# Question on Net Force

1. Sep 25, 2014

### jg871

1. The problem statement, all variables and given/known data
Problem #7
(Due 9/26 Friday by 8:00 a.m.)
Hydro, who has a mass of 45.1 kg, falls straight down 4.67 m to the water below.
a. If Hydro comes to rest .543 seconds after hitting the water, what was the net force exerted on Hydro over this time?

2. Relevant equations

3. The attempt at a solution
Looked in the book, read the sections the question pertains to, and am still lost. Any assistance with how to solve this would be appreciated.

2. Sep 25, 2014

### wannabee_engi

You can use impulse to solve this.

3. Sep 25, 2014

### jg871

(4.67/.543) *45.1?

4. Sep 25, 2014

### Staff: Mentor

No.

What's the momentum of Hydro as he first touches the water?

5. Sep 25, 2014

### jg871

4.67/.543 so 8.60?

6. Sep 25, 2014

### jg871

M/s

7. Sep 25, 2014

### Staff: Mentor

Don't just give numbers--show how you used the basic formulas.

In any case, no. Answer this: If something falls a distance X, how fast is it going at that point?

8. Sep 25, 2014

### jg871

Acceleration of gravity correct?

9. Sep 25, 2014

### Staff: Mentor

The acceleration of a falling object is g, the acceleration due to gravity. But what's the speed of the object after it falls?

10. Sep 25, 2014

### jg871

I'm struggling mightily.

11. Sep 25, 2014

### jg871

What may seem obvious is just not registering for me here

12. Sep 25, 2014

### Ering

speed = distance / time
so
4.67m / 0.543s
but how do you get the acceleration from that?

Last edited: Sep 25, 2014
13. Sep 25, 2014

### Ering

I'm also working on this question for homework.
For the velocity vs. time graph, does your constant negative line start at some velocity above zero and stop at zero? Or does it go past zero into a negative, or does it start at zero and go down?
Wasn't very good at drawing these for the test either...

14. Sep 25, 2014

### jg871

Divide by time again

15. Sep 25, 2014

### jg871

I wasn't either. Go to umd? I believe it goes into a negative but I'm confused with the class so far

16. Sep 26, 2014

### Staff: Mentor

That will give you average speed.

You are dividing the distance it falls (before it hits the water) by the time it takes to come to rest (after it hits the water). That won't help.

Separate this problem into two parts: (1) How fast is it moving when it hits the water? (2) What net force does it have after it hits the water and is slowing down?

Solve each part separately.

The acceleration due to gravity is a constant. Look it up! (It's usually called g.)