Taking another few steps back we can examine just the basics of Newtonian Mechanics - the laws of physics which govern every single physical object on this planet. Everything from a book resting on a table to a weightlifter clean and jerking 200 kg, these can all be explained with basic theories of static and dynamic force analysis.
Basics
Force can be defined through Newton's 2nd Law with the general equation F = ma; there m is mass and a is acceleration of the object. In metric units, mass is defined through kg's and acceleration through m/s^2. Thus force is defined with the units kg.m/s^2, or Newtons (N).
All objects whether moving or staying still will have a force exerted on them at all times. According to Newton's Third Law:
To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.
If you push on a wall, the wall is pushing right back on you with the exact same amount of force you applied.
Forces can come from a variety of sources: applied, frictional, gravitational; just to name a few. Here's a brief outline of how they are created.
Applied Force: A force generated by an object or person onto another object or person. This can include pushing or pulling on something. If you are pushing on a wall with 10 N of force, that is an applied force of 10 N.
Normal Force: The opposite and equal force explained by Newton's Third Law, this force counteracts applied force. In a static situation this counter force is what keeps an object from moving.
Gravitational Force: A force generated through gravitational pull. Any object through in the Earth's atmosphere experiences a constant gravitational acceleration of g = 9.8 m/s^2. Thus given the equation in the previous section if we know an object's mass we can calculate its gravitational force by using its mass and gravitational acceleration (F = mg).
Friction Force: Force on an object due to friction when being dragged across a surface. Every surface-surface interface has an associated coefficient of friction (u). Combined with the normal force (normal to the gravitational force), we can calculate friction force with the equation Ff = u*Fn.
Stay Still
One scenario of force analysis is the static situation. In Statics, there is no net acceleration of the object; everything stays still and unmoving. Even though an object at rest looks to be completely unperturbed there can still be a variety of forces acting upon it; its just that the sum of all these forces in their respective planes net is zero. If I have a 10 kg plate sitting on a table, what are the forces at play? If nothing is pushing it across the table then there are no forces acting in the horizontal direction. In the vertical direction there is the gravitational force acting upon it (Fg = 10*9.8) and attempting to pull it down to Earth. Since the plate is completely at rest then there has to be some sort of counteractive force to resist the downward gravitational force. If you even read the last section you'll know this is the normal force - the force which acts in an equal and opposite direction as postulated by Newton''s 3rd Law. Thus, to keep this 10 kg plate unmoving in the vertical direction, the normal force acting from the table to the plate must equal the force of gravity acting from the plate to the table.
Moving On
While static force analysis works for non-moving situations most of our human activity is performed while moving. Any force analysis situation we encounter is known as a dynamic scenario. Going back to Newton's 2nd Law, in dynamic situations we now have an acceleration value to attach to the equation, thus the sum of all net forces will be equal to mass multiplied by acceleration of the system (F = ma).
So how do we get in dynamic scenarios? For motion to occur we must overcome the initial force of an object resisting motion; known as Newton's 1st Law, or the Law of Inertia. Every single object, from a box on the floor to our own limb segments, has its own inertia which is related to its mass. To get an object to start moving we must apply enough external force to overcome its inertia. For those who've pushed a car knows that feeling of that initial resistance where we start applying high amounts of force and the car barely gets rocking. Up until the point where the car actually moves we haven't overcome its inertia yet. However notice as you overcome inertia the car starts moving and as you push it gets easier to move the car (although this has more to do with friction of the axle).
For these dynamic scenarios, applied forces are no longer balanced like in the static scenario. When not moving, forces in a plane of motion will net to equal zero - hence no movement. However since the dynamic scenario has the object moving, one direction of force will be greater than its opposing amount. For example, if I throw a ball straight up in the air let's look at the forces at hand. Assuming no horizontal displacement, there are no horizontal applied forces. In the vertical direction we have the gravitational force of the ball acting downward. Acting up, we have the the applied force of my hand throwing the ball. Just how much force is acting on the ball? To overcome the ball's inertia, it must be greater than the downward gravitational force. Since the ball's net displacement and acceleration is going upward, the upward throwing force applied must therefore be greater than the downward gravitational force
Taking this one step further, knowing the forces applied we can figure out the acceleration of the object. Or, knowing the acceleration we want to achieve we can calculate how much force we need to apply. The forces calculated in the static scenario shows the force required to keep an object at rest. If I'm trying to push upward on a barbell and I know this barbell weighs 100 kg (Fg = 980 N), I know an upward force of 980 N will keep this barbell at rest. However, once I apply 981 N of force I now have an imbalance in forces; my upward pressing force is greater than the downward gravitational force. Since F = ma, a positive force can be correlated to a positive acceleration - if I have a 10 kg object with a measured net force of 100 N I can calculate than in order for this to be true the acceleration must be 10 m/s^2 (a = F/m; a = 100/10), as to not violate Newton's 2nd Law.
As an aside we should look at the definition of displacement, velocity and acceleration. Displacement is defined as the net distance moved. If I start off standing in a room and walk 10 m forward, my net displacement is 10 m [forward]. Displacement can be a negative value but only because of the sign convention (the way we define positive movement).
Velocity is defined as the rate of displacement; V = D/t. If I walk those same 10 m and take 5 seconds to do so my velocity is equal to 2 m/s. If I have a negative velocity the object is also moving backwards, the same direction of the displacement.
Acceleration is defined as the rate of velocity; how quickly or slowly an object moves (a = v/t). This can be useful in determining changes in speed such as with a car, or bar velocity on a bench press. The big deal about acceleration is that I can have a negative acceleration but still have an object moving in a positive direction. How you ask? Acceleration is the rate of velocity; if I have an object moving at 2 m/s and 1 second later it moves at 4 m/s, its acceleration is 2 m/s^2 (a = v2-v1/t = 4-2/1 = 2 m/s^2). If at the next instance it goes from 4 m/s to 1 m/s one second later its acceleration is -3 m/s^2 (a = 1-4/1 = -3 m/s^2). Thus an object can still be moving forward but if it is slowing down its acceleration is negative.
Why is this important? When used in Newton's 2nd Law; F = ma; we are calculating static force as the force applied just before the object moves - a minutia of added force will get this object moving. When calculating for dynamic situations we can count initial velocity is zero (not moving) to some positive value (positive). Thus any time we start going from unmoving to moving our acceleration is going to be positive. There will be times where as we move along on a path our acceleration will be negative but I'll get into that later on.
Application
So how does this relate to what you're all here for (athletics)? Simple - everything physical you do on Earth is related to these laws. Nothing can ever disrupt these laws of motion lest something severely drastic occur (such as the Large Hadron Collider tearing a new black hole in space and disrupts Earth's gravitational pull).
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| Damn you! |
Another example of dynamic scenarios is the Dynamic Effort method of lifting, popularized by Westside Barbell. The Dynamic Effort method consists of lifting submaximal weights while exerting maximal levels of force to the bar. For example let's say your best bench press is 500 lbs. For a dynamic effort day you may use 300 lbs of weight, but move the bar with 500 lbs of applied force. Due to the unbalanced forces (300 lbs of downward weight vs. 500 lbs of upward applied force) we are positively accelerating the bar, thus fulfilling the purpose of the dynamic effort method. By learning how to move the bar with maximal acceleration a weightlifter can learn how to apply maximal force and lift near 1 rep max weights. However, if the same weightlifter were to practice low rep work with 455 lbs, he may not even apply 500 lbs of force to the bar once (instead, somewhere around 456 or more lbs, due to low velocity of the weight) and on competition day the weightlifter may no longer be able to apply 500 lbs of force to a barbell. It can be concluded that the greater acceleration we apply to the bar, the greater force we are applying. Learning to apply maximal force to the bar will help us lift heavier weights. Thus, knowing methods on how to apply maximal force can benefit the weightlifter come competition time.
As a follow up we'll be looking at the physics of weightlifting bands and how they force the lifter to increase acceleration through the lift.
References:
http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html
http://www.physicsclassroom.com/class/newtlaws/u2l2b.cfm
Hibbeler RC. Engineering Mechanics: Dynamics, 12th Ed. Pearson Education, 2010
Hibbeler RC. Engineering Mechanics: Statics, 12th Ed. Pearson Education, 2009
Simmons L. Westside Book of Methods. Westside Barbell, 2007
nice article dude, it never occurred to me that lifting lighter weight meant using more force. Interestingly moving the weight up faster is making me stronger than moving more weight slower per rep.
ReplyDeleteI'll keep checkin in on these articles, cheers!