Physics Forces: Identifying Forces PDF

EXPLORATION 1 Representing Forces In everyday language, “to force” can mean to cause something to happen; in science, a force is a push or a pull exerted by one object on another. In the International System (SI)...

EXPLORATION 1 Representing Forces In everyday language, “to force” can mean to cause something to happen; in science, a force is a push or a pull exerted by one object on another. In the International System (SI) 2 of Units, the unit for force is the newton (N), which is equal to 1 kg m/s. Force is a vector quantity because it has direction as well as magnitude. Collaborate With a partner, compare and contrast the everyday and scientific meanings of force. In your discussion, address these questions: Can there be a force if nothing happens? How might the units (kg m/s2) be related to the scientific definition? FIGURE 1: Spring scales are Identifying Forces used to measure force. Think about the downward pull due to gravity. Scientists distinguish between weight, the gravitational force acting on an object, and mass, a measure of the amount of matter. The mass of an object can be measured by comparing it to known masses using a balance; mass is not a force. Weight and other forces can be measured—for example, by how much they push or pull a spring in a spring scale. These tools work because an object’s weight is balanced by another force. A kitchen or bathroom scale can be used to measure the downward force (weight) that is balanced by an upward supporting force, such as from a table or the floor. This supporting force is perpendicular to a surface and is called the normal force. © Houghton Mifflin Harcourt Publishing Company Image Credits: ©Houghton Mifflin Harcourt INFER For each type of force in the table, match the example description with the vector image of the force. Force Description air resistance Air exerts a force against the moving box in a way that increases with the box’s speed. friction Two sliding surfaces produce a force that acts opposite to the direction of the relative motion of the surfaces. gravitational force Earth exerts a force of attraction on the box. normal force An object exerts a force on the box in a direction perpendicular to the surfaces in contact. 54 Unit 1 Physics and Engineering  Cause and Effect Exploring Friction FIGURE 2: The surfaces of the box and When an object is pushed or pulled across another object, small the ramp resist motion between them. points of the surfaces in contact push against each other and resist motion. This effect and several other effects, together, produce friction. Friction is a force that opposes motion between two surfaces that are in contact. The box in Figure 2 stays in place because static friction resists the force that would make it slide down the ramp. Static friction occurs when the two surfaces are not sliding. When the box slides along the surface, kinetic friction resists—but does not prevent—the motion. The force of kinetic friction is less than the maximum value of static friction. Unlike air resistance, static and kinetic friction do not typically depend on speed or on the area of contact. They are each proportional to the force pushing the surfaces together, which is usually characterized by the normal force. The normal force is in a direction perpendicular to a surface, so it is not always vertical. Language Arts Connection Use the two types of friction to describe what happens as you slide a heavy box across a table. You might also research the coefficient of friction. Prepare an explanation to present to the class. Using Diagrams to Analyze Force The normal force and static friction tend to oppose other forces, while forces acting in the same direction produce a greater force. Forces combine as vectors, in the same way velocities or accelerations combine. To make calculations, however, you have to determine which vectors to add together. APPLY Use vector arrows to draw the forces you expect to be present when a car is parked on flat pavement. Treat the car as a single object. © Houghton Mifflin Harcourt Publishing Company Compare the vectors you drew with those of a classmate. Each of you may have represented the car’s weight, for example, in different ways. Perhaps you drew gravitational force pulling down on the car or the heavy car pressing down on the pavement. Look at the forces on a single object: the car. You can combine these forces. It is less useful to combine the forces that act on different objects. Lesson 3 Effects of Forces 55  A free-body diagram is a way to model a situation by looking at only one object and the external forces that act on it. Figure 3a shows some forces on the car and forces on the pavement. Figure 3b includes the car’s weight but not the forces on the pavement. FIGURE 3: The first model (a) shows forces on both the car and the ground. The second, free- body diagram (b) shows only forces on the car. a b In a free-body diagram, each force on an object is represented by a vector arrow. This model treats all forces as if they were acting at a single point, called the center of mass of the object. The center of mass is usually represented as a dot and may not be exactly at the center of the object. In Figure 3b, the free-body diagram of the car, the normal force is shown as a single vector pointing upward from the center of mass. Compare Figure 3a, in which the normal forces from the ground are represented as two vectors at the tires. ANALYZE Select the correct label for each vector in the free-body diagrams. A label can be used more than once. Fgravity Fnormal Fspring Fstring Determining Net Force © Houghton Mifflin Harcourt Publishing Company The net force on an object, Fnet, is the vector sum of all external forces acting on it. You may also see net force written as ΣF, where the capital sigma, Σ, indicates a summation. In each of the free-body diagrams that you labeled, two forces are balanced. When forces on an object are balanced, the net force is zero. The forces may be large or small, but if they balance, the result is as if no force were acting on the object. INFER What can you infer about the net force when an object is unmoving—at rest? Explain your reasoning. 56 Unit 1 Physics and Engineering  PREDICT Next to each diagram, draw a vector to represent the direction and approximate magnitude of the net force. Then describe what you think will happen to the motion of the box. Scenario Free-body diagram Predicted motion box falling in air box on a spring If forces act in one dimension—that is, along a single line—they can be represented as positive and negative numbers. For example, a force of 10 N down and a force 8 N up can be written as −10 N + 8 N = −2 N, a net force of 2 N downward. When forces are in two dimensions, you can look separately at the forces FIGURE 4: Components of the in perpendicular directions, such as vertical and horizontal forces. Because force due to gravity frictional forces are parallel to surfaces in contact and normal forces are perpendicular to the surfaces, you might choose a coordinate system oriented with one axis along the surface. In Figure 4, one axis would be parallel to the ramp and the other would be perpendicular to the rampFand parallel to Fnormal the Ffriction Fnormal friction normal force on the box. You can often determine the normal force by analyzing the other forces. Suppose gravitational force is the only force pressing the object to the surface. Fgravity Fgravity For an object at rest on a horizontal surface, the normal force equals the weight of the object supported by the surface. For a slanted surface, such as a ramp, the normal force equals the component of the force due to gravity perpendicular to the surface. APPLY Suppose you place a coin on a book; the book’s cover acts as a ramp. You open the cover slowly until static friction is overcome. The coin begins to slide. What happens to the forces on the coin as the angle of the cover changes? Select all correct answers. © Houghton Mifflin Harcourt Publishing Company a. A component of the frictional force begins pushing the coin and book together. b. The frictional force decreases suddenly as the coin starts to slide. c. The gravitational force increases. d. The component of gravitational force along the surface increases. e. The component of gravitational force perpendicular to the surface decreases. f. The normal force decreases slowly, which produces a slow decrease in static friction. Evidence Notebook Construct a free-body diagram for the falling ant and the falling acorn in the example, just before each hits the ground. Use the lengths of the vectors to show your estimate of the relative magnitudes. Lesson 3 Effects of Forces 57