Calculating Average Force and Mass: Weightlifting and Shelf-Moving Examples

[defetey]

A world champion weight-lifter does 5000J of work lifting a weight from the floor to a height of 2m. What is the average force exerted on the weight? What is the mass of the weight?

So I got the first part, it's 2500N. But how would I find the mass given just that much information?

And for this question: A store clerk moved a 4.4kg box of soap without acceleration along a shelf by pushing it with a horizontal force of magnitude 8.1N. If the employee did 5.9J of work on the box, how far did the box move?

So the answer is 0.73m, which is gotten by dividing 5.9 by 8.1 (5.9=8.1*d). But am I just supposed to assume the force used is 8.1N or is it certain?

 

[berkeman]

A world champion weight-lifter does 5000J of work lifting a weight from the floor to a height of 2m. What is the average force exerted on the weight? What is the mass of the weight?

So I got the first part, it's 2500N. But how would I find the mass given just that much information?

And for this question: A store clerk moved a 4.4kg box of soap without acceleration along a shelf by pushing it with a horizontal force of magnitude 8.1N. If the employee did 5.9J of work on the box, how far did the box move?

So the answer is 0.73m, which is gotten by dividing 5.9 by 8.1 (5.9=8.1*d). But am I just supposed to assume the force used is 8.1N or is it certain?

On the first one, use F=ma, with the only a being g.

On the second one, yeah, the force is given as 8.1N, and it was held through the distance.