11 November 2018

A Lion Can't Tackle a Bear!

The Chicago Bears beat the Detroit Lions today, 34-22.  They put the game out of reach early in the second quarter when rookie wide receiver Anthony Miller (#17) slipped free of a Lion defender and scampered into the end zone for a 45-yard touchdown reception.  Check out the start of the play (click on image for a larger view).
The Bears had 2nd and 5 on the Lions 45-yard line.  Note the three-receiver set on the right side of the Bears line.  Miller is on the inside, as I've indicated.  Now do a little counting.  You'll count 11 Bears, but you'll only get to 10 when counting the Lions.  Where is the 11th Lion?  Free safety Glover Quin (#27) can't be seen in the above screen capture.  He's playing deep coverage.

Just after the snap, confusion reigned supreme in the Lions secondary.  In the screen capture below, Lions strong safety Quandre Diggs (#28) had just let Miller go by him.  And yet he was pointing toward the line of scrimmage!
You can also see Diggs pointing toward Miller, which I assume was meant for his teammate, Glover Quin.  Bears quarterback Mitchell Trubisky (#10), who enjoyed great protection from his offensive line, had to like seeing Miller break free.  He threw the ball from the Bears 47-yard line (click on image for a larger view).
Now check out all the open space around Miller when he caught the ball at the Lions 27-yard line (click on image for a larger view).
You can see that Miller had to turn around after he makes the catch.  Note that Glover Quin is still not visible in the screen capture.  I had to wait for the instant replay to see where Quin was when Miller caught the ball (click on image for a larger view).
You can see Quin on the far right of the screen capture.  And now comes the decisive physics!  The key concept here is relative velocity.  When you're sitting in a car going 60 mph on the highway, you look out the window and see the world going in the opposite direction at 60 mph.  Your windows protect you from the oncoming air and, ignoring the bumps in the road and other things that give you an occasional acceleration, you feel at rest in your car's frame of reference.  If a car passes you going 70 mph, you see that car passing you at 10 mph.  It's a person on the ground that would see you going 60 mph and the other car going 70 mph.  It's more complicated when all the motion isn't along a single direction.  I put velocity vectors on Quin and Miller as they headed toward each other (click on image for a larger view).
Quin's vector is more into the screen, so it appears shorter.  In Quin's reference frame, he doesn't see Miller moving at the same speed and direction as someone standing on the sidelines would see Miller moving.  That blue velocity vector in the above image has to be subtracted from all velocity vectors if one wishes to see the world from Quin's reference frame.  The problem for Quin is that he aimed too close to where Miller was in the above image, instead of aiming closer to where Miller would be at the point of collision.  Check out the near miss (click on image for a larger view).
A view from another angle shows just how close Quin missed Miller (click on image for a larger view).
Quin is too far behind Miller because Miller's center of mass is to the left of Quin.  The best Quin could have hoped for was an arm tackle while his linear momentum continued to send him behind Miller.  Note that Miller lowered his center of mass to increase his stability.  It's far easier to turn a refrigerator over when it's standing upright than it is when it's on its side.  Check out what happened to Quin (click on image for a larger view).
Quin could only receive a little torque from Miller and get twisted as he fell.  Miller needed only to get to about 16 mph to reach the end zone without getting tackled (click on image for a larger view).
I am in no way claiming that it's easy to calculate a relative velocity in a split second on an NFL gridiron!  It's challenging enough for high school students to add vectors, and they have a lot more time and the use of calculators!  Professional athletes get quite good at adding velocities, even though most probably don't realize that's what they're doing.  Thousands of plays in practice and in games help athletes add velocities as if doing so were second nature.  Quarterbacks do it all the time when they throw to a spot where a receiver will be by the time the ball nears the turf.

This was a fun play to analyze for today's Check Down piece Chuck Nice (of Playing with Science) and I did for TuneIn's 1st & Goal.  Click here for the audio link.

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