Question 10 of 26 Attempt 3 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 37.5 kg barbell. Joe is taller than Sam, which means Joe's arms are 68.4 cm long whereas Sam's are 61.4 cm . Calculate the work done by Joe W Joe when he raises the barbell to finish one repetition. The acceleration due to gravity is 9.81 m / s 2

0 J

Explanation:

Work done = Force × displacement

In one repetition, the barbell moves through a displacement of 0 m.

This is because in the first motion, the barbell moves through a displacement of Joe's arm length = 68.4 cm. In the second motion, the ball moves up, having a displacement of -68.4 cm.

Note that displacement is a vector quantity so the directions have to be considered. Here, we have taken downward motion to be positive and upward motion to be negative.

Total displacement = 68.4 cm + (-68.4 cm) = 0 cm = 0 m

So,

Work done = Force × 0 m = 0 J.

Bonus:

You might wonder why this value is 0 J even though Joe has expended energy. Does this mean the energy has been destroyed? No, the energy was converted to heat (and maybe sound) which is why he could have been sweating.