We usually assume the tension in a cable is the same everywhere in the cable. The force that acts in a direction opposite motion through a gas is called air resistance force.
It comes from cumulative interaction with air molecules. It increases as the velocity through the gas increases. It also increases as the area normal to the direction of motion increases. Spring force comes from the displacement of molecules.
It is always opposite the displacement of spring. Those forces do not involve physical contact between the two objects but act through the space between the two. Examples of non-contact or action at a distance forces, which are given in the list given below:. The attractive force exerting between two objects is called gravitational force. It originates with the presence of matter.
The electromagnetic force includes basic electric and magnetic interactions and is responsible for the binding of atoms and the structure of solids. The weak nuclear force causes certain radioactive decay processes and certain reactions among the most fundamental particles. The strong force operates among the fundamental particles and is responsible for binding the nucleus together. Force is measured in the dynamic method in which acceleration is imparted to a standard body by pulling on it with a stretched spring.
Although convenient for the purposes of definition, it is not always a particularly practical procedure for the measurement of forces. Acceleration is seldom easy to measure. Another method for measuring forces is base on measuring the change in shape or size of a body a spring, say on which the force is applied when the body is unaccelerated.
This may be called the static method of measuring forces. The basis of the static method is that when a body under the action of several forces, has zero acceleration , the vector sum of all the forces acting on the body must be zero.
This is, of course, just the second law of motion. A single force acting on a body would produce an acceleration; this acceleration can be made zero if we apply another force to the body equal in magnitude but oppositely directed. In practice, we seek to keep the body at rest. If now we choose some force as our unit force, we are in a position to measure forces.
The pull of the earth on a standard body at a particular point can be taken as the unit of force. Related Articles. For simplicity sake, all forces interactions between objects can be placed into two broad categories:.
Contact forces are those types of forces that result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces. These specific forces will be discussed in more detail later in Lesson 2 as well as in other lessons. Action-at-a-distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.
Examples of action-at-a-distance forces include gravitational forces. For example, the sun and planets exert a gravitational pull on each other despite their large spatial separation. Even when your feet leave the earth and you are no longer in physical contact with the earth, there is a gravitational pull between you and the Earth. Electric forces are action-at-a-distance forces. For example, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation.
And magnetic forces are action-at-a-distance forces. For example, two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters.
Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an "N. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude and due to the fact that it is mathematically difficult to predict its value. It is most noticeable for objects that travel at high speeds e.
Air resistance will be discussed in more detail in Lesson 3. The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.
For most springs specifically, for those that are said to obey " Hooke's Law " , the magnitude of the force is directly proportional to the amount of stretch or compression of the spring. A few further comments should be added about the single force that is a source of much confusion to many students of physics - the force of gravity. As mentioned above , the force of gravity acting upon an object is sometimes referred to as the weight of the object.
Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.
Mass is related to how much stuff is there and weight is related to the pull of the Earth or any other planet upon that stuff. The mass of an object measured in kg will be the same no matter where in the universe that object is located. Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not at least for purposes of our study ; and its mass will be 2 kg whether it is being pushed upon or not.
On the other hand, the weight of an object measured in Newton will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g - the gravitational field strength.
On earth's surface g is 9. On the moon's surface, g is 1. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet. So if we were to measure g at a distance of km above the earth's surface, then we would find the g value to be less than 9.
The nature of the force of gravity will be discussed in more detail in a later unit of The Physics Classroom. Always be cautious of the distinction between mass and weight. It is the source of much confusion for many students of physics. As mentioned above , the friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. For the purpose of our study of physics at The Physics Classroom, there are two types of friction force - static friction and sliding friction.
Sliding friction results when an object slides across a surface. As an example, consider pushing a box across a floor. The floor surface offers resistance to the movement of the box. We often say that the floor exerts a friction force upon the box.
This is an example of a sliding friction force since it results from the sliding motion of the box. If a car slams on its brakes and skids to a stop without antilock brakes , there is a sliding friction force exerted upon the car tires by the roadway surface. This friction force is also a sliding friction force because the car is sliding across the road surface.
Sliding friction forces can be calculated from knowledge of the coefficient of friction and the normal force exerted upon the object by the surface it is sliding across. The formula is:. The coefficient value is dependent primarily upon the nature of the surfaces that are in contact with each other.
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