4. A 10-kg block on a horizontal, frictionless surface is attached to a light spring (force constant = 1.2 kN/m). The block is initially at rest at its equilibrium position when a force (magnitude P) acting parallel to the surface is applied to the block, as shown. When the block is 8.0 cm from the equilibrium position, it has a speed of 0.80 m/s. How much work is done on the block by the force P as the block moves the 8.0 cm? (A) 8.3 J (B) 6.2 J (C) 7.0 J (D) 9.7 J (E) 3.6 J

7. Angie says that an object is in equilibrium if the net torques about the center of mass is zero. Robbie says an object is in equilibrium if the sum of external forces is zero. Which one, if either, is correct? (A) Both are correct: an object is in equilibrium if either condition holds. (B) Neither is correct: both conditions must hold simultaneously. (C) Neither is correct: the net external force and the net external torque about any axis must be zero. (D) Neither is correct: an object is in equilibrium only if its velocity is zero in all coordinate systems. (E) Both are correct: if the sum of the external forces is zero, the net torque about any axis is automatically zero, and vice versa.

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