Physics Test Focus Point PDF

From your everyday experience, you have basic understanding of the concept of force. Any time the state of motion of an object changes, a force has been applied. Force can cause a stationary object to start moving or a moving object to accelerate. You exert force to stretch or compress a spring, to...

From your everyday experience, you have basic understanding of the concept of force. Any time the state of motion of an object changes, a force has been applied. Force can cause a stationary object to start moving or a moving object to accelerate. You exert force to stretch or compress a spring, to throw or kick a ball, to pick your books from a table, to fetch water from a river or well etc. In mechanics, a force is a push or a pull exerted on a body that changes the state of motion of the body. That means force can change the velocity of a body or cause deformation by changing its shape or size. The push or pull on an object can vary considerably in either magnitude or direction. Because a force is determined by both a magnitude and a direction, it is a vector quantity. Forces, like other vectors, are represented by arrows and can be added using the familiar head-totail method or trigonometric methods. Examples of forces include friction force, normal force, and the force of gravity. Contact and Non-Contact Forces forces are acting. Some are applied directly to an object and some act on a body without touching it. Forces that involve physical contact between objects are called contact forces, because the bodies will experience the force while contact is maintained. However, forces that do not involve physical contact between objects are know as non-contact forces. Contact forces are the easiest to understand and include the simple push and pull that are experienced daily in people’s lives. Applied forces, normal forces, frictional forces, and spring forces are the types of contact forces. You might have already an experience that when you bring a piece of iron close to a magnet, but without touching it, the piece of iron will be attracted to the magnet. This magnetic force is one common example of non-contact force. Another example of a non-contact force is the gravitational force between an object and the earth. If you throw the object vertically upwards, it falls back to earth because of this force. Newton’s Laws of Motion Newton’s first law of motion is sometimes called the law of inertia. It states that a body continues to be in its state of rest or of uniform motion in a straight line unless it is acted on by unbalanced force. Newton’s first law of motion explains how inertia affects moving and non-moving objects. Inertia is a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force. In other words inertia is the tendency of an object to resist any attempt to change its velocity. Look at the pool balls in Figure 4.4. When a pool player pushes the pool stick against the white ball, the white ball is set into motion. Once the white ball is rolling, it rolls all the way across the table and stops moving only after it crashes into the cluster of colored balls. Then, the force of the collision starts the colored balls moving. Some may roll until they bounce off the raised sides of the table. Some may fall down into the holes at the edges of the table. None of these motions will occur, however, unless that initial push of the pool stick is applied. Newton’s first law of motion defines a special set of reference frames called inertial frames. An inertial frame is defined as one in which Newton’s first law of motion (also called the law of inertia) is valid. Such a reference frame is called an inertial frame of reference. If an object does not interact with other objects, it has zero acceleration in an inertial frame of reference. Any reference frame that moves with constant velocity relative to an inertial frame is itself an inertial frame. Mass is a measure of the resistance of an object to change in its state of motion. Mass is an inherent property of an object and is constant everywhere. However, weight is the magnitude of the gravitational force acting on an object and can change from one place to another. Objects with large masses have large inertia and are more resistant to changes in their state of motion. Newton’s Second Law of Motion We have discussed Newton’s first law which explains that an object either remains at rest or moves in a straight line with constant speed when there is no unbalanced force acting on it. But what happens to an object when there is nonzero unbalanced force acting on it? This question is answered by Newton’s second law of motion. To hold an object in your hand, you have to exert an upward force to oppose, or "balance," the force of gravity. If you suddenly remove your hand so that the only force acting on the object is gravity, it accelerates downward. This is one example of Newton’s second law, which states, basically, that unbalanced forces or net external force causes nonzero acceleration. When you exert some horizontal force F on the block, it moves with some acceleration a. If you apply a force twice as great, you find that the acceleration of the block doubles. If you increase the applied force to 3F, the acceleration triples, and so on. From such observations, we conclude that the acceleration of an object is directly proportional to the force acting on it. The acceleration of an object is directly proportional to the force acting on it. Thus, the greater the mass of an object, the less that object accelerates under the action of a given applied force. Mathematically, Newton’s second law can be expressed as F~ = m~a where F~ is the force acting on the body, m is the mass of the body , and ~a is the acceleration when acted on by the force F~ From the above equation, you can see that the unit of force newton can be expressed in terms of the units of mass, length, and time. 1 N = 1 kgm/S(square) Therefore, 1 N is defined as the force that, when acting on an object of mass 1 kg , produces an acceleration of m/s(square). At this stage, you may be able to guess the gravitational force F~g acting on an object by applying Newton’s second law. However, here, ~a is the acceleration due to gravity. The acceleration due to gravity is denoted by ’g’ and has a constant value of 9.8 m/s 2 on the surface of the earth and is directed towards the center of the earth. Hence, applying Newton’s second law, the magnitude of the gravitational force on an object is given by: Fg = mg This force is directed towards the center of the earth. Fg is also called weight of the object. Weight of a body is represented by ’W’. The above equation can also be written as: W = mg  Newton’s Third Law of Motion Have you ever pushed a wall or your table with your finger tip? Let you do it again in a class. What do you feel? If you push a wall with your hand with some force, the wall pushes your hand back with the same force. If you push the wall harder, the wall pushes you back with larger force and you may feel pain in your hand. This simple activity illustrates an important general principle known as Newton’s third law of motion. In fact, the force with which you push the car is equal in magnitude but opposite in direction to the force with which the car pushes you back as shown in Figure 4.6. In general, if F12 is the force exerted by object 1 on object 2 and F21 is the force exerted by object 2 on object 1, then, F12 = −F21 That means F12 is equal in magnitude but opposite in direction to F21. F12 is called action force and F21 is called reaction force, though either force can be labeled the action or reaction force. Consider a block of mass m placed on a horizontal table. The action and reaction forces are represented by F12 and F21. Let F12 be the force that the block exerts on the table. This force is equal to the weight of the block. F12 is directed vertically downwards as shown in the figure. On the other hand, the table exerts a force F21 on the block. F21 is directed vertically upwards, but has the same magnitude as F12. F21 is also called normal force. However, it has to be noted that taking F21 as action force and F12 as reaction force is also possible. NewtonâA˘Zs third law of motion states that every action has an equal ´ and opposite reaction. This means that forces always act in pairs. Key Concept: The action and reaction forces are equal in magnitude but opposite in direction and they act on different objects.