Focus on Dynamics, Part II, Kinetics

In Part I of Mechanical Engineering, Focus On Dynamics, we talked about kinematics.  in This Part II, we’ll talk about kinetics.

Kinetics is the study of the relationship that exists between forces acting on a body, the mass of the body itself, and its motion.  It’s used to predict the motion caused by given forces acting on a body or to determine the forces required to produce a motion of the body.

In kinetics forces can be attributed to the motion of an object by way of Sir Isaac Newton’s Second Law of Motion.  In other words, this law relates the kinematics of an object to the forces applied to it, and in equation form that relationship looks like this:

Force = [Mass] x [Acceleration]

“Mass” is the weight of the object, in pounds, divided by the acceleration of the Earth’s gravity.  On Earth, the acceleration of gravity is considered to be a constant and is taken to be 32.2 feet per second, per second.  (No, that’s not a typo.)

Legend has it that Newton discovered the earth’s acceleration when gravity caused an apple to fall from a tree and hit him on the head.  That may or may not be true, but Newton’s Second Law of Motion forms the foundation for the study of kinetics.

To illustrate how kinetics works, we can look at another very simple example.  Suppose you have a block of steel that weighs 50 pounds, and it is sitting still on a nice smooth, frictionless sheet of ice (see Figure 1).

Figure 1

 How hard would you have to push on the side of this block to make it accelerate across the ice at a rate of 10 feet per second, per second?  Use Newton’s second law to find out:

Force = [(50 Lb.) ÷ (32.2 ft./second²)] x [10 ft./second²]

Force = 15.5 Lb.

Force and energy calculations in kinetics can get really complicated, especially when you have to consider the effects of friction, forces acting at angles, velocity, rotation, motion constraints, collisions, etc.