Chapter 7, Problem 50A

When a weightlifter jerks a barbell over his head, is the force exerted on the barbell more than, less than, or equal to the bar-bell’s weight? Explain.

To determine

Whether the force exerted on the barbell by the weightlifter, when he jerks it over his head, is more than, less than, or equal to the barbell’s weight.

Answer to Problem 50A

More than the barbell’s weight.

Explanation of Solution

Introduction:

As per Newton’s third law of motion, for every force exerted there is an opposite and equal reaction force.

And, according to Newton’s second law of motion, if the force $F$ is applied to the mass $m$ , then the acceleration produced is given by,

  $a=\frac{F}{m}$

When the barbell is resting on the floor, the weight of the barbell acts in the downward direction. And, as per Newton’s third law of motion, the floor will exert an equal force on it in upward direction.

Now, if the weightlifter wants to lift it over his head, he needs to provide acceleration to the barbell. Considering Newton’s second law of motion, then the net force has to be in upward direction. Thus, the force exerted on the barbell has to be greater than its the weight.

Conclusion:

Hence, to provide acceleration to the barbell in upward direction, the force exerted on it has to be greater than its weight.