Consider a crate at rest on a factory floor. As a pair of workmen begin lifting it, does the support force on the crate provided by the floor increase, decrease, or remain unchanged? What happens to the support force on the workmen’s feet?
ANSWER: Suppose the mass of the crate is “m”. While it was on the floor, it had a weight of “mg” because of the normal reaction. When some people were lifting the crate, the normal reaction changes accordingly. Let’s calculate the normal reaction. STEP 1:- Let’s imagine that the people lifted the crate and the crate accelerated upwards with an acceleration of “a”. The normal reaction the crate got from the hands of the workers is, N mg = ma-------------------(1) N = mg + ma = m(g + a)---------------(2) The normal reaction changed as m(g + a), which was mg before. So the weight also increases. While going up, the weight of an object always increases if it is accelerating upwards. STEP 2:- The support force on the feet of the workmen also increases as the normal reaction increases. CONCLUSION: All this is true when the crate is accelerating upwards by the workmen. When they hold it constantly without doing anything, the normal reaction and the support force remain unchanged.