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# sg9 - Chapter 9 Study Guide for Force 9.1 Force Skill 9.1...

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Chapter 9 Study Guide for Force 9.1 Force Skill 9.1 Understand how the force exerted on an object is related to the object’s momentum. Linear momentum is conserved for an isolated system, but it is not necessarily conserved for a system interacting with its surroundings. Whenever a force is exerted on an object, it causes a change in the object’s linear momentum. How much the momentum is changed depends on the magnitude of the force and the amount of time for which the force is applied. The magnitude of the force exerted on an object depends on both the change in momentum it causes and the rate at which this change in momentum occurs. For a constant force, F = Δ p Δ t (9.1) Skill 9.2 Understand how the net force exerted on an object is obtained. When an object O interacts with more than one other object, then the resulting change in O ’s momentum is due to the combination of all the interactions. This combined force is referred to as the net force exerted on the object. To find the net force exerted on an object, simply take the vector sum of all the individual forces exerted on the object. The net force is often denoted vector F net or Σ vector F . Example: We often have to apply a constant force just to keep our car going at a constant speed. This is because there are other forces pushing back on it (due to wind and friction). The net force, however, is zero if the velocity is constant. The net force is in the direction of the change in momentum. In the case of constant inertia (which is mostly what we will be dealing with), then it is also the direction of acceleration. Look at checkpoint 9.2. 9.2 The reciprocity of forces Skill 9.3 Understand the reciprocity of forces Interaction Pairs: The forces exerted by two interacting objects on each other are equal and in opposite directions. These forces are said to form an interaction pair . Look at checkpoints 9.4-9.7. 1

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9.3 Identifying forces Skill 9.4 Be able to distinguish between contact forces and field forces. Definition of Contact Forces: Contact forces arise when things actually touch at the macroscopic level. Definition of Field Forces: Field forces are forces that don’t require physical contact. The gravitational and electromagnetic forces are the only ones we see on the macroscopic level. 9.4 Equilibrium Skill 9.5 Be able to identify interaction pairs. An object at rest or moving at constant velocity (momentum not changing) is said to be in equilibrium . An object at rest is in static equilibrium, and an object traveling with constant velocity is in dynamic equilibrium. Since in either case, its momentum is not changing, the net force acting on it is zero. Look at checkpoints 9.9 and 9.10. 9.5 Free-body diagrams Skill 9.6 Be able to construct free-body diagrams. Here are the steps for constructing a free-body diagram: 1. Draw a small circle or box that represents the system under consideration.
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