21. We connect an object of unknown mass to a horizontal ideal spring on a smooth surface. The object is oscillating with period of 0.2 s. It is known that the force constant of the spring is 100 N/m. Find the mass of the unknown object. 22. Suppose we have a cylinder of volume V = 1.0 m with a piston, filled with ideal gas. The pressure in the cylinder is p = 2.0 x 105 Pa. If the gas expands in an isothermal process with constant temperature T = 300 K, from initial volumn V = 1.0 m to the final volumn V = 2.0 m'. The universal gas constant R = 8.31 J/(molK). Find the heat Q absorbed by the ideal gas. O *=0 Fig. 1 23. Two charged particles are placed as shown in Fig. 1. q, = 0.15/C is located at the origin, and 42 = 0.35uC is located on the positive x-axis at x2 = 0.40 m. Where should a third charged particle, 93, be placed to be at an equilibrium point (such that the forces on it sum to zero)? (Coulomb's constant is k = 8.99 x 109 Nm2/C2) 24. Consider a battery that has potential difference Vt = R 1.5 V across its terminals when it is not connected to a circuit. When connected to an external resistor with R = 2.6 0, the potential difference between the two terminals B A and B reduces to V,' = 1.3 V . Find the internal resistance of the battery. R Fig. 2