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Get Full Access to Conceptual Physics - 12 Edition - Chapter 5 - Problem 15e
Get Full Access to Conceptual Physics - 12 Edition - Chapter 5 - Problem 15e

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# When the athlete holds the barbell overhead, the reaction ISBN: 9780321909107 29

## Solution for problem 15E Chapter 5

Conceptual Physics | 12th Edition

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Problem 15E

When the athlete holds the barbell overhead, the reaction force is the weight of the barbell on his hand. How does this force vary for the case in which the barbell is accelerated upward? Downward? Step-by-step solution

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ANSWER: STEP 1:- To answer the question let’s have a look on this example. When you are standing in an elevator and the elevator is not moving at all, then you apply a force downward on the elevator’s floor. At the same time the floor pushes you with the same force which is called the normal force. These two forces balance each other so you are in equilibrium. This normal force is defined as the weight of the object in physics. STEP 2:- Now imagine when the elevator is moving upwards with an acceleration of “a”. So, you push the elevator downwards with a force of “mg”, and you are getting some normal reaction “N”. by the same time you are accelerating upwards. So, the equation will be, N mg = ma-----------------(1) N = mg + ma = m(g + a). So you can see here the weight increased while accelerating upwards. STEP 3:- But when you are accelerating downwards with an acceleration of “a”, then the equation will look like, mg N = ma-------------------(2) N = mg ma = m(g a) From the above equation we can see that the weight or normal reaction gets decreased while it is accelerating downwards. When the object is in free fall, then a=g and the weight will be, W = N = m(g g) = 0. This is the condition for weightlessness. All these are true for the athlete holding the mass.

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