One of the most important concepts in Physics, force is anything that causes an object motion or change of its velocity. Force can be described in simple language as push or pull upon an object which results to its interaction with another object.
Force is divided into two major broad categories: contact force and action-at-a-distance forces.
Types of Contact Forces
As its name suggests, it is the force at the point of contact or interaction of two objects. It is further divided into six types of contact forces.
1. Friction Force
It is a kind of force that most, if not all, of us are already familiar with. It is the force exerted at the surface of an object as another is pressed or rubbed against it. The physical contact of the two objects results to intermolecular activities on their surface, and that is the friction force.
The amount of force that will be exerted depends on the nature of the surfaces; the rougher the object, the greater the force.
Most of the time, this force acts against the motion of the object. For example, when you are pushing a box across the floor forward, the force of friction on the box is toward your direction.
2. Tension Force
It is the force transmitted through a rope, string, cable, or wire when it is pulled by two objects on its opposite ends. For better understanding, imagine you are pulling a box with the use of a rope.
The box is tied on one end of the rope and you’re pulling the rope through the other. You are exerting force on the rope, then the rope pulls the box, the force applied on the box by the rope is the tension force.
You are able to cause motion to the box without direct contact with it, the force went through another medium which in this case is the rope.
This also applies to stringed musical instruments such as guitar. When the strings of a guitar are being tuned, the tension on the strings is also adjusted at the same time. This tension force is related to the sound that the strings produce.
3. Normal Force
It is also called the ‘support force’ and perhaps the most common physical force in our everyday lives. The normal force acts perpendicular to the body that applies the force whenever two objects are in direct contact with each other.
On horizontal surface, the magnitude of the normal force is always upward and equal to the weight of the object. For example, a person who weighs 60N is standing on the ground; the ground in return, exerts a 60N force upward to maintain equilibrium.
However, the force becomes complex in inclined planes where the gravitational force is not perpendicular to the plane.
4. Air Resistance Force
It is a special type of frictional force which acts opposite the direction of the body moving through the air. It is a friction force from the air that acts upon the body moving through it. This force is usually weak that it is often negligible and does not affect the speed of the object.
It becomes higher in objects with faster motion and greater mass or surface area. When you are walking at a normal pace, you experience air resistance force, but it does not significantly affect you. Once you start running, the force becomes greater and becomes more noticeable.
The concept of parachute used by skydivers is the air resistance force. Without parachute, the skydiver will fall from the sky very rapidly. The parachute increases the diver’s surface area and weight, and it slows down their speed of falling.
5. Applied Force
It is the force applied by a person or an object to another object; it could be either push, pull, or drag. The change in velocity of the object would depend on the direction and the magnitude of the force applied onto it.
Some common example of situations where force is applied through pushing are pushing the door to open, throwing a ball, or pushing a ball on a flat surface causing in to roll. An applied force is also present when you are pulling a heavy box across the room using a rope.
The dragging effect happens when two objects are in direct contact and the force applied on one also causes motion to the other object.
6. Spring Force
This is the last on this list of contact forces. This is the force exerted by a compressed or a stretched spring to the object attached to it.
According to Hooke’s Law, the magnitude of the force is directly proportional to the amount of stretch or compress of the string. The spring exerts the same amount of force that is applied into it.
A simple example is when you compress a spring using your hands; the resistance you feel is caused by the spring force.
Types of Action-at-a-Distance Forces
These types of forces are the ones which resulted from two objects which are interacting with each other even without direct physical contact.
Even though objects have large distance between them, they can still cause push or pull to each other, and this force is the action-at-a-distance force. Like the contact force, this type of force also has sub-types.
1. Gravitational Force
This force is present in every massive heavenly bodies, it is a force that attracts other objects towards them. Gravitation maintains the planets in their paths around the sun.
It is one of the four fundamental forces of nature, even though this is the weakest of the four, it has a great contribution for shaping the organization of galaxies in the universe.
On Earth, the weight of an object is always equal to the force of gravity applied onto it. On other heavenly bodies, the mass of an object is directly proportional to the gravitational force it experiences.
Everything on Earth experiences gravity that is pulling them towards the center of the Earth. On the moon, the gravity is much weaker (6x less than that on the Earth), if a person weighs 60 kg on Earth, he/she will be only 10 kg on moon. That is why astronauts need heavy suits to keep them from floating.
The gravitational force of the moon is also the one responsible for tides on Earth. During full moon, the water level is always higher because the moon is pulling it towards it.
2. Electrical Forces
Electrical force causes attraction or repulsion of two charged objects. If the objects are both either positively or negatively charged, then the two objects will move away from each other.
On the other hand, when the charges are opposite, the two objects attract. Electric forces is equal to the net electric charge produced
One example, a common activity done in schools, is when you place a charged plastic material (sheet, tube, etc) over tiny bits of paper. You charge the plastic by rubbing it, the heat caused by the friction force will cause it to be negatively charged.
The paper is a positively charged material, so when you put a charged plastic over it, the two materials will be attracted to each other.
Around the electric force, there are electric fields. Electric fields points outward from positively charges, while toward a negatively charged particle.
3. Magnetic Forces
Rotating electric charges generate magnetic fields. The fields have two magnetic fields, north and south. Like poles repel and opposite poles attract.
The force in a magnetic field depends on the magnitude and velocity of the electrically charged particle, and the strength of the magnetic field.
The earth is a huge magnet itself in essence. The molten metallic core has circulating electric currents within it and that is where the Earth gets its magnetic field.
The small magnetic needle in a compass points to the north because the needle is suspended in such a way that it can spin to align itself with the planet’s magnetic field.
Ironically, the north magnetic poles of compass needles are attracted to the Earth’s south magnetic pole, so what we call the magnetic north pole is actually not.