Newton's second law is one of the most important laws of high school physics, and it appears in various exams. If you want to grasp Newton's second law well, you must first have a deep enough understanding of it!Today, Xingbo Physics will lead you to understand Newton's second law from a different perspective.
Newton's second law
1 Contents: The magnitude of the acceleration of an object is proportional to the force on it, inversely proportional to its mass, and proportional to the reciprocal of the mass of the object, and the direction of acceleration is the same as that of the force.
2 Expressions.
1) Proportional formula: f kma, where k is the proportional coefficient and f is the resultant external force on the object.
2) In the International System of Units: f ma
3.Significance. The expression of Newton's second law, f ma, is the resultant force on the object on the left, and the effect on the right is the acceleration a on the effect of the object with mass m under the action of this resultant force, which highlights that the force is the cause of the change in the state of motion of the object and the cause of the acceleration of the object.
The unit of force
1 SI unit of force.
Newton, abbreviated as the ox, symbol n.
Definition of 2 1 n.
The force that causes an object with a mass of 1 kg to produce an acceleration of 1 m s is called 1 n, i.e., 1 n 1 kg·m s
3 Significance of the scale factor.
1) In f kma, the selection of k has a certain arbitrariness
2) In the International System of Units k 1, the expression of Newton's second law is f ma, where f, m, and a are in newtons, kilograms, and meters per square second, respectively.
The nature of Newton's second law
1.Causality.
The force is responsible for the acceleration and the object gains acceleration as long as the resultant external force on the object is not zero.
2.Vector.
The formula f kma is a vector formula, and the direction of acceleration a is always the same as the direction of the resultant external force f on the object.
3.Transient.
The acceleration a and the resultant external force f on the object always exist at the same time, change at the same time, and disappear at the same time.
4.Identity.
F, M, and A correspond to an inertial frame of reference (generally referring to the ground), the same object (or the same system).
5.Independence.
The acceleration produced by each force acting on an object follows Newton's second law;The acceleration produced by the components of each force in all directions also follows Newton's second law;The resultant acceleration of the object is equal to the vector sum of the acceleration produced by each force. The component formula of Newton's second law is.
The relationship between resultant force, velocity, and acceleration
1.The acceleration of an object is determined by the resultant force exerted and is not necessarily related to velocity.
2.If the angle between the resultant force and the velocity is an acute angle, the object accelerates;If the angle between the resultant force and the velocity is obtuse, the object will slow down.
is the definition of acceleration, a and v,
Not directly related;
is the deterministic form of acceleration.
Sample questions
Example 1According to Newton's second law, the following statement is correct: ( ) a When the resultant force experienced by an object changes, its acceleration does not necessarily change at the same timeb The net force experienced by the object must reach a certain value in order for the object to produce acceleration c The magnitude of the acceleration of an object is proportional to the magnitude of any of the forces on which it is subjectd to the mass when the mass of the object changes but the horizontal component of the net force on which it is subjected does not change.
【Answer】dAccording to Newton's second law, the magnitude of the acceleration of an object is proportional to the resultant force it is subjected to, and inversely proportional to its massThe resultant force is not zero, i.e., it has acceleration, so abc is wrong;d Newton's second law applies to an object in a certain direction, that is, when the mass of the object changes but the horizontal component of the net force on which it is subjected does not change, the magnitude of the horizontal acceleration of the object is inversely proportional to its mass, so d is correct. Therefore, choose D.