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What if ... it moving? On a moving object, we have to continuously apply a force to counter the force of friction. Otherwise, the force of friction acting against the force the direction of motion brings the object to rest. of friction disappears in Do you remember doing an activity in an earlier grade where Grade 8 the world? How you found that the force of friction depends upon the nature of Curiosity will the motion the surfaces in contact? Let us carry out a similar activity here. Chapter 5 of objects be Activity 6.1: Let us investigate impacted? 1. Collectfourcoinsof ` 10,onelargestrongrubberbandandanadhesive tape. Locate horizontal surfaces of different materials, such as wooden table top, cemented floor, laminated table top, and polished marble or tiled floor (you may also choose other surfaces). Check that the surfaces are level. 2. Stack the four coins on top of each other and secure them together with an adhesive tape around the sides. 3. Hold the rubber band slightly stretched between your forefinger and thumb on the wooden table top (Fig. 6.12a). Mark points A and B at its ends as shown in Fig. 6.12b. Make another mark C up to which you will stretch the rubber band. C C A B A B (a) (b) (c) Fig. 6.12: (a) Rubber band slightly stretched between your forefinger and thumb, (b) marks A, B, C on the surface, and (c) rubber band stretched back to point C with stack of 4 coins 4. Holding the ends of the rubber band at A and B, place the stack of coins near the middle of A and B. Now, using a finger of your other hand, push back the stack of coins till the rubber band is pulled back to the mark C (Fig. 6.12c). Then, release the stack of coins and observe its motion. Do you find that after losing contact with the rubber band, the velocity of the stack of coins decreases gradually and it comes to rest after travelling some distance? Measure the distance travelled from C and record it. Repeat this step twice. 5. Repeat steps 3 and 4 for laminated table top while ensuring that the points A, B and C are marked at the same distances as earlier. Does the stack of coins travel a larger distance than it did on the wooden table top before coming to rest? Does its velocity decrease more slowly now? 6. Next, repeat step 5 on a horizontal polished marble or tile floor. Does the stack of coins travel an even larger distance and its velocity decrease even more slowly? What conclusion do you draw from your observations? Before you release the stack of coins, it is stationary. It means that the forces acting upon it are balanced. Upon release, the force applied by you 98 Exploration|Grade 9