Physics Past Paper - Elasticity, Fluid Flow, Surface Tension PDF

# الباب الرابع: المرونة ## Elasticity - If a force or a group of forces acts on a fixed (stationary) object, it often causes a change in its dimensions or its geometric shape in general. - If the acting force is removed, and the object regains its original state, then the object is said to be p...

# الباب الرابع: المرونة ## Elasticity - If a force or a group of forces acts on a fixed (stationary) object, it often causes a change in its dimensions or its geometric shape in general. - If the acting force is removed, and the object regains its original state, then the object is said to be perfectly elastic. - If the object does not regain its original shape to any degree after the acting force is removed, then the body is said to be imperfectly elastic, meaning that it is inelastic, that is, it is a plastic body. - Solid bodies, under the influence of external forces, fall between these two properties, elasticity and plasticity, in their response to change. ## 1- Stress - Defined as the force acting on a unit area. Its units are 2.Nm. Stress is of several types: - Tensile Stress: results in an increase in length. - Compression Stress: results in a decrease in length or change in volume. - Shear Stress: results in a change in the shape of a geometric body, without a change in volume. ## 2- Strain - Refers to the relative response of the object to the acting stress. Just as stress has three types, strain, accordingly, has three corresponding types. There is longitudinal strain, which is the change in length relative to the original length. There is volumetric strain, which is the change in volume relative to the original volume (V). And there is shear strain, which is tan p , where p is called the shear angle. Strain has no units because it is a ratio. - **Relationship between stress and strain for ductile metals.** - If we act on a ductile wire that is fixed at one end with a continuously increasing direct stress, and plot the relationship between the direct stress and the resulting strain, we obtain the diagram (1-4). The following is noted from this diagram: - Stress is directly proportional to strain, until it approaches point E, which is called the elastic limit. - If the stress is removed before reaching point E, the wire returns to its original length and cross-section, meaning the wire is fully elastic in this region, and its behavior follows Hooke’s Law (as will be discussed later). # الباب الخامس: انسياب الموائع ## Flow of fluids - A fluid is a substance that does not have a definite shape, but takes the shape of its container. It is characterized by its ability to flow, therefore the term encompasses liquids and gases. - Fluids are divided into two classes: - Compressible Fluids: These are fluids whose density changes with the changing pressure acting on it, such as gases. - Incompressible Fluids: These are fluids whose density does not change with the changing pressure acting on them such as liquids. - **Steady and Unsteady Flow** - There are two types of flow: In the first type, which is steady flow, the velocity of the fluid at a given point is constant and does not change with time. The velocity can change from point to point based on cross-section of the tube. The second type is unsteady flow, in which the velocity will change at the same point from one moment to another. - **Streamline and Turbulent Flow** - In streamline flow, each part of the fluid has a specific path, and the paths do not intersect – the opposite of what happens in turbulent flow, where the paths intersect. - **Path lines or stream lines** - A path line is an imaginary line inside the fluid that provides a tangent to the flow direction at any point. - **Tube of flow** - We imagine a tube of flow inside the fluid, in the case of steady flow, we imagine a small area perpendicular to the direction of flow (S2), we draw a streamline of the fluid passing through a point on the periphery of this area, until it reaches the other cross-section (S1). This makes what is called the tube of flow, as in diagram (1-5), which is a hypothetical tube whose walls represent the stream lines. A characteristic of the tube of flow is that the fluid does not penetrate its walls, because a tangent to the wall, at any point should be the direction of flow at that point # الباب السادس: التوتر السطحي ## Surface tension - Attractive forces between the molecules of a fluid are short-range in nature, with a range of about 1.6 x 10^-9 cm. Therefore, any molecule inside the fluid is subjected to attractive forces from the molecules surrounding it from all sides, and these are located only within a circle of action. This means that the resultant of these forces is zero. However, for molecule B on the surface, as in diagram (1-1), the number of attractive molecules in the lower half of the circle of action is larger than the number of molecules in the upper half. Therefore, molecule B is subjected to a force pulling down. Similarly, all surface molecules of the fluid are subjected to a similar force in the direction of the fluid. - Briefly, the surface of the fluid tends to contract, which leads to an increase in the surface free energy of the fluid. - **Capillary action** - If we put a quantity of water in a glass container, we notice that the surface of the water near the point of contact between the water and container walls rises a little, until it completely adheres to the walls, as in the diagram. This means that the attraction between the glass and the water is greater than the attraction between the molecules of the water. On the other hand, if we put a quantity of mercury in a glass container, as in the diagram, we notice that the surface of the mercury drops in the region of the contact between it and the glass wall. If we draw a tangent to the surface of the liquid at the point of contact between it and the solid surface. <start_of_image> Diagrams are not included as I am not an image-processing tool.