College Physics II – Mechanics, Sound, Oscillations, and Waves
Definition
External forces are the forces that act on an object from outside the object's system. These forces are not generated within the object itself, but rather originate from the object's interaction with its surrounding environment.
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External forces can be contact forces, such as friction or normal forces, or non-contact forces, such as gravitational or electromagnetic forces.
The external forces acting on an object are crucial in determining its motion and acceleration, as described by Newton's Second Law.
When analyzing the motion of an object, it is important to identify and consider all the external forces acting on it, as they contribute to the net force and, consequently, the object's acceleration.
The magnitude and direction of external forces can change over time, affecting the object's motion and the net force acting upon it.
Understanding the role of external forces is essential in predicting and analyzing the behavior of objects in various physical systems, such as mechanics, electromagnetism, and astrophysics.
Review Questions
Explain how external forces are related to Newton's Third Law.
According to Newton's Third Law, for every action (external force) there is an equal and opposite reaction. This means that when one object exerts an external force on another object, the second object will exert an equal and opposite external force back on the first object. This reciprocal relationship between external forces is a fundamental principle in understanding the dynamics of interacting objects.
Describe the role of external forces in the analysis of an object's motion using a free-body diagram.
A free-body diagram is a visual representation of all the external forces acting on an object. By identifying and accurately depicting the external forces, such as gravitational, normal, and friction forces, the free-body diagram allows for a thorough analysis of the object's motion and the net force acting upon it. This analysis is crucial in applying Newton's Second Law to determine the object's acceleration and predict its future behavior.
Evaluate how changes in the magnitude and direction of external forces can affect the motion of an object.
The motion of an object is directly influenced by the net force acting upon it, which is the vector sum of all the external forces. If the magnitude or direction of any of the external forces changes, it will alter the net force and, consequently, the object's acceleration according to Newton's Second Law. This relationship is fundamental in understanding how external forces can cause an object to speed up, slow down, or change direction, making the analysis of external forces crucial in predicting and explaining the behavior of physical systems.
Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts an equal and opposite force on the first.
A free-body diagram is a visual representation of all the forces acting on an object, including external forces, which is used to analyze the object's motion and the net force acting upon it.
Net Force: The net force is the vector sum of all the forces acting on an object, including external forces. The net force determines the object's acceleration according to Newton's Second Law.