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Newtons Laws Sports Assignment

Sir Isaac Newton stated three laws which explain the relationship between force and movement. Having an understanding of Newton's laws which you can apply to sporting situations gives you a deeper understanding of sports technique.

Newton's first law of motion

"A body continues in a state of rest or uniform velocity unless acted upon by an external force."

This is also known as the 'law of inertia' and means that something will either stay still, or stay moving unless a force acts on it. For example a golf ball will remain still unless a the force applied by the golf glub makes it move. Or that same golf ball will continue to move at a constant velocity (speed in a straight line) unless a force acts on it to slow it down (e.g. wind resistance) or change its direction (e.g. gravity).

Questions: Can you give examples in sport where:

  1. A force causes a body at rest to move?
  2. A force causes a moving body to change direction?
  3. A force causes a moving body to accelerate?
  4. A force causes a moving body to decelerate?
  5. A force causes a body to change shape?

Newton's second law of motion

"When a force acts on an object, the rate of change of momentum experienced by the object is proportional to the size of the force and takes place in the direction in which the force acts."

This simply means that when the golf ball above is struck by the golf club the rate of change of momentum (or velocity) of the ball is proportional to the size of the force acted on it by the club. A popular way of describing Newton's second law of motion, particularly when doing calculations is F=MA or Force = Mass x Acceleration

Newton's third law of motion

"For every action there is an equal and opposite reaction."

Or to put it exactly how Newton put it - 'For every force that is exerted by one body on another, there is an equal and opposite force exerted by the second body on the first.' This is sometimes referred to as the law of reaction.

For example when a Tennis player hits a ball the racket exerts a force on the ball and the ball exerts an equal and opposite force on the racket. The racket exerts what is known as the action force and the ball exerts the reaction force which is felt by the increased resistance at the time the racket strikes the ball.

If the same ball then hits the floor it exerts a force on the floor and the floor exerts an equal and opposite force on the ball.

Sir Isaac Newton formulated 3 physical “laws” that became the basis for classical mechanics.  Through these laws he describe the relationship of forces, objects, and motion.  For three centuries this has been the foundation for understanding motion and physical force systems.

Keep in mind that although these 3 laws changed the way scientist looked at the world, it is by no means complete.  Further revelations dealing with quantum physics and theories of relativity have shown that these laws are only the basis for mechanics and not all-inclusive.  Nonetheless, understanding these 3 laws is a pre-requisit to studying motion and their physical systems.

Newton’s 3 Law’s of Motion

The first rule of newton’s laws is you do not talk about newton’s laws.

The second rule of newton’s laws is you do not talk about newton’s laws!

But seriously, here they are…

1) Law of Inertia

An object in a state of constant velocity tends to remain in that state of motion unless an unbalanced force is applied to it.  In other words, it is the resistance to motion changes.  There are important considerations when conceptualizing inertia.  One of these considerations is that rest is a constant velocity and can be considered to have inertia.  Another consideration is that gravity is an unbalanced force acting on all objects.

An unfortunate epiphany of Inertia

1) How Inertia Applies to Biomechanics

Consider the late swing phase of gait and the forces going forward with the lower extremity.  Just prior to heel-strike there are almost no muscles activated that bring the extremity forward, yet it is still proceeding to travel forward in space.  This is inertia.  To deal with this inertia the body deploys an eccentric contraction of the hamstrings to slow down the extremity to prepare for heel-strike and to reduce harsh reactionary forces.

Eccentric deceleration of inertia during late phase of gait

2) Force = Mass x Acceleration (F = ma)

The net force applied to a body (mass) produces a proportional acceleration.  This law describes the relationship between an object’s mass, acceleration, and the applied force.  Both acceleration and force must have the same vector direction.

This can also be viewed in different terms:

Momentum = mass x velocity.  The change of momentum of a body is proportional to the impulse impressed on the body, and happens along the straight line on which that impulse is impressed.  Momentum cannot be changed unless acted upon by an outside force; it can only be conserved.

Acceleration is proportional to the unbalanced forces acting on it and inversely proportional to the mass of the object (a = F/m)

Ken Griffey Jr. had one smooth 2nd Law of Motion

How F=ma Applies to Biomechanics

Pretty much every static and dynamic movement has a force.  Muscles are the tissues that contract and create force on the body’s levers (connective tissue, bones).  With any human movement, F=ma can be used to create a simplified calculation of force.  This equation can even be used with static positions.  Consider the static forward head posture.  Gravity and the mass of the head imposes an antero-inferior force.  To counter this force and prevent your neck from snapping off at your desk, you have to constantly contract your levator scapulae, upper trapezius, and posterior cervical muscles to counter this force.  By calculating the acceleration of gravity and mass of the head, you can begin to calculate the muscle forces necessary to prevent movement.

Mass of head and gravity combine to impose a downward force

3) Action Reaction Law

For every action there is an equal and opposite reaction.  This law describes how forces always come in pairs, meaning that anytime objects are contacting each other, they are exerting a force.  An important consideration here is the concept that gravity is ALWAYS touching every object.

Tom Cruise displays his confident control of Newton’s 3rd Law of Motion in the classic movie “Color of Money”

How Action-Reaction Applies to Biomechanics

Putting that ankle weight on a patients leg will create an increase in the force of the mass and downward pull with gravity, the reaction is that the opposing muscle will have to create a force to overcome this mass.  Another example of this law is with ground reaction forces.  Running on soft ground will result in much less impact forces than running on hard concrete.

Ground Reaction Forces During Gait

Bottom Line

Newton’s 3 laws of motion are the basis for understanding motion and the correlative force systems.  Each law can be applied to biomechanics in it’s own way.

  • Inertia = An object of a constant velocity tends to remain in that state of motion unless an unbalanced force is applied to it
  • F=MA = The net force applied to a body (mass) produces a proportional acceleration
  • Action-Reaction = For every action there is an equal and opposite reaction



Newtonian Laws





Biomechanic Relationships

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