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Honors Physics 1. 2. 3. 4. 5. 6. I can define Force. I can explain the difference between contact and field force. I can draw Force (Free-body) Diagrams I can explain the relationship between the motion of an object and the net external force acting on it. I can determine the net external force on an object. I can calculate the force required to bring an object into equilibrium. In its most easiest terms, force is a push or a pull on an object. However, in this class we will define force as “the cause of an acceleration, or the change in an object’s velocity.” Therefore, force can change the direction of an object or its speed! 1 pound (lb) = 4.448 Newtons (N) 1 Newton (N) = 0.225 pounds (lbs) 1 kilogram (kg) = 1000 grams (g) 1 kilogram (kg) = 2.20 pounds (lbs) Did you know? The abbreviation for the pound (lb) comes from libra, the Latin word for pound! Contact Forces Force that arises from the physical contact of two objects. Field Forces Force that can exist between objects, even in the absence of physical contact between the objects. Field theory was developed as a tool to explain how objects could exert force on each other without touching. According to this theory, the presence of an object affects the space around it so that a force is exerted on any other object placed within that space. The region of influence is called a “field.” For example, an object falls to Earth because of the interaction between the object and the gravitational field of the Earth. Field forces are especially important in the study of particle physics – but alas, this must wait until later! Force is a vector – it shows magnitude and direction! A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects. In a force diagram, ALL of the forces acting on an object are shown. Another name for a force diagram is a free-body diagram. A free-body diagram helps to analyze a situation and isolate an object to show the forces acting on it. Did you notice that the force diagram vectors originate from the center of the object!!??!!?? This spot is known as the “center of gravity!” The first step is to examine the motion of the object before forces are applied. A common misconception is that an object with no force acting on it will always be at rest! To test this misconception, consider sliding a bowling-ball on different surfaces. Imagine a bowling-ball on a thick carpet. If you apply a force the bowling-ball moves, however as soon as you remove the force the bowling-ball stops immediately! Now imagine pushing the bowling-ball on a waxed floor. If you push with the same force it will slide much farther before coming to a rest! Galileo, in the 1630’s realized that a block sliding on a perfectly smooth surface would slide forever in the absence of an applied force. He concluded, correctly that it is not the nature of an object to stop once it is set in motion; RATHER it is an object’s nature to maintain its state of motion. This concept was developed by Newton, in 1687 and has come to be known as Newton’s First Law of Motion. An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force. The tendency of an object not to acceleration is called INERTIA. INERTIA is the tendency of an object to maintain its state of motion. Large Inertia Small Inertia In the absence of forces, a body will preserve its state of motion. When the net external force on an object is zero, its acceleration (or change in its velocity) is zero. In order for the car to accelerate – the car needs to overcome the force of friction, the force of air resistance, and the force due to the weight of the car! To understand how a car maintains a constant velocity under the influence of so many forces, it is necessary to understand the different between external force and net external force. External Force – A single force that acts on an object as a result of the interaction between the object and its environment. Net External Force – The vector sum of all the forces acting on an object. Sometimes it is called the resultant force. Steps to solution: 1. Define the problem and identify the variables. 2. Select a coordinate system, and apply it to the free-body diagram. 3. Find the x and y components of all vectors. 4. Find the net external force in both the x and y directions. 5. Find the net external force. 6. Evaluate your answer. You leave your physics book on the top of a drafting table that is inclined at an angle of 35o. The freebody diagram shows the forces acting on the book. Find the net external force acting on the book and determine whether the book will remain at rest in this position. Force of friction = 11 N Force of table on book = 18 N Force of gravity on book = 22 N