How Does a Scale React When a Weight Lifter Pumps and Throws a Barbell?

[Melchior25]

Homework Statement

A weight lifter stands on a bathroom scale. He pumps a barbell up and down. What happens to the reading on the scale as this is done? Suppose he is strong enough to actually throw the barbell upward. How does the reading on the scale vary now? (Select all that apply.)

True Decreases when barbell is pumped up.
False Increases when barbell is pumped up.
True Increases when barbell is pumped down.
False No change when barbell is thrown.
True Decreases and then increases when barbell is thrown.
False Increases and then decreases when barbell is thrown.
False No change when barbell is pumped up and down.
False Decreases when barbell is pumped down.

Could someone please double check my answers to this question and let me know if I have any wrong. Thanks...

 

[Dick]

You seem to have some concept backwards. When he pushes the barbell UP, he has to exert a force on the barbell greater than it's weight to accelerate it upward. By Newton's third law, the barbell then exerts the same force on him, hence one greater than it's weight. The scale indicates the total of the weight of the lifter and the downward force that the barbell exerts on him. Conversely, to bring the barbell downwards he has to exert less force on it than it's weight. Try rethinking your answers.

 

[Moetasim]

I cannot provide a definitive answer without knowing the exact weight and strength of the weight lifter and the barbell. However, based on the information provided, the following statements appear to be correct:

1. False - The reading on the scale will not decrease when the barbell is pumped up. The weight lifter's body weight and the weight of the barbell are still being supported by the scale, so the reading will not decrease.

2. True - The reading on the scale will increase when the barbell is pumped down. This is because the weight of the barbell is being added to the weight of the weight lifter, causing an increase in the overall weight being supported by the scale.

3. False - If the weight lifter is strong enough to throw the barbell upward, the reading on the scale will not increase. This is because the weight lifter's body weight is still being supported by the scale, and the barbell is no longer in contact with the scale.

4. True - When the weight lifter throws the barbell upward, the reading on the scale will decrease and then increase. This is because when the barbell is thrown upward, it is no longer in contact with the scale, causing a decrease in the overall weight being supported. However, as the barbell falls back down, it will once again be in contact with the scale, causing the reading to increase.

In summary, the reading on the scale will only change when the weight lifter is actively lifting or lowering the barbell. When the barbell is thrown upward, the reading on the scale will temporarily decrease, but will return to its original value once the barbell falls back down.