If the same net force is applied to a 7 kg object and a 21 kg object, then

Newton’s second law is a = F/m, so with the same net force, acceleration is inversely proportional to mass. The 7 kg object has one-third the mass of the 21 kg object, so it accelerates three times faster. Therefore, the correct choice is “the 7 kg object accelerates three times faster than the 21 kg object.”

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