Question

3)A bullet hits a hanging plate target at rest. The bullet gets stuck on the target and the plate's center of mass gets raised to a height h. Consider no friction at the hanging point of the target. which statement is true?

a)The initial kinetic energy of the bullet is less than the gravitational potential energy of the plate at height h. b) The initial kinetic energy of the Buller is greater than the gravitational potential energy of the plate at height h. c) The initial kinetic energy of the bullet is the same as the gravitational potential energy of the plate at height h.

4)A disk with mass 20 g is rotation about its perpendicular axis through its center. A bug with mass 5g is on the disk midway between the axis and the rim. The bug takes off gently. Which of the statements Is true after the bug leaves the disk?

a) The angular momentum of the system decreases and its angular speed increases. b)The angular momentum of the system increases and its angular speed increases. c) The angular momentum of the system increases and its angular speed decreases. d)The angular momentum of the system decreases and its angular speed decreases. e)The angular momentum of the system remains constant and its angular speed increases. f) The angular momentum of the system remains constant and its angular speed decreases.

Answer #1

3. Ans : initial kinetic energy of bullet is greater than the potential energy of the target at height h.

Since the bullet stuck on the target and moves with it after collision, the collision is perfectly inelastic.

Hence kinetic energy is not conserved. Initial kinetic energy is greater than final kinetic energy. Hence greater than the gravitational potential energy at height h.

4. Ans: Angular momentum of the system remains constant and angular speed increases.

Since there is no external torque on the system , angular momentum should be conserved. When the bug flies off, total moment of inertia decreases,so angular velocity should be increased to keep angular momentum a constant.

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