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Learning Introductory Physics with Activities

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

Section 3.11 Challenge - Laws of Motion

Subsection Explanation Tasks

Explanation 3.11.1. Three Cars - Redux.

Recall Explanation - Three Cars. Answer this question again. As part of your explanation, discuss how your answer, your reasoning, or your understanding has changed since you first thought about this task.
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Explanation 3.11.2. The Angled Block.

In the two situations depicted below, an angled block is at rest on a ramp. In each case, a hand pushes on the block with constant force. In Case A, the hand pushes on the upper horizontal surface of the block. In Case B, the hand pushes on the upper angled surface with a constant force of the same magnitude. In both cases, the block remains at rest on the ramp.
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Figure 3.11.1. A hand pushes on different surfaces on a block on a ramp.
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Part A: Is the magnitude of the net force on the block in Case B greater than, less than, or equal to the magnitude of the net force in Case A?
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Part B: Is the magnitude of the frictional force exerted on the block by the ramp in Case B greater than, less than, or equal to the magnitude of the frictional force in Case A?
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Subsection A*R*C*S Activities

For each A*R*C*S activity, you are expected to complete and label all of the A*R*C*S Steps. For your first A*R*C*S activity, each step will walk you through the A*R*C*S process. A simple example is provided in A*R*C*S Example: Rectangular Park II.
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A*R*C*S 3.11.3. The Block upon the Ice.

You are pulling a small, heavy block of mass \(40 \mathrm{~kg}\) across an icy lake. You pull on the rope at the angle shown in such a way that the normal force on the block by the ice is zero. Find the magnitude of the tension and the magnitude of the acceleration. Assume the gravitational force on the block points downward with a magnitude equal to \(mg\text{,}\) where \(m\) is the block’s mass and \(g = 10 \mathrm{~m/s^2}\text{.}\)
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Figure 3.11.2. A block is pulled across an icy pond.
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1. Analyze and Represent
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  1. Identify known and unknown quantities with both a symbol and a number.
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  2. Identify and justify any assumptions.
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  3. Choose an appropriate system and draw a-free-body diagram for the system.
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2. Calculate
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  1. Represent physics principles that will help you solve for the tension and the acceleration.
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  2. Determine a symbolic equation for each unknown quantity in terms of known variables.
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  3. Plug numbers into your symbolic answer.
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3. Sensemake
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  1. Check the units of your symbolic answer.
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  2. Compare your numerical answers to appropriate numerical quantities with the same units.
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  3. Use covariational reasoning: how should your symbolic equations depend on \(m\) and \(\theta\text{?}\)
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References References

[1]
Tug of War image inspired by teamwork by Lucas Helle from Noun Project (CC BY 3.0)
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