Cycle 2 :: 2017-18
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Benito
Consider africtionless double-Inclined plane AOB placed on a berital surface XY
shown in figure. A glass marble is released from point on plane
Orches height
on plane OB. Now, the plane OB is die sed OC and then to OD and the marble is
released from the same height . The height reached by the ball de
1. maximum in position OB
2. maximum in position OC
3. maximum in position OD
4. same in all the positions of the
plane
VO/
Y
0
lens and its
Answers
Answer:
By observing the motion of objects on an inclined plane Galileo deduced that objects move with a constant speed when no force acts on them. He observed that when a marble rolls down an inclined plane, its velocity increases [Fig.1(a)]. When marble falls under the unbalanced force of gravity as it rolls down and attains a definite velocity by the time it reaches the bottom. Its velocity decreases when it climbs up as shown in Fig. 1(b). Fig. 1(c) shows a marble resting on an ideal frictionless plane inclined on both sides.
Galileo argued that when the marble is released from left, it would roll down the slope and go up on the opposite side to the same height from which it was released. If the inclinations of the planes on both sides are equal then the marble will climb the same distance that it covered while rolling down. If the angle of inclination of the right-side plane were gradually decreased, then the marble would travel further distances till it reaches the original height. If the right-side plane were ultimately made horizontal (that is, the slope is reduced to zero), the marble would continue to travel forever trying to reach the same height that it was released from.
The unbalanced forces on the marble in this case are zero. It thus suggests that an unbalanced (external) force is required to change the motion of the marble but no net force is needed to sustain the uniform motion of the marble. In practical situations it is difficult to achieve a zero unbalanced force. This is because of the presence of the frictional force acting opposite to the direction of motion. Thus, in practice the marble stops after travelling some distance. The effect of the frictional force may be minimised by using a smooth marble and a smooth plane and providing a lubricant on top of the planes.
Explanation:
In his first experiment, Galileo studied the motion of marbles on an inclined plane. He observed that when a marble rolls down an inclined plane, its velocity increases, as shown in Fig. 9.6 (a). Here the marble falls under the unbalanced force of gravity. The velocity of the marble decreases when it rolls up the inclined plane (against the force of gravity), as shown in Fig. 9.6 (b). From these observations, Galileo argued that the velocity of a marble rolling on flat horizontal surface should remain constant.
By observing the motion of objects on inclined planes, Galileo deduce
Fig. 9.6. Motion of a marble (a) down the inclined plane and (b) up the inclined plane.
To test his ideas further, Galileo used double inclined plane. He observed that when the inclinations of the planes on both sides are equal, then the marble rolled down from one plane from certain height will climb to the same height on the other plane [Fig. 9.7 (a)].
By observing the motion of objects on inclined planes, Galileo deduce
Fig. 9.7.
By observing the motion of objects on inclined planes, Galileo deduce
Fig. 9.8. Motion of a marble on a double inclined plane.
If the angle of inclination of the right side plane were gradually decreased, then the marble would travel larger and larger distances to reach the same height [Fig. 9.7 (b) and (c)]. Ultimately, if the right side plane were made horizontal, the marble would continue to travel forever to reach the same height from which it was released. No unbalanced force acts on the marble in this case.
The above experiments suggest that an unbalanced force (external force) is required to change the motion of an object while no unbalanced force is needed to keep an object moving with a constant velocity. Galileo concluded the following law of inertia :
A body moving with a certain speed along a straight line path will continue to move with the same speed along the same straight line path in the absence of external forces.
In actual practice, the bodies stop due to the force of friction which always acts opposite to the direction of motion.
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