# Laws of Physics

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 are 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!