Question

Two friends, Joe and Sam, go to the gym together to strength train. They decide to start off with an exercise called flat bench press. Lying flat on a bench with his arms in an extended position, each friend slowly lowers (to his chest) and then raises a barbell back to the starting position. They start off by using a 31.5 kg barbell. Joe is taller than Sam, which means Joe's arms are 70.0 cm long whereas Sam's are 59.0 cm . Calculate the work done by Joe ????Joe when he raises the barbell to finish one repetition. The acceleration due to gravity is 9.81 m/s2 .

Answer 1

Answer:

216.31 (the work done by gravity is -216.31) positive for going up.

Explanation:

We look at this question first by getting the right equation for work.

Which should be... W = F x D.

From this, we can do everything, we need the Force (F) first - the question tells us that Joe is lying on his back and moves his arms upward to raise the barbell. This means that he is countering the force of graving on the object.

What is the formula for the force of gravity on an object near the earth?

Right here --- $F_{grav}$ = mg

m = the mass and...

g  =  the acceleration due to gravity which is 9.81 m/s2

Before we plug things in though, we need to convert everything to SI units,

the weight is in kg - so we're good to go there, but the length of Joe's arms are in "cm" we need m or meters. Converting 70 cm to m = .7 m.

Now, we just put it all together - (31.5kg)(9.81m/s2)(.7m) =  216.31 J or 216.31 N m.