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Chapter 5: Problem 19

Identify the force present and explain whether work is being (a) You lift a book off the top performed in the following cases: of a desk. (b) Air is compressed in a bicycle pump.

Short Answer

Expert verified
(a) The force present is the applied force (or normal force) that counters gravity acting on the book. Since the applied force and the upward displacement are in the same direction, work is being done on the book as we lift it off the desk. (b) The main force acting here is the applied force by the person compressing the pump. The air molecules' displacement occurs in the same direction as the applied force (inwards). Hence, work is being done on the air as it is being compressed inside the bicycle pump.

Step by step solution

01

Identify the forces

In this scenario, the main force acting on the book is gravity, which pulls the book downwards. When we lift the book, we are applying an upward force (called the applied force or normal force) that counters the force of gravity.
02

Determine the displacement

When we lift the book off the desk, it is being displaced upward. The displacement occurs in the same direction as the applied force we are applying to it.
03

Evaluate if work is being done

In physics, work is defined as the product of force and displacement. When a force is applied on an object and the object gets displaced in the direction of the applied force, then work is being done. In this case, since the applied force and the displacement are both in the same direction (upwards), work is indeed being done on the book as we lift it off the desk. (b) Air being compressed in a bicycle pump
04

Identify the forces

In this case, the main force acting is the applied force by the person who is compressing the pump. This force is directed inwards, which pushes the air molecules closer together, increasing the air pressure inside the pump.
05

Determine the displacement

When the air molecules are being compressed inside the pump, they experience a decrease in volume (i.e., their positions get closer together). This displacement is in the same direction as the applied force.
06

Evaluate if work is being done

Again, work is the product of force and displacement. In this case, the applied force and the air molecules' displacement are both in the same direction (inwards, to compress the air). Therefore, work is being done on the air as it is being compressed inside the bicycle pump.

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Most popular questions from this chapter

Write balanced equations that describe the formation of the following compounds from elements in their standard states, and then look up the standard enthalpy of formation for each substance in Appendix C: (a) \(\mathrm{CH}_{3} \mathrm{OH}(l),\) (b) \(\mathrm{CaSO}_{4}(s),\) (d) \(\mathrm{P}_{4} \mathrm{O}_{6}(s),\) (c) \(\mathrm{NO}(g)\).

Suppose that the gas-phase reaction $2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow\( \)2 \mathrm{NO}_{2}(g)$ were carried out in a constant-volume container at constant temperature. (a) Would the measured heat change represent \(\Delta H\) or \(\Delta E\) ? (b) If there is a difference, which quantity is larger for this reaction? (c) Explain your answer to part (b).

Calculate \(\Delta E\) and determine whether the process is endothermic or exothermic for the following cases: \((\mathbf{a}) q=0.763 \mathrm{~kJ}\) and \(w=-840 \mathrm{~J}\). (b) A system releases \(66.1 \mathrm{~kJ}\) of heat to its surroundings while the surroundings do \(44.0 \mathrm{~kJ}\) of work on the system.

(a) When a 0.47-g sample of benzoic acid is combusted in a bomb calorimeter (Figure 5.19), the temperature rises by \(3.284^{\circ} \mathrm{C}\). When a 0.53 -g sample of caffeine, $\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{~N}_{4} \mathrm{O}_{2}\(, is burned, the temperature rises by \)3.05^{\circ} \mathrm{C}\(. Using the value of \)26.38 \mathrm{~kJ} / \mathrm{g}$ for the heat of combustion of benzoic acid, calculate the heat of combustion per mole of caffeine at constant volume. (b) Assuming that there is an uncertainty of \(0.002^{\circ} \mathrm{C}\) in each temperature reading and that the masses of samples are measured to \(0.001 \mathrm{~g},\) what is the estimated uncertainty in the value calculated for the heat of combustion per mole of caffeine?

Atomic hydrogen (H) is used in welding (AHW). The atoms recombine to hydrogen molecules with a large release of heat according to the following reaction: $$ 2 \mathrm{H}(g) \longrightarrow \mathrm{H}_{2}(g) $$ (a) Using the thermodynamic data in Appendix C, calculate the enthalpy change for this reaction per mole of \(\mathrm{H}_{2}\). (b) Which has the higher enthalpy under these conditions, \(2 \mathrm{H}(g)\) or \(\mathrm{H}_{2}(g) ?\)
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