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Download Types of Forces and Free Body Diagrams Adapted from Prentice
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Types of Forces and Free Body Diagrams Adapted from Prentice Hall’s Science Explorer and Holt Physics Forces are Vectors Because the effect of a force depends on its magnitude (strength) and direction, force is a vector quantity. Diagrams that show force vectors as arrows are called free body diagrams. Applied Force An applied force is a push or pull on an object. When you push on a book, or pull on a rope you apply a force to to that object. Remember according to Newton’s Second Law, force is equal to mass X acceleration! F = ma Gravity / Weight You already know that a bowling ball weighs more than a tennis ball. But how do you know? If you imagine holding one ball in each hand, you can imagine the downward forces acting on your hands. Because the bowling ball has more mass than the tennis ball, the force of gravity pulls more strongly on the bowling ball, and it pushes your hand down with a stronger force than does the tennis ball. The force of gravity exerted on the ball by Earth, Fg, is always directed toward the center of Earth. The magnitude of this force, Fg, is a quantity called weight. When the mass is known, the weight of an object can be calculated using the equation Fg = ma, where a is the acceleration due to gravity,or 9.8 m/s2. Air Resistance Objects falling through air experience a type of friction called air resistance. Air resistance is an upward force. Air resistance is not the same for all objects. The greater the surface area of an object, the greater the air resistance. That is why a leaf falls more slowly than an acorn. Air resistance increases with velocity. So as a falling object speeds up, the air resistance against it increases. Eventually, the air resistance equals the force of gravity. Remember that when forces are balanced, there is no acceleration. So although the object continues to fall, its velocity no longer increases. This velocity, the greatest velocity an object reaches, is called terminal velocity. Normal Force Imagine a television set at rest on a table. We know that the force of gravity is acting on the television. How can we use Newton’s laws to explain why the television does not continue to fall toward the center of Earth? An analysis of the forces acting on the television will reveal the forces that cause equilibrium. First, we know that the force of gravity, Fg, is acting on the object. Because the television is in equilibrium, we know that another force, equal to Fg but in the opposite direction, must be acting on it. This force is the force exerted on the television by the table. This force is called the normal force, Fn. The word normal is used because the direction of the contact force is perpendicular to the surface, and one meaning of the word normal is “perpendicular.” Figure(a) (previous page) shows the forces acting on the television. The normal force is always perpendicular to the contact surface but is not always opposite in direction to the force of gravity. Figure(b) (previous page) shows a free-body diagram of a refrigerator on a loading ramp. The normal force is perpendicular to the ramp, not directly opposite the force of gravity. Friction When two surfaces rub, the irregularities of one surface get caught on those of the other surface. The force that one surface exerts on another when the two rub against each other is called friction. Friction acts in a direction opposite to the object’s direction of motion. Without friction, the object would continue to move at constant speed forever. Friction, however, opposes motion. Eventually friction will cause an object to come to a stop. The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together. Rough surfaces produce greater friction than smooth surfaces. The force of friction also increases if the surfaces push harder against each other. If you rub your hands together forcefully, there is more friction than if you rub your hands together lightly. Tension 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. Free Body Diagrams A free-body diagram is used to analyze only the forces affecting the motion of a single object. First draw a diagram to represent the isolated object under consideration. The next step is to draw and label vector arrows representing all external forces acting on the object. The arrows point in the direction of the force. The relative size of the arrows shows the relative magnitude (strength) of the different forces (a longer arrow means a bigger force).
