# Kinetic Energy

1. Oct 9, 2007

### Phykid

1. The problem statement, all variables and given/known data
Our homework set today was to find out why is it that when you hit someone at 30mph they have a 20% chance of dying. Yet when you hit someone at 40mph they have a 90% chance of dying.

2. Relevant equations
KE = 1/2 M V^2

3. The attempt at a solution
First of all i found out that 1mile = 1.6km; which means 1 mile = 1600m
I then did 60x60 to find out the amount of seconds in 1 hour: which was 3600 seconds
I then divided 30 by 3600 to get the meters/second; which was 8.333 [reoccuring] m/s
Now I found out the Kinetic Energy. We were allowed to choose which car we used, and I used my fathers. This weighs 2090kg (Mazda 5)

KE = 2090 x (8.333^2) = 145127.278
145127.278 / 2 = 72563.63901 J

This is where the problem came

I then tried it with 40mph

40 / 3600 to get the m/s; which turned out as 0.0111 [reoccuring]
I now thought how the heck could that be right? But I continued & tried to do the equation.

Ke = 2090 x (0.011^2) = 0.25289
0.25289 / 2 = 0.126445

Okay now I'm very confused. Can anyone help me out?

When 30mph KE is 72563.63901 how is 40mph 0.0111?

Thanks,
Phykid

2. Oct 9, 2007

### Staff: Mentor

The calculation methods for 30 mph and 40 mph are identical, so recheck your arithmetic. Redo both conversions from mph to m/s.

You forgot the 1.6 factor in going from miles to km.

Last edited: Oct 9, 2007
3. Oct 9, 2007

### Phykid

I'm confused, sorry :uhh:

EDIT: Ignore that, someone posted same time as me

4. Oct 9, 2007

### Staff: Mentor

Redo your conversion of 30 mph to m/s. Do it carefully, step by step.

5. Oct 9, 2007

### Phykid

So how would I go about including the 1.6 into it?

Thanks,
Phykid

6. Oct 10, 2007

### Phykid

Bump :O

7. Oct 10, 2007

### learningphysics

8. Oct 10, 2007

### Phykid

Brilliant!

9. Oct 10, 2007

### Staff: Mentor

I hope you realize that in your first post you correctly described how 1 mile = 1.6 km = 1600 m (approx). But then for some reason you forgot to use that bit of info when you did your conversion from mph to m/s.

10. Oct 10, 2007

### Phykid

I didnt until you pointed it out in your first post :)
thanks