Why Is the Calculated Force Excessive for Lifting the Barbell?

[treyh]

An athlete whose mass is 86.5 kg is performing weight-lifting exercises. Starting from the rest position, he lifts, with constant acceleration, a barbell that weighs 470 N. He lifts the barbell a distance of 0.50 m in a time of 2.0 s.


I cannot seem to find the answer to this. This is what i did:

- I found the total force exerted on the feet by doing: 86.5*9.8 + 470N = 1317.7N
- Then I used d = a*t^2 and got that the acceleration is 25m/s^2
- Then i found the mass of the barbell by doing 470N/9.8 and got 47.96kg.
- After using F=ma i added the two forces and got the answer 2516.7N.

This isn't the right answer. What did i do wrong? Any help is appreciated.

Thanks.

 

[tiny-tim]

hi treyh!

An athlete whose mass is 86.5 kg is performing weight-lifting exercises. Starting from the rest position, he lifts, with constant acceleration, a barbell that weighs 470 N. He lifts the barbell a distance of 0.50 m in a time of 2.0 s.

i don't understand the question …

is his body remaining stationary (apart from his arms)?

or is his centre of mass also moving up 0.50 m ? ​

 

[PeterO]

An athlete whose mass is 86.5 kg is performing weight-lifting exercises. Starting from the rest position, he lifts, with constant acceleration, a barbell that weighs 470 N. He lifts the barbell a distance of 0.50 m in a time of 2.0 s.


I cannot seem to find the answer to this. This is what i did:

- I found the total force exerted on the feet by doing: 86.5*9.8 + 470N = 1317.7N
- Then I used d = a*t^2 and got that the acceleration is 25m/s^2
- Then i found the mass of the barbell by doing 470N/9.8 and got 47.96kg.
- After using F=ma i added the two forces and got the answer 2516.7N.

This isn't the right answer. What did i do wrong? Any help is appreciated.

Thanks.

I just read the original post very carefully, but the only question mark I found was at the end of " What did I do wrong?"

I am tempted to reply: " You didn't ask a question?"

The line I highlighted red above is interesting. If an object falls under the influence of gravity - with an acceleration a little under 10 m/s^2 - it covers just under 20m in 2 seconds. This barbell covered only 0.5 m in 2 seconds yet you said the acceleration was 25 m/s^2 - more than double the acceleration due to gravity ??