Một mẫu phóng xạ có hoạt độ ban đầu là 10 mCi. Sau khoảng thời gian 4 giờ, hoạt độ của mẫu còn lại 8 mCi.
(a) Tính hằng số phân rã và chu kỳ bán hủy của đồng vị phóng xạ trong mẫu.
(b) Có bao nhiêu nguyên tử hạt nhân có trong mẫu lúc đầu?
(c) Tính hoạt độ của mẫu sau khi phân rã được 30 giờ?

Boxing gloves are padded to lessen the force of a blow. (a) calculate the force exerted by a boxing glove on an opponent’s face, if the glove and face compress 7.50 cm during a blow in which the 7.00-kg arm and glove are brought to rest from an initial speed of 10.0 m/s. (b) calculate the force exerted by an identical blow in the gory old days when no gloves were used and the knuckles and face would compress only 2.00 cm. (c) discuss the magnitude of the force with glove on. does it seem high enough to cause damage even though it is lower than the force with no glove?

As the distance from a magnet increases, what happens to the magnet force?

Suppose that three astronomical objects, 1, 2, and 3 are observed to lie on a line, and the distance from object 1 to object 3 is d. object 1 has four times the mass of object 3, and seven times the mass of object 2. find the distance between objects 1 and 2 for which the net force on object 2 is zero.