Newton's laws "easy to understand"
For the study of movement are used as a basis Newton's laws. In it, the relationships between movements and forces are established.
In these laws the phenomena of nature with respect to motion are explained. Observing nature, the principle of inertia was reached, when observing that bodies that are in motion maintain it by themselves without being pushed by anyone.
The inertia of a body can be overcome by exerting a force on it, the body presenting an acceleration. The second law establishes the relationship to determine the acceleration that a body experiences under the action of a force.
Being the Newton's three laws, the bases of mechanics, are exposed, in a simple way, these principles: of inertia, of mass and the principle of action and reaction, with easy to understand exercises.
BASIC CONCEPTS "to understand Newton's laws"
Mass:
The mass of a body is the amount of matter that makes it up. It is measured in kilograms (kg) or pounds (lb). [1]
Movement:
Change of position of a body, with respect to a reference system. [two]
Uniform line movement:
It is that movement of a body at constant speed (magnitude and direction), with a straight path. [3]. See figure 1.
Acceleration:
Change in the speed of an object per unit of time.
Force:
Action exerted by one body on another, producing movement or deformation.
Newton's First Law "Principle of Inertia"
Inertia is the property of matter, by which, if a body is in motion, it tends to keep moving, if it is at rest it tends to stay at rest. See figure 2. The greater the mass of a body, the greater its inertia.
The principle of inertia, established by Isaac Newton, postulates that "if no force acts on a body, or several forces that cancel each other act, then the body is at rest or in uniform rectilinear motion". [4]. See figure 3.
The unpleasant sensation in the stomach that is felt when an elevator starts abruptly, is due to inertia, to the resistance of the body to move. Inertia is also observed when the driver of a vehicle accelerates and the passengers of the vehicle lean backwards, if the driver suddenly brakes, the passengers lean forward, tending to continue with the movement they had.
Newton's Second Law "Principle of Mass"
To overcome the inertia of a body, a force can be applied. Newton's second law establishes the relationship between the applied force, the mass of the object and the acceleration it acquires.
In figure 4, you have two horses that exert the same force on a cart, but in the cart on the right there is more mass, so the cart will move slower, with less acceleration.
In figure 5, there are two carts that have the same mass. Greater force is exerted on the cart on the right as it has two horses, so the cart will move with greater acceleration than the one on the left.
Does Newton's second law state that "The acceleration that a body acquires, under the action of a force, is directly proportional to the force in inversely proportional to its mass". See figure 6.
Exercise 1 What acceleration does the blue car in figure 7 acquire when it is pulled with a force of 2000 N? The car has a mass of 1.000 kg.
Solution:
Applying Newton's second law, the acceleration is the quotient between the applied force and the mass of the car
Thus the car will have an acceleration of 2 m / s2. For every second that passes, its speed will increase by 2m / s.
Weight of an object
The weight of a body is the force with which the earth pulls it towards it. If an object is dropped freely, it acquires an acceleration of approximately 9,81 m / s2, known as "acceleration of gravity (g)".
Weight is a force that is always directed toward the ground. By Newton's second law, it is given by: Weight = mg
Anywhere on the planet the mass of a body is the same, it does not vary, however, the acceleration of gravity varies from one point to another on the earth, therefore, the weight also varies. This is because the Earth behaves as if all its power of attraction were accumulated in its center, the closer to the center it is located, the greater the force of attraction, the greater the weight. See figure 8.
Exercise 2 What is the mass of a woman weighing 600 N?
Solution
Newton's second law is applied to determine the weight of a body, as shown in Figure 9.
Exercise 3 Determine the weight of a man whose mass is 70 kilograms, when he is located at:
a) The sea. At sea level the acceleration of gravity is g = 9,81 m / s2
b) At the north pole, where gravity is g = 9,83 m / s2
c) At the equator, with g = 9,78 m / s2
Solution
Figure 10 shows the calculation of man's weight at sea level, at the north pole and at the equator. Since gravity is different, the weights are different, but the mass remains constant.
Newton's Third Law "Principle of Action and Reaction"
Newton's third law states that "Whenever a body exerts a force (action) on another body, it reacts with an equal and opposite force applied to the first body". [5].
In figure 11 this principle can be observed: when a person on boat A pushes boat B. With the oar, boat B moves to the right, while boat A moves to the left by the force of reaction of boat B on boat A.
Exercise 4 Determine the force with which the table pushes the book.
Solution:
By law of action and reaction (Newton's third law), the force exerted by the book on the table is the same as the force exerted by the table on the book, only it is in the opposite direction. Since the magnitudes of the forces are of the same magnitude, but in the opposite direction, the sum of the forces is zero and the book remains at rest (Newton's first law). See figure 13.
CONCLUSIONS:
El principio de inercia establece las relaciones entre los movimientos y las fuerzas que se aplican sobre un cuerpo. Si la fuerza es nula, el movimiento es rectilíneo y uniforme, o el cuerpo se mantiene en reposo. Si la fuerza sobre el cuerpo no es nula hay una aceleración (cambio de velocidad).
El principio de masa, la segunda Ley de Newton, establece la relación entre la fuerza aplicada, la masa del objeto y la aceleración que experimenta. La aceleración es directamente proporcional a la fuerza aplicada, e inversamente proporcional a la masa del cuerpo.
El principio de acción y reacción, o tercera Ley de Newton, enuncia que la fuerza ejercida de un cuerpo A sobre un cuerpo B, es igual en magnitud y opuesta en dirección a la ejercida por el cuerpo B sobre el cuerpo A.