For example, my book says that if I lower a box to the ground, the box does positive work on my hands and my hands do negative work on the box. So, if work occurs when a force causes displacement, how does negative work happen?
Are my hands displacing anything? In the context of classical mechanics as you describe, negative work is performed by a force on an object roughly whenever the motion of the object is in the opposite direction as the force. This "opposition" is what causes the negative sign in the work.
Such a negative work indicates that the force is tending to slow the object down i. When the angle has these ranges, the force has a component perpendicular to the direction of motion, and a component opposite to the direction of motion. The perpendicular component contributes nothing to the work, and the component opposite the motion contributes a negative amount to the work.
Work is the component of force parallel to the direction of motion times the displacement. That component of force could of course point in the opposite direction of motion anti-parallel. The work done by the force is positive in the first case and negative in the second. For instance, the direction of the force of gravity on a freely falling body dropped from rest points towards the center of the Earth which is also the direction of displacement while falling.
The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object? It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer. Demonstrating Work What is Work? How does Physics define it? Actually, in order to do some work in Physics, you have to fulfill three conditions: 1 You have to apply a force on a body 2 It will have to move 3 But the direction of movement must not make 90 degrees angle with that of the force applied.
Previous Post Centrifugal force: The myths and the reality. Next Post Measuring my own land located in Bangladesh. About Contact Online Tuition. Loading Comments The force of gravity, electrical forces, and magnetic forces were examples of forces that could exist between two objects even when they are not physically touching.
In this lesson, we will learn how to categorize forces based upon whether or not their presence is capable of changing an object's total m echanical energy. We will learn that there are certain types of forces, that when present and when involved in doing work on objects will change the total mechanical energy of the object.
And there are other types of forces that can never change the total mechanical energy of an object, but rather can only transform the energy of an object from potential energy to kinetic energy or vice versa. The two categories of forces are referred to as internal forces and external forces. Forces can be categorized as internal forces or external forces. There are many sophisticated and worthy ways of explaining and distinguishing between internal and external forces.
Many of these ways are commonly discussed at great length in physics textbooks - particularly college-level physics textbooks. For our purposes, we will simply say that external forces include the applied force, normal force, tension force, friction force, and air resistance force. And for our purposes, the internal forces include the gravity forces, magnetic force, electrical force, and spring force. While this is a simplistic approach, it is an approach that will serve us well in our introduction to physics.
The importance of categorizing a force as being either internal or external is related to the ability of that type of force to change an object's total mechanical energy when it does work upon an object.
If the work is positive work , then the object will gain energy. If the work is negative work , then the object will lose energy. The gain or loss in energy can be in the form of potential energy , kinetic energy , or both. Under such circumstances, the work that is done will be equal to the change in mechanical energy of the object. This principle will be discussed in great detail later in this lesson.
Because external forces are capable of changing the total mechanical energy of an object, they are sometimes referred to as nonconservative forces.
In such cases, the object's energy changes form. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy.
Yet, the sum of the kinetic and potential energies remains constant. This is referred to as energy conservation and will be discussed in detail later in this lesson. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential or vice versa ; yet the total amount of mechanical is conserved. Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are sometimes referred to as conservative forces.
In the following descriptions, the only forces doing work upon the objects are internal forces - gravitational and spring forces. Thus, energy is transformed from KE to PE or vice versa while the total amount of mechanical energy is conserved. Click the mouse to check your answers.
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