# Force basics .help!

1. Sep 21, 2014

### Tracee

I need help. I have no physics background so need the simplest of answers.
I have a question to answer - about the damage that might be caused to a person falling 0.4m off a step.
The questions doesn't ask for specific calculations but ask for concepts of force, acceleration and gravity to be applied and some equations and diagrams to show the results. My reseach so far suggests I need to work out the net force at stopping point - does this sound correct. Once I have that I can research further on the possible damage from this force.
Also, I imagine I'd have to give the person a weight of say 70kg to work this out.
I did do a calcualtion of time based on 70kg and 0.4m ending with 0.28 seconds. I have no idea if this is correct.
Help!!!!!!

2. Sep 21, 2014

### Staff: Mentor

.28 seconds is about right for the time it takes for someone to fall .4m. If you've been able to get that far, you should be able to figure how fast they're moving when they hit the ground.

When they hit the ground, their speed will fall to zero. What force is required to do that? That's the impact force that does the damage, and it depends on how quickly they're brought to a stop - concrete stops you more quickly over a shorter distance than a nice soft pad.

3. Sep 21, 2014

### 256bits

The type of falling can also determine the "damage".

If the person walks off the step, not realizing there is no more step to place a foot, he may drop down with his whole body with the posibility doing a foot plant, which could minimize the damage as the shock of hitting the ground can be taken up by flex in the knees and other joints.

On the other hand, if the person is tipping over with his feet still on the step, his whole body will be doing a rotation about that point of where feet and step are. His upper body will the first to hit the ground with the damage possibly being life threatening, especially if the person's head impacts a hard surface.

4. Sep 21, 2014

### Tracee

Thanks all. I will keep with it and see how I go, but honestly it's a unfamiliar world to me.
Just hope I get the right formulas!
I think I need to base it on a single fall to the ground at rate of gravity. Am I right in thinking I need to give the person a weight?

5. Sep 21, 2014

### Staff: Mentor

Yes. Not because it affects the speed with which he hits the ground (it doesn't - look for the apocryphal but memorable story of Galieo dropping weights from the leaning tower of Pisa) but because it affects the force that the ground needs to apply to the person to stop his downwards motion. All of that force will be concentrated on just one unlucky portion of the anatomy.

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