[填空题]
A 15-g lead bullet is ty 3ha87wj l)wtested by firing : ;e(wfduyl5n:fc:u2 t y/vbq it into a fixed block of wood with a mass of 1.05 kg. The block and imbedded bullet together absorb all the heat generated. After thermal equilibrium has be::; yc(ebv/f5ul wqt d:uy2fnen reached, the system has a temperature rise measured as 0.020 $C^{\circ} $. Estimate the entering speed of the bullet. m/s
