In the following descriptions, the only forces acting on objects are internal forces – gravitational and springing forces. Thus, energy is converted from KE to PE (or vice versa) while retaining the total amount of mechanical energy. Read each description and indicate whether the energy is converted from KE to PE or from PE to KE. Click with the mouse to check your answers. When muscle force is applied to the edges of a scale, it bends. A considerable amount of tension and compression is built into the balance. The muscular force acting on the scale is the external force. This force has a size that makes the ladder bend but does not break it. This is because an inner force provides support and prevents it from breaking. When a rubber band is pulled or stretched, it tends to develop a pulling force. The elastic object returns to its original shape as soon as the pulling force is removed. Since the interaction exists in the object or system, the force is called internal. However, the force exerted to stretch the ligament and cause a shape-changing movement is called external force.
When a footballer hits a ball, he exerts considerable force on the ball. The action-reaction force pair is not completely present in the observation system. Therefore, the force acting on the ball is called the external force. This external force acting on the ball makes it move. The distance travelled by the object is proportional to the intensity with which the force is exerted on the ball. If the work is performed by external forces (non-conserving forces), all the mechanical energy of the object is modified. The work done can be positive or negative work, depending on whether the force performing the work is directed against the movement of the object or in the same direction as the movement of the object. If the force and displacement go in the same direction, positive work is done on the object. When positive work is performed on an object by an external force, the object gains mechanical energy. If the force and displacement go in the opposite direction, negative work is performed on the object; The object then loses mechanical energy.
A force acting on an object from the outside is called an external force. Any force that acts on a structure from the inside is called an internal force. Two or more objects interacting with each other form a system. An external force results from the interaction between a system and the environment. On the other hand, an internal force is an interaction that exists within a system. External forces cause movement in an object, while an internal force resists movement. Work is said to occur when an external force acts on an object, while work done due to an internal force is zero. A force is neither external nor internal, it depends on the system of contemplation. If the pair of action-response forces is present in the system, the applied force is said to be internal; otherwise, it is external. The following descriptions include external forces (frictional, applied, normal, drag, and tensile forces) working on an object.
Read the description and indicate whether the object has gained energy (positive work) or lost energy (negative work). (NOTE: If it is a part, see the section on work.) Then indicate whether the gain or loss of energy caused a change in the kinetic energy, potential energy, or both of the object. Click the buttons to view the answers. External forces are forces caused by external agents outside the system. Internal forces are forces that are exchanged by objects in the system. The applied force of the racquet makes the ball gain speed. Thus, the external or non-conservative force changes the kinetic energy of the ball. There are a variety of ways to categorize all kinds of forces.
In a previous unit, it was mentioned that all types of forces can be classified as contact forces or as ranged action forces. Whether a force was classified as a ranged force depended on whether this type of force could exist even when objects were not physically touching. Gravity, electric forces, and magnetic forces were examples of forces that could exist between two objects, even if they did not physically touch each other.
