12 Force
Julio Gea-Benacloche and Amanda Shelton
What is Force?2
One simple way to think about a force is to identify it as some type of external influence on an object. The way that external factor influence the object can have a multitude of variables that influence it (friction, gravity, speed, velocity, direction, acceleration, mass, etc.) but at its core it remains as some type of external influence on an object. We measure force through the unit of measure of the Newton (N).
Newton’s Laws of Motion2
Most people have heard of Sir Isaac Newton, the founder of calculus, and many have heard of his Laws of Motion – but what are they again?
- Newton’s First Law: the law of inertia
- An object at rest will stay at rest and an object in motion will stay in motion (at a constant velocity), unless acted upon by an external force.
- Newton’s Second Law: F = ma
- A force acting on a body with mass will produce and acceleration proportional to that force.
- In this equation: F = force; m = mass; and a = acceleration
- Newton’s Third Law: the law of action and reaction
- For every action, there is an equal and opposite reaction.
Mechanical Energy2
One way that Newton’s Third Law can be applied in action is through forces derived from potential energy. The easy way to think about potential energy is to think of it as a spring. When you change the length of the spring you create what’s called potential energy. Whether you lengthen or shorten the spring you are changing its position – it wants to return to it’s natural state (the original position) and once you release the spring from the held position, that release of positional energy creates a reaction to return the spring to that natural state by converting that potential energy into kinetic energy, which is what helps to create movement.
We can also think about potential energy as a function of gravity. This similarly would be impacted by the object position and the potential for change as it changes position down a hill or around an access. If we come back to the human body and creating movement, we can think about how we can create potential energy through the change of position of our body (or a specific body part).
Human Movement, Potential Energy, and Kinetic Energy
Let’s create some kinetic and potential energy. Take your hand and lift it up over your head.
- How did you move your hand up over your head? You created kinetic energy through muscle action (which will get into in a later section).
- Now that your hand is over your head – what is keeping it there?
- What forces are acting on your arm to keep your hand over your head? Gravity? The muscles in your shoulder, upper arm, elbow, and forearm?
- Does your arm contain potential energy?
- How could you convert that potential energy into kinetic energy?
As we participate in various body movements, we are constantly going through a conversion of mechanical energy between potential and kinetic energy along with other types of energy like elastic energy, chemical energy, and thermal energy.
Elastic energy is often described as a potential energy function (as an example, think of our spring analogy from earlier) and is included within the system of our mechanical energy. The elasticity of a body creates an energy conversion on a macroscopic level (large scale). This can also be thought of in sport occurring outside of the body through something like a pole vault.
The elasticity (via elastic energy) of the pole itself, in relation to the mass of the person on one end, acceleration of the person, gravity, and its deformation as its positioned against a stop before the jump creates potential energy which converts to kinetic energy as a function of mechanical energy as a system.
Gea-Banacloche, J. (2019). University Physics I: Classical Mechanics. Open Educational Resources. Retrieved from https://scholarworks.uark.edu/oer/3
the resistance that one surface or object encounters when moving over another
the force that attracts a body toward the center of the earth, or toward any other physical body having mass
the rate at which an object covers distance
the speed of something in a given direction
a vector quantity that is defined as the rate at which an object changes its velocity
the quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field
an absolute unit of force in the International System of Units (SI units), abbreviated N. The force necessary to provide a mass of one kilogram with an acceleration of one meter per second per second.
stored energy that depends upon the relative position of various parts of a system
energy which a body possesses by virtue of being in motion, energy stored by an object in motion
sum of the kinetic energy, or energy of motion, and the potential energy, or energy stored in a system by reason of the position of its parts
the potential energy created through elastic deformation
the energy stored in the bonds of chemical compounds including atoms and molecule which is release when a chemical reaction occurs
energy that is caused by motion of the particles within the object or system related to its temperature