Lately I’ve been studying a book of basic physics. Don’t ask me why. I’m wading through Newton’s Laws of Motion. The First Law—an object at rest tends to stay at rest and an object in motion tends to stay in motion in a straight line at a constant speed—is pretty easy to comprehend.
But the Second Law thickens the swamp. It’s expressed tersely as an equation
The book says this means Force equals mass times acceleration. As I noodle on this, I get the impression that the more mass something has, the more force you need to get it moving. On the flipside, if something is already moving and it has enough mass it can cause something not moving to move. And the greater the mass of the moving object, the faster the acceleration the smaller object when the big one hits it. Make sense?
It does to me because it puts me in a memory from nearly half-a-century ago.
Mammaw, my paternal grandmother, has a two-handed white-knuckle grip on the steering wheel of a newly bought Oldsmobile Delta 88. I’m in the front passenger seat. We’ve just shopped at the Western Supermarket in Five Points West. I’m along to help her load and unload the groceries.
She has safely backed the car out of the parking space and, with great concentration, shifts from reverse to drive. Now we’re rolling at a speed of 5 mph—tops.
I notice, coming up on our right an abandoned empty shopping cart resting just off the curb in front of the store. The new car is apparently wider than my grandmother thinks.
The Oldsmobile, moving at no more than 5 mph, hits the shopping cart, and I kid you not, that sucker is launched like a missile—way, way faster than 5 mph. It slams into a huge white refrigerated chest of ice bags and rebounds like a ping-pong ball.
I turn around in my seat in time to watch the cart skitter on its side into the middle of the pavement we just passed over.
She’s busy making the left turn to head back home. Not only did she never see the cart, she never knew she hit it! Several people are gathering round the wounded cart. They’re pointing to us in the get-away car. But the traffic light just ahead of us burns green.
“What did you want?” Mammaw asks me.
“Nothing,” I say, “just speed up.” She does, and we glide through the intersection as the light turns red. I exhale. We’re home free.
So, back to Newton. As I see it, the massively superior mass of the Delta 88 met an object at rest, a puny, wiry little cart and set it in motion. The cart then met another object of superior mass, the ice chest, which, being superior, remained inert, thus turning the cart’s motion on itself. Voilà! Newton’s Second Law!