Impulse and momentum principles – Understanding through sports

While there are some similarities and many differences between cricket and baseball, the most interesting item of difference for me is the way the ball is let go by the bowler in cricket and the pitcher in baseball.

I belong to the era when the West Indies great speedsters Charlie Griffith and Wesley Hall were terrorizing batsmen with their thunderbolts. They both started their run ups from almost near the boundary line (that is what it sounded like from the duration the crowd roared throughout the run up – no TVs). Australian Graham McKenzie was the name on the lips of those who admired pace bowlers (count me in as one) slightly later. The bowling action – over arm, and no particular whiplash moment.

I have also had a chance to watch pitchers like Nolan Ryan of Houston Astros – baseball’s equivalent of Wesley Hall, with no insult to either one. In baseball the ball is released in a sequence of jerks – shoulder, elbow and the wrist, but so fast that a high-speed camera would miss it. But, it is a jerky motion.

I don’t think there were any speed camera’s during Hall’s time. Let us take it he threw it in the low 90s, miles per hour (when the ball left his hand). But, Ryan’s has been clocked at 108 miles per hour. Perhaps the two decades difference between the two, including improvements in training and other technical analysis of motion, could account for, say, 2-3 mph. But, beyond that?

I tend to believe the difference between the speed of the ball in the two sports is due to how the motion of the ball is created. When the bowler arrives at the crease to release the ball, the ball carries its portion of the momentum of the bowler’s run up. No such build up for the pitcher. He has to start with planting firmly his right leg, if he is a right handed pitcher. Of course, he twists his body and unwinding it through the pitching motion also contributes to the ball’s initial momentum. The unwinding of body twist contributes to the speed of the ball in cricket also.

But all said, the duration in which the ball is imparted its final momentum is vastly different between the two modes of releasing the ball – very high in the case of cricket, and very low in the case of baseball. This, as per the definition of impulse implies that the force on the arms (the shoulder, elbow and wrist joints) is very high in the case of baseball pitching. However, the pounding the leg takes through the long run up and the final landing before releasing the ball does a lot of harm to the legs (ankle sprain), hips and lower spine of a cricket bowler.

The above is a layman’s interpretation of what is called bio-physics. If experts spot fundamental errors, I request they correct the explanation.

Those readers of my generation would immediately recognize how John McEnroe, one of the greatest tennis players and who was contemptuous of physical training, generated speed, while serving and while executing a forehand shot. He strung his racquet in the mid to high 40s, pound force. The strings deflected and essentially “cupped” the ball and then over time (still only a fraction of a second) the ball was released and its momentum increased.

McEnroe had one of the lethal serves. His service motion, besides hiding where the serve is going to land, released all the energy built up by his body-twist released by unwinding from a strange starting stance. When he was returning the opponent’s return in a rally, the same mechanism worked, the string “cupping” (not illegally) the ball, small force acting over a longer period.

But, Bjorn Borg, McEnroe’s contemporary and again one of the greatest tennis players did precisely opposite of what the latter did – strung his racquet at low to mid 60s, pound force. His right forearm was thick, suited his top-spin heavy ground shots (both flanks, but two handed back hand). The ball hardly stayed on his racquet. But, the force imparted to the ball was great. Large force, small duration – same result as McEnroe’s.

This is a way of learning, situate two instances in opposite quadrants and see how the result is the same.

One more instance for similar learning, from volleyball. The fingers, on both hands of a setter flex backwards while receiving the feed, again to “cup” the ball, as McEnroe did, but not allowing the ball to touch the palm (a “palming” foul is called). The setter then chooses a short-set, a high-set, a back-set or may even set for a back line smash. It is the time the ball is in touch with the fingers that allows such variations. Here also, additional momentum is imparted to the ball by the setter jumping (always) while setting. It is not only the fingers that do the job, but the whole body, though the ball is controlled by the fingers. The change in the momentum, called impulse, is great, by a combination of longer duration of smaller force – a la McEnroe.

If you are in the habit of watching James Bond movies (I am), at least in one of them, The spy who loved me, towards the end, the good guys throw grenades and they use, it may surprise you as it did me, round arm motion of a sort, much unlike a baseball throw.

When I goggled why, I got a response that implies that rapid change in the momentum may make the grenade explode even in the action of throwing it. I give below what I copied.

The ideal grenade throw is not a throw, but rather an overhand lobbed toss with a high-arc. The way they teach you in basic is to get in the prone with your grenade in your hands. It is physically impossible to throw a grenade like a baseball and live to tell the story.

The “live to tell the story” is the clue – immediate explosion. Rapid change in momentum, high impulse, take your pick.

I somehow like to engage my students when I am teaching Engineering Mechanics some such out-of-textbook examples of a principle. I do not know whether the students like it. But I intend to continue.

Raghuram Ekambaram