Physics-
General
Easy
Question
A U shaped smooth wire has a semi-circular bending between A and B as shown in the figure 10.3. A bead of mass ‘m’ moving with uniform speed v through the wire enters the semicircular bend at A and leaves at B. The average force exerted by the bead on the part AB of the wire is,
- 0
- none of these.
The correct answer is:
Choosing the positive X-Y axis as shown in the figure, the momentum of the bead at A is 
. The momentum of the bead at B is 
.
Therefore, the magnitude of the change in momentum between A and B is
i.e.Dp = 2mv along positive X-axis.
The time interval taken by the bead to reach from A to B is
Dt = 
= 
.
Therefore, the average force exerted by the bead on the wire is
Fav = 
= 
=
Therefore (B) .
Note : 1.By mistake, if consider the change in the magnitude of the momentum will be equal to zero. Then the change in momentum he will attain wrong choice (A) .
2.If someone accounts carelessly d = r instead r = d/2 then he will lead to wrong choice (C) .
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