Thus, in practice the marble stops after traveling some distance. This is because of the presence of the frictional force acting opposite to the direction of motion. In practical situations it is difficult to achieve a zero unbalanced force. 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. The unbalanced forces on the marble in this case are zero. 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. 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 inclinations of the planes on both sides are equal then the marble will climb the same distance that it covered while rolling down. 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. 5(c) shows a marble resting on an ideal frictionless plane inclined on both sides. Its velocity decreases when it climbs up as shown in Fig. In the next chapter, you will learn that the marble falls under the unbalanced force of gravity as it rolls down and attains a definite velocity by the time it reaches the bottom. He observed that when a marble rolls down an inclined plane, its velocity increases. However, if this force is removed completely, the object would continue to move with the velocity it has acquired till then.īy observing the motion of objects on an inclined plane Galileo deduced that objects move with a constant speed when no force acts on them. And the change in its speed (or in the direction of motion) would continue as long as this unbalanced force is applied. Thus, to accelerate the motion of an object, an unbalanced force is required. If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion. An object moves with a uniform velocity when the forces (pushing force and frictional force) acting on the object are balanced and there is no net external force on it. It thus appears that an object maintains its motion under the continuous application of an unbalanced force. In order to keep the bicycle moving, we have to start pedaling again. This is again because of the friction forces acting opposite to the direction of motion. ![]() What happens when we ride a bicycle? When we stop pedaling, the bicycle begins to slow down. If the children push the box harder still, the pushing force becomes bigger than the friction force. This is because the friction force still balances the pushing force. 4(b), the children push the box harder but the box still does not move. It balances the pushing force and therefore the box does not move. ![]() This friction force arises between two surfaces in contact in this case, between the bottom of the box and floor’s rough surface. What happens when some children try to push a box on a rough floor? If they push the box with a small force, the box does not move because of friction acting in a direction opposite to the push. 2).įigure-3 Two forces acting on a wooden block ![]() We also know that a force can change the shape and size of objects (Fig. They move because we make a force act on them.įrom your studies in earlier classes, you are also familiar with the fact that a force can be used to change the magnitude of velocity of an object (that is, to make the object move faster or slower) or to change its direction of motion. Pushing, hitting and pulling of objects are all ways of bringing objects in motion (Fig. It can only be explained by describing what happens when a force is applied to an object. ![]() However, we always see or feel the effect of a force. What is it? In fact, no one has seen, tasted or felt a force. The concept of force is based on this push, hit or pull. We ordinarily experience this as a muscular effort and say that we must push or hit or pull on an object to change its state of motion. In our everyday life we observe that some effort is required to put a stationary object into motion or to stop a moving object. Figure-1 Pushing, pulling, or hitting objects change their state of motion.
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