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A student attaches a ruler to a block of wood sitting on a horizontal surface as shown in Fig. 3-20a. The surface exerts a large frictional force on the block. A nail is fastened at the 0-in, mark. The student pulls the rubber band tight at the 5-in . mark; he then pulls it tighter and when it reaches the 8-in. mark, the block of wood just starts to move (Fig. 3-20b). Before the block begins to move, the net force on the block when the rubber band is stretched 7 in. is (A) greater than (B) equal to (C) less than (D) unrelated to the net force on the block when the rubber band is stretched 6 in.

A student attaches a ruler to a block of wood sitting on a horizontal surface as shown in Fig.   3-20a. The surface exerts a large frictional force on the block. A nail is fastened at the 0-in,         mark. The student pulls the rubber band tight at the 5-in . mark; he then pulls it tighter and         when it reaches the 8-in. mark, the block of wood just starts to move (Fig. 3-20b). Before the          block begins to move, the net force on the block when the rubber band is stretched 7 in. is
(A) greater than                                                               (B) equal to
(C) less than                                                         
(D) unrelated to the net force on the block when the rubber band is stretched 6 in.

Grade:upto college level

4 Answers

Navjyot Kalra
askIITians Faculty 654 Points
8 years ago
The correct option is (C) less than.
The block of wood just starts to move when rubber band reaches the 8-in mark. Since the block of wood will not move in between the mark 5-inches to 8-inches, therefore the magnitude of applied force (Fapp) on the block of wood is smaller than the frictional force (f).
So the net force acting on the block of wood (Fnet) when the rubber band is 7-mark is
Fnet = f- Fapp
or,
Fapp = f -Fnet
Therefore the net force on the block when the rubber band is stretched 7 in. is lesser than the net force on the block when the rubber band is stretched 6 in.
From the above observation we conclude that, option (C) is correct.
Paarth Arkadi
22 Points
8 years ago
I think the option is B since the frictional force( static friction )  adjusts itself to the applied force . Hence since the body is at rest  if the applied force is 1N then friction force is 1N , if  applied force is 0.5 N the frictional force is 0.5N  assuming the maximum value( limmiting value ) of force is 2N and the object does is at rest. Since the net force is asked and not the applied force hence the net force remains zero till the block does not move
Paarth Arkadi
22 Points
8 years ago
I think the option is B since the frictional force( static friction )  adjusts itself to the applied force . Hence since the body is at rest  if the applied force is 1N then friction force is 1N , if  applied force is 0.5 N the frictional force is 0.5N  assuming the maximum value( limmiting value ) of force is 2N and the object does is at rest. Since the net force is asked and not the applied force hence the net force remains zero till the block does not move
Paarth Arkadi
22 Points
8 years ago
I think the option is B since the frictional force( static friction )  adjusts itself to the applied force . Hence since the body is at rest  if the applied force is 1N then friction force is 1N , if  applied force is 0.5 N the frictional force is 0.5N  assuming the maximum value( limmiting value ) of force is 2N and the object does is at rest. Since the net force is asked and not the applied force hence the net force remains zero till the block does not move

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