Understanding Force, Newton's Laws of Motion

Force is a fundamental concept in physics that describes an 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
Force is a vector quantity, meaning it has both magnitude and direction
It is measured in the SI unit of newtons (N)
Forces can be categorized into contact forces and non-contact forces (field forces)

Isaac Newton formulated the laws of motion that describe the relationship between force and motion
Newton's first law states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a force
This law introduces the concept of inertia
Newton's second law states that the force acting on an object is equal to the mass of that object multiplied by its acceleration (F = ma)
This law provides a mathematical relationship between force, mass, and acceleration
Newton's third law states that for every action, there is an equal and opposite reaction
When one object exerts a force on another object, the second object exerts an equal and opposite force on the first object

Contact forces involve direct physical contact between objects
Examples of contact forces include: Applied force, Frictional force, Tension force, Normal force, Air resistance force, Spring force
Non-contact forces (field forces) act through space without direct physical contact
Examples of non-contact forces include: Gravitational force, Electric force, Magnetic force

Gravitational force is the attractive force between objects with mass
The gravitational force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them
This is described by Newton's law of universal gravitation: F = G(m1m2)/r^2, where G is the gravitational constant, m1 and m2 are the masses of the objects, and r is the distance between their centers
On Earth, the gravitational force acting on an object is often referred to as its weight

Electromagnetic force is the force between electrically charged objects
It includes both electric and magnetic forces
Electric force is caused by the interaction of electric charges
Like charges repel each other, and opposite charges attract each other
Magnetic force is caused by the movement of electric charges
Magnets have north and south poles, and like poles repel each other, while opposite poles attract

Strong nuclear force is the force that holds protons and neutrons together in the nucleus of an atom
It is the strongest of the four fundamental forces
It operates over very short distances

Applied force is a force that is applied to an object by a person or another object
If a person is pushing a box across the floor, they are applying a force to the box

Frictional force is the force that opposes motion when two surfaces are in contact
There are two types of friction: static friction and kinetic friction
Static friction prevents an object from starting to move
Kinetic friction opposes the motion of an object that is already moving

Tension force is the force transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends
For example, if a weight is hung from a rope, the tension in the rope is equal to the weight of the object

Normal force is the force exerted by a surface on an object in contact with it
It is always perpendicular to the surface
If an object is resting on a table, the normal force is equal to the weight of the object

Air resistance force is the force that opposes the motion of an object through the air
It depends on the speed, shape, and size of the object
As an object falls through the air, the air resistance force increases until it equals the weight of the object, at which point the object reaches its terminal velocity

Spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it
The spring force is proportional to the amount of compression or extension of the spring
This is described by Hooke's law: F = -kx, where k is the spring constant and x is the displacement of the spring from its equilibrium position

Net force is the vector sum of all the forces acting on an object
If the net force on an object is zero, the object is said to be in equilibrium
If the net force on an object is not zero, the object will accelerate in the direction of the net force