Introduction A force is a push or a pull acting upon an object. When we talk about different types of physical forces we will use the word exert. Exert means put to use or exercise. (For example, all objects exert gravitational forces.) All forces exist as a result of the interaction between objects. These interactions occur on a macroscopic level between objects that can be seen with the naked eye as well as on a microscopic level, between elementary particles of matter. Elementary particles are the smallest structural units (building blocks) of matter. They are very small and can be seen only through a very powerful microscope. There are many types of physical forces. All objects in the universe are usually affected by several different forces at once. This chapter looks at air resistance force. What is air resistance force? Air resistance force is the force of air pushing against a moving object. Air resistance force is a type of frictional force (refer to Chapter 5 of this unit). Like all frictional forces, the force of air resistance always opposes the motion of an object. Usually, the air resistance force is not very strong. When you walk, for example, you are affected by air resistance force, but it does not really slow you down. If you start running, the air resistance force will become more noticeable. Air resistance force depends on the speed and the size of the surface area of a moving object. See Image 1 Combination of different physical forces All objects are affected by several different physical forces at once. The force that is the strongest will define the change of an object. Change might mean a change of shape, position or speed of this object. Now, lets look at an example of the combination of different physical forces, where the air resistance force is very strong. Example: a skydiver falling through the air Object of study: a skydiver Forces affecting an object: chemical bonding forces, gravitational forces, the force of atmospheric pressure and the force of air resistance. Chemical bonding forces All objects are made of very small structural units, called molecules. Molecules hold on to each other with chemical bonding forces. Solid materials have stronger chemical bonding forces than gases and liquids. Gravitation force Gravitation is a force of attraction between all objects in the universe. All objects in the universe are affected by the forces of gravitation. The larger the object is, the stronger its force of gravitation. On Earth, the largest object is the Earth itself, so all objects are attracted or pulled down towards it. In our example, a skydiver falls down through the air towards the Earths surface because he or she is affected by the gravitational force of the Earth, or its gravity. Motion under the influence of gravitational force only is called a free fall. Atmospheric pressure All objects on Earth are affected by the force of atmospheric pressure. The force of atmospheric pressure is created by the weight of air. Air is not the name of a chemical element or gas. Air is a non-scientific term for a mixture of many different gases. The atmosphere is a layer of different gases surrounding our planet. Even though air seems weightless, it does have a weight. The Earths atmosphere is about 500 kilometres deep. That is a lot of air! This air pushes down, with its weight, everything on Earth, creating atmospheric pressure. Air resistance force Air resistance is the result of collisions of an objects surface with air molecules. Air resistance depends on the speed of the object as well as the size of its surface. That is the reason why skydivers have to use a parachute. The word parachute comes from the French words para, protect or shield, and chute, fall. If a skydiver does not use a parachute, he or she will hit the ground at very high speed. A parachute increases the surface area of a skydiver. The terminal speed of the skydiver is the speed at which the downward pull of gravity is balanced by the equal and upward opposing force of air resistance. Summary In our example, the air resistance force was increased by increasing the surface area of a falling object by attaching a parachute. The large surface area of a parachute increases the air resistance force, which slows down the motion of a skydiver, making his landing slow and safe.
Posted on: Wed, 21 Jan 2015 02:00:54 +0000
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