Brick Scene

The Physics behind The Brick Scene: Home Alone 2

In case you've forgotten the scene......

So let's take a look at what's going on here...

Marv was able to withstand getting hit directly in the face by three bricks falling from a four-story high building. The only sign of injury are small marks on his forehead, blurry vision, and slurred speech - which is not even remotely accurate to the injuries that would occur in real life. In real life, the impact velocity would be enough to certainly cause a concussion and he would be left unconscious - not slightly dazed. Let's break down the physics in play to see why.

First off, here's a few numbers.....

- The top of the building Kevin is throwing from is four stories high, or 12 m.
- A brick that size would weigh approximately 4.5 lbs (or 2.04 kg)
- The human brain can withstand crash impacts of 1100 pounds-force without severe brain injury or deformation to the skull

So would Marv actually survive these injuries?

To do this we must find the impact velocity and force. To make these calculations simpler, we will assume the brick was dropped straight down and there was no initial force (only effected by gravity). So lets find the time it will take to drop...

Givens:
Acceleration = 9.8 m/s2
Height = 12m 
Mass= 2.04 kg 

d= v1t + 1/2(a)(t2)
12 = 0t + 1/2 (9.8) (t2)
12 = 4.9t2
12/4.9 = t2
2.49 = t2
1.56s = t 

It will take 1.56 seconds for the brick to reach Marv from the top of the building. So now we can find the impact velocity.

v2 = v1 + at 
v2 = 0 + (9.8)(1.56)
v2 = 15. 29 m/s

So the impact velocity of  the brick is 15.29 m/s, which will certainly cause greater injuries than Marv sustained. So now we know how fast the brick will be going when it comes into contact with Marv, but how much force is that? Will he actually be able to survive? 

To calculate this we must find the amount of kinetic energy (Newtons) that the brick possesses when it hits Marv. Based on the Law of Conservation of Energy, we know that the kenetic energy at the bottom will be equal to the gravitational potential energy at the top. So to solve for this we will use the following formula...

Eg = mgh 
      = (2.04)(9.8)(12)
       = 240 Newtons 

     Eg= Ek
240N= 240N

So Marv will be hit with 240 Newtons of force at 15.29 m/s, but will he survive. To find this out we must solve for pound-force. 

1 Newton = 0.224809 Pounds of force

So, 

240N = 54 pounds-force

So Marv will be hit with 54 pounds of force. While this won't kill him, it will cause greater injuries than what he has sustained. He would likely suffer from brain injuries (ie: concussions) and would have more visible injuries then small markings.