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it is about the forces that fluids exert on objects in them or moving through them. Unlike solids, liquids and gases can flow and change shape quickly and easily without separating, so they are classified as fluids. The fluids we are most concerned about in sport biomechanics are air and water. Air is the medium we move through in all land-based sports and human activities, and water is the medium we move through in all aquatic sports and activities. • Two types of forces are exerted on an object by a fluid environment: a buoyant force due to the object’s immersion in the fluid and a dynamic force due to its relative motion in the fluid.
Buoyant Force: Force Dueto Immersion • Two types of forces are exerted on an object by a fluid environment: a buoyant force due to the object’s immersion in the fluid and a dynamic force due to its relative motion in the fluid. The dynamic force is usually resolved into two components: drag and lift forces. The buoyant force, on the other hand, always acts vertically. A buoyant force acts upward on an object immersed in a fluid. You are probably most familiar with this principle in connection with objects in water.
Fill a bathtub or a deep sink with water. Take a basketball (or any other large inflatable ball) and try to push it into the water. Now try it again with a tennis ball (or racquetball). Which is easier to push under water? It’s easier to push the tennis ball under water. You may also notice that the farther you push the ball into the water, the larger the force you have to push with. The force that pushed upward on the tennis ball or basketball was buoyant force. Buoyant force seems to be related to the size of the object immersed in water and to how much of the object is immersed.
The size of the buoyant force is equal to the weight of the volume of fluid displaced by the object. • Specific Gravity and Density • Whether or not something floats is determined by the volume of the object immersed and the weight of the object compared to the weight of the same volume of water. Specific gravity is the ratio of the weight of an object to the weight of an equal volume of water. Something with a specific gravity of 1.0 or less will float. Another measure that can be used to determine if a material will float is density. Density is the ratio of mass to volume. The density of water is about 1000 kg/m3. The density of air is only about 1.2 kg/m3.
Buoyancy of the Human Body • Muscle and bone have densities greater than 1000 kg/ m3 (specific gravities greater than 1.0), whereas fat has a density less than 1000 kg/m3 (specific gravity less than 1.0). These differences in density are the basis for the underwater weighing techniques used to determine body composition.
Someone who has low body fat can still float, because the lungs and other body cavities may be filled with air or other gases that are much less dense than water.
Dynamic Fluid Force: ForceDue to Relative Motion • Buoyant force is the vertical force exerted on an object immersed in a fluid. It is present whether the object is at rest or is moving relative to the fluid. When an object moves within a fluid (or when a fluid moves past an object immersed in it), dynamic fluid forces are exerted on the object by the fluid. The dynamic fluid force is proportional to the density of the fluid, the surface area of the object immersed in the fluid, and the square of the relative velocity of the object to the fluid. Equation summarizes this relationship:
The dynamic fluid force is proportional to the density of the fluid, the surface area of the object immersed in the fluid, and the square of the relative velocity of the object to the fluid. • The dynamic fluid force that results from motion within a fluid is commonly resolved into two components: the drag force and the lift force.
Drag Force • Drag force, or drag, is the component of the resultant dynamic fluid force that acts in opposition to the relative motion of the object with respect to the fluid. A drag force will tend to slow down the relative velocity of an object through a fluid if it is the only force acting on the object. The size of the drag force is thus proportional to the acceleration (slowing down) of the fluid molecules as they pass the object, as well as to the mass of the molecules that are slowed down. The greater the decrease in velocity of the molecules and the faster the rate of this decrease, the greater the total drag.
Drag forces are produced by two different means: surface drag and form drag. Surface drag may be thought of as equivalent to the sum of the friction forces acting between the fluid molecules and the surface of the object (or between the fluid molecules themselves). Form drag may be thought of as equivalent to the sum of the impact forces resulting from the collisions between the fluid molecules and the object.
Surface Drag Surface drag is also called skin friction or viscous drag. As a fluid molecule slides past the surface of an object, the friction between the surface and the molecule slows down the molecule. On the opposite side of this molecule are fluid molecules that are now moving faster than this molecule, so these molecules are also slowed down as they slide past the molecules closest to the object. These molecules, in turn, slow down the molecules next to them. So the surface drag is proportional to the total mass of the molecules slowed down by the friction force and the average rate of change of velocity of these molecules.
A projectile is an object upon which the only force acting is gravity. There are a variety of examples of projectiles. An object dropped from rest is a projectile (provided that the influence of air resistance is negligible). An object that is thrown vertically upward is also a projectile (provided that the influence of air resistance is negligible). And an object which is thrown upward at an angle to the horizontal is also a projectile (provided that the influence of air resistance is negligible). A projectile is any object that once projected or dropped continues in motion by its own inertia and is influenced only by the downward force of gravity.
By definition, a projectile has a single force that acts upon it - the force of gravity. If there were any other force acting upon an object, then that object would not be a projectile.
Trajectory- the path taken by a projectile is called the trajectory. • Range- the horizontal distance covered by the projectile is called the range. • Angle of release- the angle at which a projectile is released or projected in the air is called the angle of release. It may also be defined as the acute angle between the horizontal and the velocity vectors at the instant of release. • Height of Projectile- the maximum vertical distance above the ground that a projectile reaches during its flight path is called the height of projectile.
Relative height of release- the vertical distance between the point of release and the point of landing is called Relative height of release. This can be positive or negative. If the point of release is higher than the point of landing the Relative height of release is taken as positive. If it is taken as negative when the point of landing is higher that the point of release. This is simply called the height of release. • Time of flight- the total time taken by a projectile to complete its trajectory is called the time of flight or the time for which is projectile remains in air.
Time of Ascent- the time taken by a projectile to reach the peak height of its trajectory is called the time of Ascent. • Time of Descent- the time taken by a projectile to land from the peak height is called the time of ascent.
To study the behaviour of projectile, the effect of air resistance is excluded or neglected, but this factor considered later. • When the release and landing are the same level • Examples- a football kicked above the ground • A golf ball hit for a long shot • A hockey ball hit in a rising shot • Certain gymnastic exercises.
Characteristics • Time of ascent is equal to the time of descent
In situation where range is the major consideration, an object or implement should be released at 45 degree.
