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We see that the friction is caused by friction and you find it somewhat easier the interlocking of irregularities in the to move the box already in motion than two surfaces. It is obvious that the force to get it started. of friction will increase if the two surfaces are pressed harder. You can 9.3 Friction : A Necessary experience it by dragging a mat when Evil nobody is sitting on it, and when a Recall now some of your experiences. Is preson is sitting on it. it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy, or has a thin layer of cooking oil on it; would it become easier or more difficult to hold it? Just think : would it be possible to hold the glass at all if there is no friction? Recall also how difficult it is to move on a wet muddy track, or wet marble floor. Can you imagine being able to walk at all if there were no friction? You could not write with pen or pencil if there were no friction. When Fig. 9.6 :You have to push on the box to keep it moving your teacher is writing with chalk on the blackboard, its rough surface rubs Recall your experience when last time off some chalk particles which stick to you moved a heavy box from one place to another (Fig. 9.6). If you have no such experience, get that experience now. What is easier — to move the box from rest, or to move it when it is already in motion? The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. When the box starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding Fig. 9.7 : A nail is fixed in the wall due to friction is slightly smaller than the static friction FRICTION 115 2024-25