![]() The force of air resistance is often observed to oppose the motion of an object. The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The friction force is discussed in more detail later on this page. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below: As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Though it is not always the case, the friction force often opposes the motion of an object. There are at least two types of friction force - sliding and static friction. The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. For instance, if a person leans against a wall, the wall pushes horizontally on the person. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. The normal force is the support force exerted upon an object that is in contact with another stable object. (Caution: do not confuse weight with mass.) The force of gravity on earth is always equal to the weight of the object as found by the equation: Fgrav = m * g where g = 9.8 N/kg (on Earth) All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. By definition, this is the weight of the object. ![]() The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. Gravity Force (also known as Weight) F grav The applied force is the force exerted on the desk by the person. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The free body diagram for the bottom of the loop (in the image above) gives us a net force equation of Fnet = Fn - Fg.An applied force is a force that is applied to an object by a person or another object. This means that at the top, the car (and the person in it) experienced an apparent weight that is less than their actual weight, so they felt lighter than they actually are. At the top of the loop, the normal force and gravity were pointing in the same direction, but since the centripetal acceleration in uniform motion is constant, the normal force actually had a smaller magnitude than the gravitational force. Since the normal force is greater than the gravitational force, a person on the car at the bottom of the loop will feel heavier than they actually are. Remember that the normal force contributes to the apparent weight of an object - how heavy the object feels. Although it is not depicted in the picture, the normal force is the net force on the car, so its magnitude is greater than that of the weight of the car to produce a net force. The free body diagram above depicts the roller coaster at the bottom of the loop, where Normal Force is pointed upwards, toward the center of the circle, and the gravitational force is pointed downwards. For example, in the case of a planet orbiting the sun, the gravitational force is the centripetal force, so it will point towards the center of the circle no matter where in the orbit the planet is. The centripetal force will always point toward the center of the circle, but you may be surprised to find that the gravitational force will not always point downwards. From there, you can create your coordinate system and break down any forces that are not aligned with your coordinate system into x and y components. Your positive x or y axis should always be in the direction of the centripetal force, toward the center of the circle. The first step in drawing a free body diagram for an object in uniform circular motion is to select an appropriate coordinate system. Drawing a free body diagram for an object this way can be useful to more specifically analyze the motion of an object. The applied force (F_a) on the box could be a rope that is pulling on the box at the center of its right side, which is why the applied force vector is placed on the right side of the box. The normal force of the floor on the box is acting at the point where the box meets the floor. The weight of the box (W) is acting at the center of mass of the box, where the weight force always acts on an object. In this image, the force of static friction is acting at the point where the box meets the floor, and that is where it is drawn.
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