# Q & A: Newton’s laws

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Q:
Eighth grade science teacher with two more questions. 1. Newton’s second law states that the force on an object is equal to the mass times the acceleration. Is free fall an exception to that since all objects in free fall accelerate at the same rate regardless of mass? 2. Can you explain the forces involved in arm wrestling both from the action - reaction standpoint and balanced/unbalanced forces? Since unbalanced forces only count when the force is exerted on the same object, exactly how does someone win in arm wrestling? Aren’t the forces on two different objects (arms)? My unit on physics is almost finished. Thanks for your help
- Marsha
Sevier Middle School, Kingsport, TN
A:
1. Very perceptive question! In the contect of Newton's laws, the equal accelerations of all different objects in a gravitational field is sort of a remarkable coincidence. It arises because the 'm' that appears in a= F/m is exactly proportional (we call it the same) as the 'm' that appears in F=mg, so they cancel to give a=g.
Why should gravity alone have this special property? In Einstein's General Relativity, that property (things in free fall don't accelerate differently) ceases to be a coincidence and instead becomes a starting ingredient for a whole different picture of space and time. And in this new picture, there is no 'force' of gravity, but only that surprising space-time geometry. So you're onto a big idea!

2. I'll probably miss some of the forces, but obviously you're right that the two arms exert equal forces opposite directions on each other. So what determines which way the linked arms accelerate? It's set by forces from the outside- the forces from the shoulders and perhaps from the contact between the elbows and the table. Whoever can marshal bigger forces of this sort can determine the direction of the net force on the arms, i.e. who wins.

Mike W.

(republished on 07/12/06)