Question

A weight lifter lifts a 310 N set of weights from ground level to a position over his head, a vertical distance of 1.65 m. How much work does the weight lifter do, assuming he moves the weights at constant speed?

Answer 1

Assuming he moves the weights at constant speed

Acceleration is the derivative of velocity(which for simplicity sake, is basically speed).
If the velocity is constant, it's derivative is 0.

So there's 0 acceleration when moving at a constant speed.

we know that F=ma. If the acceleration is 0, that must mean the force is 0.

Remeber this appplies to the NET force. So how many forces do we have. Well we have the 390N set acting down, which would be the force of gravity. We also have an upwards force of the lifter. Well if the net forces add to 0, the force of the lifter lifting the weights has to be 390N as well, to cancel out the gravitational force.

Work=Force*distance.

We know the force, it's 310N. We know the distance, it's 1.65m

310N*1.65m=511.5 J

The lifter does 511.5 J of work