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Chapter 7: Problem 1

Give an example of something we think of as work in everyday circumstances that is not work in the scientific sense. Is energy transferred or changed in form in your example? If so, explain how this is accomplished without doing work.

Short Answer

Expert verified
Holding a heavy object at a constant height, such as carrying a suitcase without moving it, is an example of an everyday task that isn't considered work in the scientific sense. In this case, our muscles use chemical energy (ATP) to generate force and maintain the object's position against gravity, with the energy being transformed into thermal energy as the muscles generate heat. This energy transfer happens without any movement or displacement in the direction of the force (gravity), so it doesn't qualify as work according to the scientific definition.

Step by step solution

01

1. Identify an everyday example not considered work in the scientific sense.

An example of something we think of as work in everyday circumstances but is not considered work in the scientific sense is holding a heavy object at a constant height, like carrying a heavy suitcase without any movement.
02

2. Define work in the scientific sense.

In the scientific sense, work (W) is a transfer of energy that occurs when a force (F) is applied to an object, causing it to move some distance (d) in the direction of the force. Mathematically, work can be expressed as: \( W = F \cdot d \cdot \cos(\theta) \), where \( \theta \) is the angle between the force and the direction of motion.
03

3. Explain why the example is not considered work in the scientific sense.

Holding a heavy object at a constant height does not involve any movement or displacement in the direction of the force (which is gravity, acting downwards). Since there is no movement or displacement in the direction of the force, there is no work being done in the scientific sense, even though our muscles are exerting effort to maintain the object's position.
04

4. Discuss energy transfer or transformation in the example.

In this example, even though work is not being performed in the scientific sense, energy is still involved. Our muscles use chemical energy stored within the body (in the form of ATP) to generate force and maintain the heavy object's position against gravity. This chemical energy is transformed into thermal energy as our muscles generate heat during the exertion.
05

5. Explain how energy is transferred or changed without doing work.

In the example of holding a heavy object without any movement, energy is transferred and changed internally within our body without performing work in the scientific sense. The chemical energy stored in ATP molecules is transformed into thermal energy as our muscles exert force to counteract gravity and maintain the object's position. This transformation of energy does not involve movement or displacement in the direction of the applied force, so it doesn't qualify as work according to the scientific definition.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Energy Transformation
Energy transformation occurs when energy changes from one form to another. This is common in nearly all physical processes around us. For instance, while holding a heavy suitcase steady, your body transforms chemical energy stored in food into thermal energy. This happens even without actual movement of the suitcase.

In daily activities, various forms of energy transformation occur:
  • Chemical energy in food becomes kinetic energy when you move.
  • Electrical energy changes to thermal energy in a heater.
  • Solar energy is transformed into chemical energy by plants through photosynthesis.
Understanding energy transformation helps explain how energy is conserved and why we get tired even while doing tasks that don鈥檛 involve visible motion, like holding something still.
Scientific Work Definition
In physics, the concept of work is precise and may differ from everyday thinking. Scientifically, work is calculated only when a force moves an object over a distance. If there is no movement in the direction of the force, no work is done.

The formula to calculate work is:
\[ W = F \cdot d \cdot \cos(\theta) \]
where:\
  • \( W \) is the work done.
  • \( F \) is the force applied.
  • \( d \) is the displacement.
  • \( \theta \) is the angle between force and displacement.
This definition means holding an object still, like a suitcase, does not qualify as work in the scientific sense, despite the effort involved. The angle is 90 degrees when force and displacement directions differ, resulting in zero work according to the formula.
Force and Displacement
Force and displacement are key elements in the scientific definition of work. Force is any interaction that, when unopposed, changes the motion of an object. Displacement refers to any change in an object's position. When force and displacement act in the same direction, maximum work is done. If perpendicular, as in holding an object still, no scientific work occurs. This explains why holding a suitcase with no movement doesn鈥檛 count as work even if you exert force. Summarizing:
  • Force without displacement = no work.
  • Displacement without force = no work.
  • Force + displacement in parallel = work done.
Understanding this helps clarify why certain tasks, like standing still while carrying something, can be tiring yet not considered work by scientific standards.
Chemical Energy
Chemical energy is vital as it powers many daily activities. Stored in molecules like glucose, it is released through chemical reactions. In activities involving physical exertion, chemical energy is converted internally without causing visible work. For example, holding a suitcase steady uses chemical energy stored in your muscles to generate force. This conversion doesn鈥檛 lead to displacement, yet it transforms chemical energy into thermal energy, which is why you might feel warm or tired.

Key points about chemical energy:
  • Stored in food and fuels.
  • Released during chemical reactions, like digestion or burning.
  • Transforms into other energy forms, such as thermal and kinetic energy.
Understanding the role of chemical energy helps explain why an effort can still consume energy without resulting in scientific work.

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Most popular questions from this chapter

Give an example of a situation in which there is a force and a displacement, but the force does no work. Explain why it does no work.

(a) What is the average useful power output of a person who does \(6.00 \times 10^{6} \mathrm{J}\) of useful work in \(8.00 \mathrm{h}\) ? (b) Working at this rate, how long will it take this person to lift \(2000 \mathrm{kg}\) of bricks \(1.50 \mathrm{m}\) to a platform? (Work done to lift his body can be omitted because it is not considered useful output here.)

(a) Calculate the work done on a 1500 -kg elevator car by its cable to lift it \(40.0 \mathrm{m}\) at constant speed, assuming friction averages \(100 \mathrm{N}\). (b) What is the work done on the lift by the gravitational force in this process? (c) What is the total work done on the lift?

As a young man, Tarzan climbed up a vine to reach his tree house. As he got older, he decided to build and use a staircase instead. since the work of the gravitational force \(m g\) is path independent, what did the King of the Apes gain in using stairs?

Boxing gloves are padded to lessen the force of a blow. (a) Calculate the force exerted by a boxing glove on an opponent's face, if the glove and face compress 7.50 cm during a blow in which the 7.00 -kg arm and glove are brought to rest from an initial speed of \(10.0 \mathrm{m} / \mathrm{s}\). (b) Calculate the force exerted by an identical blow in the gory old days when no gloves were used, and the knuckles and face would compress only \(2.00 \mathrm{cm} .\) Assume the change in mass by removing the glove is negligible. (c) Discuss the magnitude of the force with glove on. Does it seem high enough to cause damage even though it is lower than the force with no glove?
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