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انجليزي - عربي
For example, the force acting on a freely falling projectile is the force exerted by Earth on the projectile Fg, and the magnitude of this force is mg . The reaction to this force is the force exerted by the projectile on Earth, Fg = -Fg . The reaction force Fg must accelerate Earth toward the projectile just as the action force Fg accelerates the projectile toward Earth . However , because Earth has such a large mass , its acceleration due to this reaction force is negligibly small .
You directly experience Newton's third law whenever you slam your fist against a wall or kick a football with your bare foot . You should be able to identify the action and reaction forces in these cases .
As another example of Newton's third law , consider the helicopter Most helicopters have a large set of blades rotating in a horizontal plane above the body and another , smaller set rotating in a vertical plane at the back . Other helicopters have two large sets of blades above the body rotating in opposite directions . Why do helicopters always have two sets of blades ? In the first type of helicopter , the engine applies a force to the blades , causing them to change their rotational motion .
According to Newton's third law, however, the blades must exert a force on that engine of equal magnitude and in the opposite direction . This force would cause the body of the helicopter to rotate in the direction opposite the blades. A rotating helicopter would be impossible to control , so a second set of blades is used . The small blades in the back provide a force opposite to that tending to rotate the body of the helicopter , keeping the body oriented in a stable position . In helicopters with two sets of large , counter-rotating blades , engines apply forces in opposite directions so that there is no net forces rotating the helicopter .
As we mentioned earlier , Earth exerts a force Fg on any object . If the object is a Tv at rest on a table , the reaction force to Fg is the force the TV exert on Earth , Fg . The TV dose not accelerate downward, because it is held up by the table . The table , therefore , exerts on TV an upward force n called the normal force . ( It is called the normal force because it is normal , or perpendicular , to the table's surface ) This is the force that prevents the TV from falling through the table ; it can have any value needed , up to the point of breaking the table . The normal force is an elastic force arising from the sag of the table's surface a sag caused by the heavy TV . The heavy TV . The normal force balanced the the weight . the reaction to n is the force exerted by the TV on the table , n . Therefore , we conclude that .
