|
Newton's first law states that every object will remain at rest
or in uniform motion in a straight line unless compelled to change its
state by the action of an external force. This is normally taken as the
definition of inertia. The key point here is that if there is
no net force acting on an object (if all the external forces cancel
each other out) then the object will maintain a constant velocity.
If that velocity is zero, then the object remains at rest. If an
external force is applied, the velocity will change because of the
force.
The second law explains how the velocity of an object changes when it
is subjected to an external force. The law defines a force to be
equal to change in momentum (mass times velocity) per change in
time. Newton also developed the calculus of mathematics, and the
"changes" expressed in the second law are most accurately defined in
differential forms. (Calculus can also be used to determine the velocity
and location variations experienced by an object subjected to an
external force.) For an object with a constant mass m, the second
law states that the force F is the product of an object's mass
and its acceleration a:
F = m * a
For an external applied force, the change in velocity depends on the
mass of the object. A force will cause a change in velocity; and
likewise, a change in velocity will generate a force. The equation works
both ways.
The third law states that for every action (force) in nature there is
an equal and opposite reaction. In other words, if object A exerts a
force on object B, then object B also exerts an equal force on object A.
Notice that the forces are exerted on different objects.
So, how does Newton's Laws of Motion revealed in a
baseball swing?
Newton's First Law of Motion
An object at rest tends
to stay at rest and an object in motion tends to stay in motion with the
same speed and in the same direction unless acted upon by an unbalanced
force.
The initiation of a
batters swing begins with the hips, the biggest and strongest muscles in
the body which include the legs and torso. The bat is at rest until the
force of the hips, legs and torso are applied at heel drop.
Hence...the hips lead the hands!
Newton's Second Law of Motion
The acceleration of an
object as produced by a net force is directly proportional to the
magnitude of the net force, in the same direction as the net force, and
inversely proportional to the mass of the object.
In terms of an equation, the net force is equated to the product of
the mass times the acceleration.
Fnet = m * a
or
Fnet = m(bat) * a (the speed
the bat is traveling)
Description of Force
An applied force is a force which is applied to an object by a person
or another object. If a person is pushing a desk across the room, then
there is an applied force acting upon the object. The applied force is
the force exerted on the desk by the person
Fapp
Using the large muscles in the body to generate maximum bat speed and
acceleration. By using the core muscles to launch the swing and move
the bat around the body a heavier and longer bat can be used. Using a
bigger, heavier bat (mass) and the core muscles to generate speed
(acceleration) players are optimizing their force.
In this entire discussion, the emphasis has been on the "net force."
The acceleration is directly
proportional to the "net force;" the "net force" equals mass times
acceleration; the
acceleration in the same direction as the "net force;" an
acceleration is produced by a "net force." The NET FORCE. It is
important to remember this distinction. Do not use the value of merely
"any 'ole force" in the above equation; it is the net force which is
related to acceleration. The net force is the vector sum of all the
forces. If all the individual forces acting upon an object are known,
then the net force can be determined.
In physics,
circular motion is rotation along a circle: a
circular path or a circular orbit. The rotation
around a fixed axis of a three-dimensional body involves
circular motion of its parts. We can talk about circular
motion of an object if we ignore its size, so that we
have the motion of a point mass in a plane.
Examples of circular motion are: an artificial
satellite orbiting the Earth in geosynchronous orbit, a
stone which is tied to a rope and is being swung in
circles (cf. hammer throw), a racecar turning through a
curve in a racetrack, an electron moving perpendicular
to a uniform magnetic field, a gear turning inside a
mechanism.
A special
kind of circular motion is when an object rotates around
its own center of mass. This can be called spinning
motion, or rotational motion.
Circular motion involves acceleration of the moving
object by a
centripetal force which pulls the moving object towards
the center of the circular orbit. Without this
acceleration, the object would move inertially in a
straight line, according to Newton's first law of
motion. Circular motion is accelerated even though the
speed is constant, because the object's velocity vector
is constantly changing direction.
Rotational motion is similar to circular
motion, except the object involved is a rigid body in
which all points rotate around the center of mass of the
object and not around a fixed point.
Pure rotational motion is circular movement in
which all points in the body move in circles, and that
the centers of these circles all lie on a line called
the axis of rotation. Pure Rotation is caused by an
arrangement called a 'force couple'. This is where two
equal and opposite forces act on the object from an
equal [perpendicular] distance apart.
Rotational hitters are taught to tilt their body and
swing up through the ball maximizing their rotational
force. The bat is pulled by the hips into the center of
the circular orbit, path of the pitch and rotates around
the axis. The bat path rotates through the ball and
around the body, circular orbit!
Newton's Third Law
A force is a push or a
pull upon an object which results from its interaction with another
object. Forces result from interactions!
According to Newton, whenever
objects A and B interact with each other, they exert forces upon each
other. Hit the ball square. When you sit in your chair, your
body exerts a downward force on the chair and the chair exerts an upward
force on your body. There are two forces resulting from this interaction
- a force on the chair and a force on your body. These two forces are
called action and reaction forces and are the subject of
Newton's third law of motion. Formally stated, Newton's third law is:
"For every action, there is an equal and opposite reaction."
The statement means that in every interaction, there is a pair of
forces acting on the two interacting objects. The size of the forces on
the first object equals the size of the force on the second
object. The direction of the force on the first object is opposite
to the direction of the force on the second object. Forces always
come in pairs - equal and opposite action-reaction force pairs.
Rotational mechanics teach hitters to hit the ball level to the path
it is on, equal and opposite to the pitched ball. The hips are working
equal and opposite, the shoulders are working equal and opposite and the
elbows are as well. Front hip back, back hip forward - front shoulder
up, back shoulder down - front elbow up, back elbow down. It's all
connected to each other and the results are rotational mechanics! |
