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A body can be subjected to more than one force at a time. We must be careful while applying Newton's second law. The vector addition of forces should include all forces, which act on the body.
However, the list of force vectors, being added, must exclude the forces that the body applies on other bodies. Also, internal forces such as the intermolecular forces are not considered as internal forces can not change the velocity of the body. For example, we can not raise ourselves in air, using internal muscular force.
Generally we associate force with acceleration. This is, however, not always the case. When the object is a rigid body and free to move, then indeed acceleration is the only outcome of the application of force. However, if the object is not a rigid body, then a part of force may be used to deform the object and a part of it may be used to accelerate the object. If the body is constrained to move as well, then only deformation will result.
Force and its effect are current measurements at a particular position. What it means that if we apply a force of 2 N on a body of mass 2 kg for 1 second, then force will produce an acceleration of 1 (as given by F=ma) exactly for 1 second. The moment force is removed, the acceleration of the body becomes zero. This is the case with a projectile. We need a force to impart an initial velocity. For the rest of flight when the projectile is airborne, there is only gravity (neglecting air resistance) that determines the acceleration of the projectile.
A bob of mass 0.1 kg is hung from a mass-less string of 1 m length. The bob is set in oscillation. Then, the string is cut and the trajectory of the bob is observed. The bob takes different trajectory depending on the position of bob when the string is cut. It is found that the path is parabolic for the position(s) of bob :
(a) at mean position
(b) at extreme position
(c) while it is moving up towards extreme position
(d) while it is moving down towards mean position
Force has no past or future. The moment the string is cut, there is no tension force on the bob. Only gravity acts on the bob. If the bob is at extreme position, then its velocity is zero and there is downward acceleration due to gravity. As a result, the bob falls vertically down.
Path of pendulum bob
In all other cases, the bob has certain velocity. Its direction is horizontal at mean position. It makes certain angle with horizontal when bob is moving either towards extreme position or mean position. In all these cases, velocity is in a direction which is not vertical. The bob is worked down by gravity acting downward. As such, there is downward acceleration due to gravity. This results in a parabolic trajectory.
Hence, options (b), (c) and (d) are correct.
Study of force and related phenomena is facilitated with certain classifications. One of the basic classifications is based on our experience with force. In our day to day life, we experience force when objects or particles are in contact. At the same time, we are aware that there are gravitational and electromagnetic forces, which come in to being without contact. These forces can change velocity of an object acting from a distance.
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