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Forces Reference Sheet A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Contact Forces A contact force results when the two interacting objects are perceived to be physically contacting each other. Air Resistance Force The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas. Applied Force An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person. Frictional Force The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force—sliding friction and static friction. Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. Normal Force The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a table, the table surface is exerting an upward force upon the book in order to support the weight of the book. On occasion, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person. Tension Force The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. The Charles A. Dana Center at the University of Texas at Austin 1 Reference Sheet: Forces Spring Force The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. A force that restores the object to its rest or equilibrium position always acts upon an object that compresses or stretches a spring. Action at a Distance Forces Action at a distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation. Gravitational Force The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. Magnetic Force A magnetic force is the attraction or repulsion that arises between electrically charged particles because of their motion. A magnetic force is the basic force responsible for the action of electric motors and the attraction of magnets for iron. Electromagnetism is an interaction between electricity and magnetism, as when an electric current or a changing electric field generates a magnetic field, or when a changing magnetic field generates an electric field. Inertia An object’s tendency to resist changes in its state of motion is described as inertia. If a force does not act on an object in motion, the object will continue in motion. If an object is at rest, it will continue in that resting position unless a force acts it on. The resistance to a change in motion varies with mass, and the greater the mass of an object, the greater is its tendency to resist a change in motion. Therefore, an objects’ inertia is tied to its mass. Force Diagrams For this session, diagrams will be used to show the relative magnitude (strength) and direction of all forces acting upon an object in a given situation. The length of the arrow in the force diagram reflects the relative magnitude of the force. The direction of the arrow shows the direction of the force. Each force arrow in the diagram is labeled to indicate the type of force. Represent the object of interest with a black dot and draw the force arrow from the dot. The forces acting on an object determine the object’s motion. If the forces are balanced, the object will continue doing what it is doing. If the forces are unbalanced, the object’s motion will change depending on the direction of the unbalanced force. The Charles A. Dana Center at the University of Texas at Austin 2 Reference Sheet: Forces Example A: Object resting on a table. Normal Force Object Force of Gravity Balanced Forces: force. The object will not move. The normal force is equal to the gravitational Example B: Object resting on a table. Lifting Force Force of Gravity Object Unbalanced Forces: The lifting force is greater than the gravitational force. The object moves upward. The Charles A. Dana Center at the University of Texas at Austin 3 Reference Sheet: Forces Example C: Object resting on a table. Normal Force Object Force of Gravity Unbalanced Forces: The force of gravity is greater than the normal force. The object moves downward. Example D: Object being pushed across a table. Normal Force Applied Force Resistance Force Force of Gravity Object Unbalanced Forces: The applied force is greater than the resistance force. The object moves to the right. The Charles A. Dana Center at the University of Texas at Austin 4 Reference Sheet: Forces Example E: Object being pushed across a table. Normal Force Applied Force Object Force of Friction Force of Gravity Unbalanced Forces: The applied force is greater than the resistance force. The object moves to the left. The Charles A. Dana Center at the University of Texas at Austin 5