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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 5.5 Using Action-Reaction Pairs

Subsubsection Practice Activities

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Activity 5.5.1. The Book Stack.

A stack of two books is at rest on a table.
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Book 2 (blue) on top of book 1 (orange) on a flat table.
Figure 5.5.1. A stack of two books.
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Use Force Analysis to identify all Action-Reaction (Newton’s 3rd Law) Pairs and identify any forces that are equal in magnitude.
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Activity 5.5.2. The Block Race.

Block A is accelerated across a frictionless table by a hanging \(10 \mathrm{~N}\) mass. An identical block B is accelerated by a constant \(10 \mathrm{~N}\) tension in the string.
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Left: Block A connected to a string which runs over a pulley to a hanging block with a 1 kg hanging mass with gravitational force of about 10 N. Right: Block B connected to a string which runs over a pulley to a hand that acts a constant downward tension of about 10 N.
Figure 5.5.2. Two Blocks connected to strings.
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(a)

Before you begin, predict which block you think will have a larger acceleration.
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(b)

Use Force Analysis to determine the acceleration of each block. Sketching free-body diagrams for each object is essential!
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A*R*C*S 5.5.3. The Pair of Blocks.

Blocks A and B are connected by an ideal string via a massless pulley. The coefficient of kinetic friction is \(\mu\text{.}\)
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Block A connected to a string which runs over a pulley to a hanging block.
Figure 5.5.3. Two Blocks connected by a string over a pulley.
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Use the A*R*C*S Steps to determine the acceleration of each block.
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This situation is a particularly good one for special-case analysis: what are some cases you might want to try?
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A*R*C*S 5.5.4. The Pair of Pulleys.

Blocks A and B are connected by an ideal string via two massless pulleys.
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Block A connected to a string which runs over two pulleys before being connected to the ceiling. Block B is suspended from the second pulley.
Figure 5.5.4. Two Blocks connected by a string to two pulleys.
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Use the A*R*C*S Steps to determine the acceleration of each block.
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Hint.
The magnitudes of the blocks’ accelerations are different. How can you relate them?
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