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April 19, 2010 | Exam Preparation Blog

Obey the Law! The Laws of Motion, That Is.

Newton's LawIf you’ve reviewed kinesiology and/or biomechanics in your studies, you should already be familiar with our mathematician friend Isaac Newton.  As we talk about Newton’s three main laws of motion you will want to refer to the following pages in your manual:

 ACE’s Essentials of Exercise Science: pg 106 – 112

Most of us can name off at least one of Newton’s three motion related laws, if not what they stand for. When taken together, Newton’s three laws of motion provide a better understanding of the relationships among forces, mass, and human movement – at individual joints or of the body as a whole.

Newton’s first law, the law of inertia, states that a body at rest will stay at rest and a body in motion will stay in motion unless acted on by an outside force. Law one also states that a body’s inertia is proportional to its mass (a heavier object is harder to get moving than a lighter object). So how does this relate to the fitness professional? Weight lifting! Starting a bicep curl with a 15 pound weight is going to be harder than starting a bicep curl with a 2 pound weight.

Newton’s second law, the law of acceleration, states that a force acting on a body in a given direction is equal to the body’s mass multiplied by the body’s acceleration in that direction.  (Force = mass x acceleration) How can we apply that to fitness?  With soccer, the more force you exert during a kick, the faster the acceleration of the ball. This means when the ball leaves your foot, it is going to travel faster and farther. A dainty kick (little force) means less acceleration. An ‘I mean business’ kick (more force) will mean greater acceleration. In this case the mass, the soccer ball, remains the same in both examples.

We can also see Newton’s second law in terms of weight training. If you ‘cheat’ by initiating a lifting movement with momentum from a body part other than the one you are focusing on, the momentum of the moving weight may be greater than the muscle’s ability to decelerate and stop it. We’ve all seen the individual at the lat pull down machine using a half crouch and a great heaving motion to start pulling the weight down towards their chest. Often at the end of the exercise, this weight goes crashing back down to the stack with a great speed because of this compensation used to initiate the lift. (the lifter didn’t use the intended muscles to get the movement started, so those intended muscles can’t slow down/decrease the movement at the end of the motion).

So how do Newton’s first law and second law relate? Newton’s first law tells us that force is required to accelerate an object. Newton’s second law answers the question of how much force is required.

Finally, Newton’s third law is commonly referred to as the law of reaction – every applied force is accompanied by an equal and opposite reaction force. Say it with me ‘every action has an equal and opposite reaction’.  With fitness we talk about this in regards to impact forces when jogging, doing plyometrics, or step class. Step training remains popular even though the magnitude of the ground-reaction forces associated with each step place the student as risk for a variety of overuse injury. Using a ten inch step is going to result in greater impact forces than using a 2 inch step. When we ‘land’ we exert a force on the ground. The earth exerts the same force back up at us, which our body must then dissipate through its shock-absorbing structures (knee meniscus for example). Many aerobic shoes have additional shock absorbing material under the forefoot, as this is where the force first impacts.

So we’ve talked about Newton’s first, second and third laws. Rather than memorizing what the law says, focus instead on how that law applies to fitness and exercise. Also keep in mind that Newton and his laws are part of the background science part of your certification. Important in a fundamental sense, yes, but not something you want to spend a large amount of time on.



By April Merritt
