# Nfl helmet to helmet hits

1. Oct 22, 2010

### oagles5

ok the other day sports science did a show about nfl helmet to helmet hits

on the show the combined the speeds of the players each player was running 10 mph so they said the speed at impact was 20 mph and then did the number on a 20 mph impact

i posted in a sports forum that this was wrong..i said because of newtons 3rd law you do not add the speed of the players.. the impact should have been done at 10 mph

i had many mixed reviews .....with ppl saying i was wrong that you would add the speed of both player

can you please tell me who is right

2. Oct 22, 2010

### rcgldr

You're probably thinking of a car collision case where two cars hitting head on at 10 mph each would be similar to either hitting a solid wall at 10 mph, where almost all of the momentum change between wall and car occurs with the car. A better analogy would be two cars hitting head on at 10mph each would be similar to one car at 20 mph hitting a car moving at 0 mph head on.

Another issue is how elastic (less energy reduction) and stiff (g force) the coliisions are.

3. Oct 22, 2010

### oagles5

This question has been asked before and I think resulted in incorrect answers. If two cars hit head on at 60mph, my assertion is it is equivalent to hitting a solid cement wall at 60mph, NOT 120mph.

The reason for this is the resulting force is not about approaching speed but about the deceleration. When a car hits a wall its speed goes from 60mph to 0mph. It is this force that smashes your face.

When a car hits another car head on, both cars come to a full stop in the same way -- each car goes from 60mph to 0mph -- resulting in the same force to your face.

My assertion is that the popular claim that hitting a car head on is like hitting a brick wall at twice the speed is WRONG. It is like hitting a PARKED CAR at twice the speed. And this is because the parked car will "give" and slide with you in the direction of the collision. In other words, hitting a parked car at 60mph is actually like hitting a brick wall at 30mph.

so this would not apply with the football hit ? i figured the part were it says you go 60 to 0 would apply to the football hit

and i thought because it was 2 masses force would be divided in to the 2 players......im sorry im not big into this stuff im just trying to get a better understanding thank you for the help

Last edited: Oct 22, 2010
4. Oct 22, 2010

### Andy Resnick

Using forces to analyze collisions is a bad idea- energy is the appropriate metric for this, for a variety of reasons- even better than momentum. And the total energy goes as the *square* of velocities.

5. Oct 22, 2010

### oagles5

that's the link to the show it's about 2 min long .....can someone tell me if it's done right

Last edited by a moderator: Sep 25, 2014