# Question on Net Force

## Homework Statement

Problem #7[/B](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?

## 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.[/B]

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You can use impulse to solve this.

(4.67/.543) *45.1?

Doc Al
Mentor
(4.67/.543) *45.1?
No.

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

No.

What's the momentum of Hydro as he first touches the water?
4.67/.543 so 8.60?

4.67/.543 so 8.60?
M/s

Doc Al
Mentor
4.67/.543 so 8.60?
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?

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?
Acceleration of gravity correct?

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

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

I'm struggling mightily.
I'm struggling mightily.
I'm struggling mightily.
What may seem obvious is just not registering for me here

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?
The acceleration of a falling object is g, the acceleration due to gravity. But what's the speed of the object after it falls?
speed = distance / time
so
4.67m / 0.543s
but how do you get the acceleration from that?

Last edited:

## Homework Statement

Problem #7[/B](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?

## 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.[/B]

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...

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

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

Doc Al
Mentor
speed = distance / time
That will give you average speed.

so
4.67m / 0.543s
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.

but how do you get the acceleration from that?
The acceleration due to gravity is a constant. Look it up! (It's usually called g.)