# 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?

1. 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.
2. Newton’s Second LawF = 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
3. 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).
• 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

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