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Forces Force – A push or a pull upon an object resulting from the object's interaction with another object. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. A force causes an object to move, change speed, change direction, or stop. People often think of force as something you apply using your muscles. When you push or pull on an object, you apply a force on it. You also apply force when you throw a baseball or kick a soccer ball, or sit on a soccer ball. These forces are examples of contact forces – are those types of forces that result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces. Field forces, or non-contact forces, are another class of forces. These forces do not involve physical contact between the agent and the receiver, but act through space. The force of gravity, namely the gravitational attraction between two masses such as the earth and you, or the sun and the planets, or between stars, is one such force. Another is the electric force, often observed as static electricity, which causes objects that are similarly charged to repel each other (and oppositely charged ones to attract). Yet another is the magnetic attraction between magnets and steel. Examples of contact and field forces are listed in the table below. Contact Forces Frictional Force Tension Force Normal Force Air Resistance Force Applied Force Spring Force Action-at-a-Distance Forces Gravitational Force Electrical Force Magnetic Force Type of Force Description of Force (and Symbol) Applied Force Fapp An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person. Gravity Force (also known as Weight) Fgrav The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. 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) and m = mass (in kg) Normal Force Fnorm The normal force is the support force exerted upon an object that is in contact with another stable object. 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. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person. Friction Force Ffrict The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force sliding and static friction. Though it is not always the case, the friction force often opposes the motion of an object. 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. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below: Ffrict = µ • Fnorm Air Resistance Force Fair The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas. Tension Force Ftens The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. Spring Force Fspring The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an "N." To say "10.0 N" means 10.0 Newton of force. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated: 1 Newton = 1kg m s2 A force is a vector quantity, which means it has both a magnitude (size) and direction. Because a force is a vector that has a direction, it is common to represent forces using diagrams in which a force is represented by an arrow. The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction that the force is acting. (Such diagrams are known as free-body diagrams) Furthermore, because forces are vectors, the effect of an individual force upon an object is often canceled by the effect of another force. For example, the effect of a 20-Newton upward force acting upon a book is canceled by the effect of a 20-Newton downward force acting upon the book. In such instances, it is said that the two individual forces balance each other; there would be no unbalanced force acting upon the book. Other situations could be imagined in which two of the individual vector forces cancel each other ("balance"), yet a third individual force exists that is not balanced by another force. For example, imagine a book sliding across the rough surface of a table from left to right. The downward force of gravity and the upward force of the table supporting the book act in opposite directions and thus balance each other. However, the force of friction acts leftwards, and there is no rightward force to balance it. In this case, an unbalanced force acts upon the book to change its state of motion. If either all the vertical forces (up and down) do not cancel each other and/or all horizontal forces do not cancel each other, then an unbalanced force exists. The existence of an unbalanced force for a given situation can be quickly realized by looking at the free-body diagram for that situation. Free-body diagrams for three situations are shown below. Note that the actual magnitudes of the individual forces are indicated on the diagram. In each of the above situations, there is an unbalanced force. It is commonly said that in each situation there is a net force acting upon the object. The net force is the vector sum of all the forces that act upon an object. That is to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out. At this point, the rules for summing vectors (such as force vectors) will be kept relatively simple. Observe the following examples of summing two forces: A net force (i.e., an unbalanced force) causes an acceleration. Combine your understanding of acceleration and the newly acquired knowledge that a net force causes an acceleration to determine whether or not a net force exists in the following situations. GRAVITY Newton realized that everything pulls on everything else. He discovered that a force of gravity acts between all things–involving only mass and distance. Every mass attracts every other mass with a force that is directly proportional to the product of the two interacting masses. This statement is known as the Law of Universal Gravitation. Expressed in symbol shorthand, m1 and m2 are the masses, and d is the distance between their centers. Thus, the greater the masses m1 and m2, the greater the force of attraction between them. The greater the distance of separation d, the weaker is the force of attraction–weaker as the inverse square of the distance between their centers. Gravitational Force – attraction between two or more objects that have mass. a. Always attracts objects, never repels. b. The closer the objects are, the stronger the force. Gravity gets weaker with distance the same way a light gets dimmer as you move farther away from it. The greater the distance from the Earth’s center, the less the gravitational force on an object. In using Newton’s equation for gravity, the distance term d is the distance between the centers of the masses of objects attracted to each other. Note in Figure 7.6 that the girl at the top of the ladder weighs only 1/4 as much as she weighs at the Earth’s surface. That’s because she is twice the distance from the Earth’s center. c. The more massive the object, the stronger the force. d. Has the ability to occur over great distances. Mass is the amount of stuff (atoms or molecules) used to make-up some piece of matter. Mass is the amount of matter in an object. We can measure the force of gravity. The amount of gravitational force between two objects is called Weight. Weight is the force due to gravity that acts on an object’s mass. Mass remains the same regardless of location, but weight varies with location due to changes in the gravitational force. If the gravitational force cause weight than the following is true: Weight is Force (of gravity) and Force (of gravity) is Weight Suppose you tie a string around a 2-pound bag of sugar and hang it on a weighing scale similar to the scales found in grocery stores. A spring in the scale stretches until the scale reads 2 pounds. The stretched spring experiences a “stretching force” called tension. The same scale in a science lab is likely in units of newtons. This scale will show the weight of the bag of sugar as 9 newtons rather than 2 pounds. Both pounds and newtons are units of weight. Units of weight in turn are units of force. The bag of sugar is attracted to the Earth with a gravitational force of 2 pounds–or equivalently, 9 newtons. We measure all forces in a unit called NEWTONS. 1 Newton = ¼ pound. 1 Kg = 9.8 N A book lies at rest on a desk. What forces act on the book to make up this zero net force? One is the force of gravity–the weight of the book. Where is the upward force coming from? It is coming from the desk that is supporting the book. We call this upward force the support force, or the normal force. * The support force must equal the weight of the book. Hold a stone above your head and drop it. It accelerates during its fall. When air resistance doesn’t affect the motion of a falling object, we say the object is in free fall. Interestingly, the amount of acceleration is the same for all freely falling objects in the same vicinity. We find that a freely falling object gains speed at the rate of 9.8 m/s each second (~10 m/s per second). In the figure to the right, we imagine a freely-falling boulder with a speedometer attached. As the boulder falls, the speedometer shows that the boulder goes 10 m/s faster each second. This 10 m/s gain each second is the boulder’s acceleration. To calculate the force of gravity on an object we use the equation: F = mg. Force (weight) = mass x acceleration of gravity. F m g F = Force (N) m = mass (Kg) g = acceleration due to gravity (9.8m/s2 on earth) All free falling objects have the same acceleration. The weight of a 1-kg stone (or 1 kg of anything) is 10 N at the Earth’s surface. The weight of 10 kg of matter, such as the boulder, is 100 N. The force acting on a falling object is its weight. The acceleration of the stone is and for the boulder, For free fall the downward net force is weight. Only weight! But when air is present, the downward net force = weight – air drag. What happens to the net force if air drag builds up to equal weight? Then acceleration becomes zero. Does this mean the object comes to a stop? No! What it means is the object no longer picks up speed. We say the object has reached terminal velocity. Tides 1. The revolution of the moon around the Earth and the rotation of the earth cause the tides a. Due to the moons gravity 2. There are two high and two low tides per day 3. Although the Sun is bigger the moon has a greater effect on tides due to its proximity a. Sun is too far and does not have as great of an effect as the moon but still does have some effect. The part of the Earth beneath the crust is molten–fluid. Because of this we have Earth tides–actual rises and falls in the Earth’s crust. Earth tides, however, are much smaller than ocean tides. There are also atmospheric tides. These regulate the cosmic rays that reach the Earth’s surface. The greatest fluctuation of tides between high and low (ocean, Earth, or atmospheric) occurs during the alignments that make a new and full moon. The fluctuations in atmospheric tides produce changes in the intensity of cosmic rays reaching the Earth’s surface–which in turn affects some life forms.