Understanding Force: Properties, Laws, and Specific Examples
Properties of Force
A force is an influence acting on an object that causes it to change its state of motion. We notice it when we push or throw an object.
Property 1: Interaction Between Objects
A force is always applied by one physical object to another.
Property 2: Magnitude and Direction
A force is characterized by its magnitude and direction. These are required to specify a force. The magnitude can be expressed in kilopond (Kp). The direction is the direction in which the force would tend to move the object and is indicated by an arrow. Magnitudes characterized by a magnitude and a direction are called vectors.
The kilopond or kilogram-force is the force with which the earth attracts a standard kilogram located at sea level and 45° latitude.
Property 3: Newton’s Third Law of Motion
When object A exerts a force F on object B, object B simultaneously exerts a force R on object A. The force R is of equal magnitude but opposite in direction to F. Forces always act in pairs. The two forces of each pair are sometimes referred to as forces of action and reaction. It implies that one of the forces appears in response to the other.
Remarks: The forces of action and reaction act upon two different objects. A force is always applied by one object to another; each object exerts a force on the other.
Property 4: Vector Sum of Forces
If two or more forces act simultaneously on the same object, their effect is the same as that of a single force equal to the vector sum of the individual forces.
S = F1 + F2
Newton’s first law is the basis of statics, and the second law is the basis of dynamics.
Newton’s First Law of Motion (Special Case)
For an object to remain at rest, i.e., in equilibrium, the vector sum of all forces acting on it must be zero. This is only one condition necessary for an object to be at rest. If an object is to remain at rest, the net force must be zero. If the net force is not zero, the object cannot remain at rest. It can move even when the net force on it is zero. There is another condition that also must be satisfied to remain at rest.
Note: The three laws of motion were set out by Newton.
Specific Forces
Everyday forces and their peculiar features.
Force of Gravity
Gravity is the force with which the earth attracts all objects. For example, a block of lead experiences this force, which is approximately the same value at any point on land. It can vary by 0.5% between the pole and the equator. The direction of the force points to the center of the earth. Thus, the direction of the force of gravity on the lead block at the pole is different from the direction on the block at the equator. The lead is attracted to the center of the earth by the force of gravity. By Newton’s third law, the block must attract the earth with a force of equal magnitude and opposite direction to the force exerted by the earth on the block. This implies that the power to attract is a special property of the earth and must be present in all objects. The pound is defined in terms of the relationship 1 lb = 4.45 N.
Spring Force
Fg = Kx. Fg + Fk = 0. Fk = -Fg. For any force F, the force -F has the same magnitude and opposite direction to F.
Contact Force
Consider a block resting on a table, in equilibrium. According to the first law, another force besides the force of gravity must act on it. The surface is slightly deformed by the block, and it exerts an upward force Fc. This force, which is perpendicular to the surface, is called the contact force. The sum of Fg, the force of gravity on the block, and Fc, the contact force, is zero. It follows that Fc must be of opposite direction to Fg:
Fc = -Fg
Contact forces are exerted by solid objects. They are real forces and accompanied by small distortions in the surfaces. A contact force differs from the spring force by the degree of distortion that takes place. The surface of a solid body exerts a force with only insignificant changes occurring. A solid body acts somewhat like a tense spring and can exert contact forces in different circumstances without experiencing a significant change in its appearance. This should not deceive us into thinking that these forces are less real.