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Unit 2 Breakdown

You are on Lesson 2 of 8

1. Unit 2.1 | Understanding and Applying Newton’s Law in Depth
2. Unit 2.2 | Common Linear Forces, Equations, and Misconceptions [Current Lesson]
3. Unit 2.3 | Drawing and Understanding Force Body Diagrams
4. Unit 2.4 | Deriving Equations to Solve Linear Force Problems
5. Unit 2.5 | Advanced Force Problems – Tension and Elevators
6. Unit 2.6 | Advanced Force Problems – Ramps (Inclines)
7. Unit 2.7 | Advanced Force Problems – Pulley System
8. Unit 2.8 | Advanced Force Problems – Multi-Body System

In this lesson you will learn: 

• What are linear forces and why they are important
• How we use forces in Physics
• Six common linear forces and their equations

The Importance of Forces

Forces is the most important concept to learn in Physics. 

In fact, the rest of Physics is just learning about different types of forces and how they work. 

Think about planets. From this we study gravitational forces.

Now zoom down to the microscopic level. From this we study the forces that holds atoms together. We call this the “strong force” because it is one of the strongest force to exist!

What is a force?

Simply put, a force is a push or pull.

And it doesn’t have to be a physical push or pull. Magnets, for example, can push and pull without needing to touch.

A force has a direction and a magnitude. Thus it’s a vector quantity.

Reflect back to Newton’s second law: F = ma. This equation tell us that a forces cause acceleration. 

In this Unit we ONLY cover linear forces — forces that act in a straight line.

Pushing a Box

Think about pushing a box from rest.

As long as your pushing force, is greater than the frictional force, the box will speed up. In other words, you are accelerating the box.

All forces are responsible for accelerating objects.

Units

Newtons (N) is the standard unit for force.

Example: 100 N of pulling force.

Let’s derive the unit of force using Newton’s 2nd Law:

F = m x a → F = [kg] x [m/s²].

*Replace variables on the right side of the arrow with standard units .

So the unit, “Newton,” is a simple way of writing kg·m/s².

Forces to Know

In order to excel in Physics, you MUST memorize a few basic forces and their formulas.

Forces can be anything! It doesn’t only have the forces given below. There’s no right or wrong way to write these as long as you know what the label means.

For example. Let’s say you want to describe an engine pushing a car. We can “make up” a label called F_engine.

The forces given below are the most common ones you’ll come across in problems.

• Applied forces (F_A or A)
• Weight (mg or W or F_w)
• Tension (T or F_T)
• Normal force (N or F_N)
• Static Friction (f_s)
• Kinetic Friction (f_k)
• Spring Force (F_spring)

Now let’s cover each force in depth.

Applied Forces

Generally written as F_A.

An applied force is a general force. There are no strict rules or equations for it.

Usually an applied force is responsible for pushing or pulling something.

For example. A car speeds up (accelerates). This means there is a force being applied. We can call this force F_engine, we can call it F_A, or any other label that describes it best.

Weight

Let me state three commonly misunderstood facts:

1. Weight is a force. 
2. Mass and weight are NOT the same
3. Gravity is NOT a force.

Let’s start with gravity.

Gravity

Gravtiy = g = 9.81 \frac{m}{s^2}

Look at the units carefully. The units of gravity is m/s², which is a unit of acceleration.

Thus, gravity is an acceleration NOT a force. 

Mass

Mass is what you, or an object, is made up of. Your mass is the same everywhere in the universe.

The only way you can loose mass is if you trim you hair, go on a diet, etc. 

We measure mass is in kilograms (kg). For example: A person that has a mass of 70 kg.

Helpful tip: the number on the scale is your mass NOT your weight!

The force of Weight

Weight is a force that we get when we multiply mass by by acceleration due to gravity. 

Weight = mg

This comes from F = ma. We use gravity (g) in place of a general acceleration (a).  

Weight can also be written as F_w or W or F_g.

Because gravity is a vector that always points down, your weight is a vector that will also always point directly down.

All objects, on earth, will have weight.  

For example: Suppose I have a mass of 50kg. My weight will be:  50 kg × 9.81 m/s² = 490.5 Newtons.

Weight depends on gravity. The less the gravity, the less we will weight. Thus in space, where there is no gravity, we are weightless.

Tension

Tension forces can be anything involved with ropes, strings, or cables. Here’s a few examples in which you will find tension:

• Elevators
• Hanging signs
• Pulling objects with a cord
• Hanging objects
• Yo-yos
• Guitar or stringed instruments

If your problem involves any of the above, then you will likely be dealing with a tension force. 

There is no set equation for tension. You will have to derive it based on each problem. We will cover this in the next lessons. 

You can write tension as T, F_T or F_tension.

Normal Force

This is the force exerted by ANY surface, perpendicular (90°) to that surface.

All surfaces, floors, tables, will exert a normal force. 

There is even a normal force acting on your right now! If you are sitting, the normal force is coming from your seat that’s pushing at an equal and opposite to your weight (according to Newton’s third law).

Like tension and applied forces, there is NO set equation for Normal force. You will have to derive an equation for normal forces for every problem. 

Misconception on Normal Forces

A lot of students think that normal forces are opposite to weight.

This is not always true. Instead, normal forces are always perpendicular to the surface.

Here’s an example that demonstrates this: Look at the box on the ramp below. While weight points straight down, notice how the normal force doesn’t point opposite to weight. Rather it points perpendicular to the surface.

Friction

There are two types of friction (f) — static friction and kinetic friction.

While they have two different meanings they share the same equation:

f = \mu N

Where N is the normal force, and µ (pronounced ‘mew’) is the coefficient of friction. 

The coefficient of friction tells you how rough a surface is. 

For example a µ = .1 would be very slippery, like ice. And a µ of .8 would be very rough, like a highway. 

Static vs Kinetic Friction 

[ WATCH THIS VIDEO] 

Spring Force

The spring force comes from Hooke’s Law. It states that the force exerted on a spring is directly proportional to how much you stretch or compress it. Here’s the equation

F_{spring} = kx

Where k is the spring constant and x is the distance compressed/stretched

Anything involving a spring will have a spring force.

Lesson 2.2 Recap

You learned the seven most common linear forces in Physics, their equations, which problems to apply it to, and some misconceptions.

Furthermore, you learned about the significant difference between mass and weight.

Naming forces turned out to be ambiguous, but simple. For example weight can be written as F_w or W or mg.

Finally, you now understand the difference between static and kinetic friction.

Later, in Lesson 2.4, you will use these forces to derive custom equations.

Lesson 2.3 Preview

In the next lesson we will use our knowledge of forces to create simple force diagrams called FBDs. Drawing FBDs quickly and accurately will assist us in solve complex problems with ease!