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A method for determining the speed of a bullet is to shoot it into a block of wood and see how far the block slides on a surface. (See Problem 3.) Assuming (incorrectly) that the magnitude of the work done by friction is equal to the force of friction on the block times the distance the block slides, the calculated value of the bullet velocity will be (A) lower than the actual value, because there will also be a change in the internal energy of the block and surface, (B) higher than the actual value, because there will also be a change in the internal energy of the block and surface, (C) correct, because errors caused by ignoring changes in internal energies are cancelled by the error in the assumption about the work done by friction. (D) wrong, because friction invalidates conservation of momentum as well.

A method for determining the speed of a bullet is to shoot it into a block of wood and see how far the block slides on a surface. (See Problem 3.) Assuming (incorrectly) that the magnitude of the work done by friction is equal to the force of friction on the block times the distance the block slides, the calculated value of the bullet velocity will be
(A) lower than the actual value, because there will also be a change in the internal energy of the block and surface,
(B) higher than the actual value, because there will also be a change in the internal energy of the block and surface,
(C) correct, because errors caused by ignoring changes in internal energies are cancelled by the error in the assumption about the work done by friction.
(D) wrong, because friction invalidates conservation of momentum as well.

Grade:upto college level

1 Answers

Navjyot Kalra
askIITians Faculty 654 Points
8 years ago
The correct option is (B).
The bullet comes to rest in the block, in which the block moves some distance. As the magnitude of the work done by friction is equal to the force of friction on the block times the distance block slides, thus the work done on the block because of friction is 100 % of the energy dissipated because of friction. Therefore the calculated value of the bullet will be higher than the actual value, because there will also be a change in the internal energy of the block and surface. From the above observation we conclude that, option (B) is correct.

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