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.

 

[tomcue]

I would approach these questions by first identifying the relevant equations and principles involved. In the first question, we are dealing with work, force, and height. The equation for work is W = Fd, where W is the work done, F is the force applied, and d is the distance moved. In this case, we are given the work done (5000J) and the height (2m), so we can rearrange the equation to solve for the force: F = W/d. Plugging in the values, we get F = 5000J/2m = 2500N. This is the average force exerted on the weight.

To find the mass of the weight, we can use Newton's Second Law, which states that force is equal to mass times acceleration (F = ma). Since the weight is being lifted at a constant velocity, there is no acceleration, so the equation becomes F = mg, where g is the acceleration due to gravity (9.8m/s^2). Rearranging the equation, we get m = F/g. Plugging in the value for the force (2500N) and the acceleration due to gravity (9.8m/s^2), we get m = 2500N/9.8m/s^2 = 255.1kg. Therefore, the mass of the weight is 255.1kg.

In the second question, we are dealing with work, force, and distance. The equation for work is the same as before (W = Fd), but in this case, we are given the work (5.9J) and the force (8.1N). We can rearrange the equation to solve for the distance: d = W/F. Plugging in the values, we get d = 5.9J/8.1N = 0.73m. Therefore, the box moved a distance of 0.73m.

As for the question about assuming the force used, it is important to note that the given force (8.1N) is the magnitude of the horizontal force applied. In order to find the distance moved, we need to use the magnitude of the force, not its direction. Therefore, we can assume that the force used is 8.1N without taking into account its direction.