<p>Recall that when a force tends to act in or against the direction of motion of an object we say that the force is doing work on the object. Specifically, work is defined mathematically in terms of the force and the displacement of the object.</p>
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<h2>Investigation: Is work done?</h2>
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<p>Decide whether or not work is done in the following situations. Remember that for work to be done a force must be applied in the direction of motion and there must be a displacement. Identify which two objects are interacting, what the action-reaction pairs of forces are and why the force described is or isn't doing work.</p>
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<p>Max pushes against a wall and becomes tired.</p>
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<p>A book falls off a table and free falls to the ground.</p>
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<p>A rocket accelerates through space.</p>
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<p>A waiter holds a tray full of plates above his head with one arm and carries it straight across the room at constant speed.</p>
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<p>Max pushes against a wall and becomes tired. Answer: No work is done because there is no displacement.</p>
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<p>A book falls off a table and free falls to the ground. Answer: Yes work is done because there is a displacement in the direction of the force of gravity.</p>
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<p>A rocket accelerates through space. Answer: Yes work is done because there is a net force acting for there to be a net acceleration. If there is an acceleration then there is a displacement.</p>
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<p>A waiter holds a tray full of meals above his head with one arm and carries it straight across the room at constant speed. (Careful! This is a tricky question.) Answer: No work is done because there is no net force in the direction of the displacement.</p>
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<img alt="18c9799dbc1474fea65872e84a149d32.png" src="https://data.ulearngo.com/assets/3aa4f05d-0672-48f7-9b5f-ff7a448ab22d"/>
<p>For each of the above pictures, the force vector is acting in the same direction as the displacement vector. As a result, the angle \(\theta =0°\) because there is no difference in angle between the direction of applied force and the direction of displacement.</p>
<p>The work done by a force can then be positive or negative. This sign tells us about the direction of the energy transfer. Work is a scalar so the sign should not be misinterpreted to mean that work is a vector. Work is defined as energy transfer, energy is a scalar quantity and the sign indicates whether energy was increased or decreased.</p>
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<p>If\(\vec{F}_{\text{applied}}\) acts or has a component acting in the same direction as the motion, then positive work is being done. In this case the object on which the force is applied gains energy.</p>
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<p>If the direction of motion and \(\vec{F}_{\text{applied}}\) are opposite, then negative work is being done. This means that energy is lost and the object exerting the force gains energy. For example, if you try to push a car uphill by applying a force up the slope and instead the car rolls down the hill you are doing negative work on the car. Alternatively, the car is doing positive work on you!</p>
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<h3>Tip:</h3>
<p>The everyday use of the word "work" differs from the physics use. In physics, only the component of the applied force that is parallel to the motion does work on an object. So, for example, a person holding up a heavy book does no work on the book.</p>
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<p>As with all physical quantities, work must have units. Following from the definition, work is measured in \(\text{N·m}\). The name given to this combination of S.I. units is the joule (symbol J). </p>
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Physics is a science discipline concerned with the nature and properties of matter and energy. Physics topics include mechanics, heat, light and radiation, sound, electricity, magnetism, the structure of atoms, and so on.

Physics

Explore the concepts of mechanical energy, potential and kinetic energy, work, power, and conservation of energy through examples like pendulum, roller coaster, and inclined plane.


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