Physics V: Forces

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Physics V: Forces

Author: oLahav

This series of lessons is designed to help you learn, or review, the fundamentals of physics. After going through motion, we shall now introduce forces.

What's a force?

A force in physics has nothing to do with the Star Wars thing that can make you fight like a Jedi. Here's a definition:

Force (f): a push or a pull in a certain direction. Measured in N. There are 4 types of forces: gravitational, electromagnetic, weak-nuclear and strong-nuclear. Gravitational forces have to do with mass, electromagnetic forces deal with charged particles, and nuclear forces hold atoms and nuclear particles together. Force is measured with Newtons, N. A Newton is a simple way of writing kilogram metres per seconds square. In other words, $N=\frac{kg * m}{s ^2}$ .This will make more sense when we look at Newton's Second Law.

What can forces do for me?

Forces are responsible for motion. There are a few main forces that we have to look at to understand this:

Gravity (g):
we've already looked at gravitational acceleration, but in reality gravity is the force that pulls objects towards the Earth causing that acceleration. The magnitude of this force is, not surprisingly, 9.8 N.
Applied (app):
the applied force represents any type of push or pull that doesn't naturally occur on an object. For example, pushing a box is an applied force.
Friction (f):
an important force, friction is the force that resists motion. For example, if you slide a book on a desk, it will eventually stop. This is because there is friction between the desk and the surface of the book, which resists the book's motion. Friction always happens opposite to the direction of the motion.
Tension (T):
This sort of force is the force experience by a rope that is tied to a suspended object. For example, when you tie something to a string, the force that prevent the thing from falling down is tension.
Normal force (n):
The force created by a surface that opposes gravity. For example, when you stand on the floor, the floor applies a normal force on you which cancels gravity out and prevents you from moving downwards.

How do we use forces?

If you know your vector addition, you're in luck. Analyzing vectors requires a close look at angles and magnitudes.

The key here is to use what's called aFree Body Diagram,

which has the object and all the various forces that act on that object. For example:

Then, you add all the forces up, and cancel out all the ones that cancel each other out. The result will give you the

Net Force.That is acting on the object:

Note that this tells us nothing about where the object is moving, or how fast. But it does tell us something about the direction of the acceleration. We can further analyze this to conclude that since the Net Force is primarily frictional, the object has to be moving to the left, but decelerating at the same time. Looking at magnitudes and masses, we can actually calculate this deceleration.