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Ch 07: Newton's Third Law
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
All textbooksKnight Calc 5th EditionCh 07: Newton's Third LawProblem 4c
Chapter 7, Problem 4c

Block A in FIGURE EX7.4 is sliding down the incline. The rope is massless, and the massless pulley turns on frictionless bearings, but the surface is not frictionless. The rope and the pulley are among the interacting objects, but you'll have to decide if they're part of the system. Draw a free-body diagram for each object in the system. Use dashed lines to connect members of an action/reaction pair.

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Step 1: Identify the objects in the system. The system includes Block A, the rope, the pulley, and the incline surface. Since the rope and pulley are massless and frictionless, their forces will only act as intermediaries between other objects.
Step 2: Draw the free-body diagram for Block A. Represent Block A as a point or box. Include the following forces: (1) gravitational force acting downward, represented as \( F_{g} = m_{A} g \), (2) normal force from the incline acting perpendicular to the surface, \( F_{N} \), (3) frictional force opposing the motion, \( F_{f} \), and (4) tension force from the rope acting upward along the incline, \( T \).
Step 3: Draw the free-body diagram for the pulley. Represent the pulley as a circle. Include the following forces: (1) tension force from the rope on one side, \( T \), and (2) tension force from the rope on the other side, \( T \). Since the pulley is massless and frictionless, these forces will balance each other.
Step 4: Draw the free-body diagram for the rope. Represent the rope as a line. Include the tension forces acting at both ends of the rope, \( T \). The rope transmits the force between Block A and the pulley.
Step 5: Use dashed lines to connect members of action/reaction pairs. For example, connect the normal force \( F_{N} \) exerted by the incline on Block A with the reaction force exerted by Block A on the incline. Similarly, connect the tension forces between the rope and Block A, and between the rope and the pulley.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Free-Body Diagram

A free-body diagram (FBD) is a graphical representation used to visualize the forces acting on an object. In an FBD, the object is isolated from its surroundings, and all forces, including gravitational, normal, tension, and frictional forces, are represented as arrows pointing in the direction they act. This tool is essential for analyzing the dynamics of the object and understanding how these forces interact.
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Newton's Third Law of Motion

Newton's Third Law states that for every action, there is an equal and opposite reaction. This principle implies that forces always occur in pairs; when one object exerts a force on another, the second object exerts a force of equal magnitude and opposite direction back on the first. Understanding this law is crucial for identifying action/reaction pairs in the free-body diagrams of the objects involved.
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System and Surroundings

In physics, defining the system and its surroundings is vital for analyzing interactions. The system includes the objects being studied, while the surroundings encompass everything else that can exert forces on the system. In this context, determining whether the rope and pulley are part of the system affects how forces are represented in the free-body diagrams and influences the overall analysis of the motion.
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Related Practice
Textbook Question

A 1000 kg car is pushing an out-of-gear 2000 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push horizontally against the ground with a force of 4500 N. Rolling friction can be neglected. What is the magnitude of the force of the car on the truck?

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Textbook Question

A weightlifter stands up at constant speed from a squatting position while holding a heavy barbell across his shoulders. Identify the "system" on your interaction diagram.

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Textbook Question

Block A in FIGURE EX7.4 is sliding down the incline. The rope is massless, and the massless pulley turns on frictionless bearings, but the surface is not frictionless. The rope and the pulley are among the interacting objects, but you'll have to decide if they're part of the system. Draw an interaction diagram.

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Textbook Question

How much force does the astronaut exert on his chair while accelerating straight up at 10 m/s2?

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Textbook Question

Blocks with masses of 1 kg, 2 kg, and 3 kg are lined up in a row on a frictionless table. All three are pushed forward by a 12 N force applied to the 1 kg block. How much force does the 2 kg block exert on the 1 kg block?

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Textbook Question

A heavy steel cable attached to a motor is lifting a girder. The girder is speeding up. Draw an interaction diagram.

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