, so that gets mixed in.
The end is a very messy unit, so just giving it a name is convenient. Force is commonly described using the unit of the newton.
The newton can be thought of with regard to force as the meter is thought of for displacement or distance. However, the newton can also be understood in connection to its definition.
A newton is the force required to accelerate (unopposed) a 1 kg object at a rate of 1 (m/s)/s.
The abbreviation for the newton is N.
1 N = 1 kg•(m/s)/s or 1 N = 1 kg•m/s2
There are occasions when, in handling units, it is necessary to substitute kg•(m/s)/s for N, so attending closely to the above is a good idea.
Forces Are Vectors
As with acceleration and velocity, forces are vectors. Thus, a quick exploration of vectors is useful.
A vector has both a magnitude and a direction.
Whereas speed has only a magnitude (such as child running around in a circle at a rate of 2 m/s), velocity has both magnitude and direction. Velocity can describe a car traveling in a particular direction at a particular rate.
Forces, likewise must be understood to have a direction. The force of gravity is, for instance, down. The force of opposition created by the desk, would then be in the opposite direction—up. The force of the finger on the pencil would be perpendicular to the force of gravity (assuming the desk was level). The force of friction that opposes the pencil sliding would be in the opposite direction of the push on the pencil.
So, a few key points:
A vector has both a magnitude and a direction.
Magnitude of a vector can be thought of as the size or power.
Direction of a vector is… well, it is the direction of its effect. (There is your "duh" moment for the day!)
Forces are vectors, thus they must be understood to have both a magnitude and a direction.
Forces Add Up (as do all vectors)
Another important aspect of forces as that they add up. For instance, if two forces pull in the opposite direction and have the same magnitude, then they force cancels out. Forces pulling in opposite directions can be thought to have opposite signs, mathematically. As a result, if they have the same magnitude, they cancel out.
Returning to the example of the laptop or pencil sitting motionless on the desk. Because the force of gravity (down) is equal to the force of opposition caused by the molecules in the desk (up), the laptop does not move. Neither does the pencil.
When all of the forces on an object are added up, the result is called the net force. As a definition, then, net force is the sum (both magnitude and direction) of all the forces acting on an object. When the net force is zero, the object is said to be in balance or in equilibrium.
Consider a balloon in the air… Gravity pulls it down, but a person can be under it and blow up. The force of the moving air upward can exceed the force of gravity downward, and thus cause the balloon to move upward. This is because the net force on the balloon is not zero.
An object will respond to the net force which is the result of adding up all the forces acting upon it.
When the net force is zero (when all the forces cancel out) the object does not accelerate.
When there is no acceleration, then the velocity of the object does not change, so if it is in motion, it stays in motion. If it is at rest, it stays at rest.
The principles of force are directly related to Newton's Laws of Motion. These are important foundations for the study of force and motion. There are many ways to describe them. Consider this:
First Law: An object will not change velocity unless acted upon by a force. (It will neither start move, stop moving, nor change magnitude or direction of motion.)
Second Law: If an object accelerates (changes magnitude or direction of its velocity, including starting to move or stopping), the force required is equal to the mass of the object times the rate of acceleration. Thus:
F = ma
where F is net force, m is mass of the object, and a is the rate of acceleration.
Third Law: When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction. (Thus the cliche, "Every action has an equal and opposite reaction.")
Understanding forces is a major part of physics and engineering. From designing comfortable furniture to building spaceships, knowing how to work with forces is fundamental.
There are many types of forces in play in every situation. Learning how to work with forces becomes vital to understanding how the world works.