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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 4a
Chapter 7, Problem 4a

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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Step 1: Identify the objects involved in the problem. The key objects are Block A, the incline, the rope, the pulley, and the surface. Since the rope and pulley are massless and frictionless, their effects are limited to transmitting forces without adding mass or friction to the system.
Step 2: Determine the interactions between the objects. Block A interacts with the incline through friction and the normal force. The rope interacts with Block A by exerting a tension force. The pulley interacts with the rope by redirecting the tension force. The surface interacts with Block A through friction.
Step 3: Decide which objects are part of the system. Typically, the system includes Block A and the incline, as these are the primary objects whose motion and forces are analyzed. The rope and pulley can be considered external to the system since they only transmit forces without contributing mass or friction.
Step 4: Draw the interaction diagram. Represent each object as a node in the diagram. Draw arrows between nodes to indicate forces. For example, draw an arrow from the incline to Block A to represent the normal force, and another arrow to represent friction. Draw an arrow from the rope to Block A to represent tension, and from the pulley to the rope to represent the redirection of tension.
Step 5: Label each interaction clearly in the diagram. Include the type of force (e.g., tension, normal force, friction) and the direction of the force. Ensure the diagram visually represents all interactions and helps clarify the relationships between the objects in the system.

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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 the context of Block A sliding down an incline, the FBD would illustrate forces such as gravity, normal force, and friction. This helps in analyzing the net force and understanding the motion of the block.
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Newton's Second Law

Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, expressed as F=ma. This principle is crucial for determining how Block A will accelerate down the incline, considering the forces identified in the FBD.
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System and Surroundings

In physics, defining the system and surroundings is essential for analyzing interactions. The system includes the objects of interest, such as Block A and the pulley, while the surroundings encompass everything else, including external forces like friction. Understanding this distinction helps in accurately drawing the interaction diagram and identifying relevant forces.
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Related Practice
Textbook Question

A tennis racket swung over the server's head hits a tennis ball horizontally. 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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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

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

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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 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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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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