Force Revisited

I feel like I did a rush job on forces previously and wanted to come back to them, go into slightly more detail. No examples, only the first three laws. Nice and “easy”!

Newton’s First Law

A body in motion remains in uniform motion, and a body at rest remains at rest, unless acted on by a non-zero net force.

We define uniform motion as an object that is at a constant velocity, i.e. there is no change in the velocitie’s magnitude or direction (no acceleration).

Pretend you are in space, and you’ve got a tiny thruster. If you turn on your thruster, for a second, just a moment you will start moving. Now unless you reverse direction you will remain in motion forever.

Newton’s Second Law

A quick note, I’m using calculus notation so we’ll have derivatives. If you are in a non-calc class, you can replace the derivatives with Δ’s. Is this sacrilege, yeah but is sacrilege within reason.

The second law is an equation describing the sum of forces, and in it’s most general form

We make an assumption here for intro physics, and that is mass is constant (and that’s a fair assumption). Recall that the first derivative of velocity is acceleration (or the difference of velociy with repsect to the difference of time). With that the equation simplifies to:

There is an addendum here that’s dependant on who is teaching, what book you are using, and potentially who you might be getting help from. The force we are describing here is the net force, or the sum of forces acting on an object. For this, we will use the following terms interchangeably:

Here’s where I’ve found people getting confused. Pretend you are sky diving, there are two forces acting on you (for the sake of argument), 1) gravity, 2) the drag from the parachute. Neighter gravity or drag is equal to ma, their sum is.

Newton’s Third Law

If an object A exerts a force on an object B, B will exert and equal and oposite force on A.

The mass, size, area, and volume don’t matter. Lets play with two ideas for a moment to explore this a bit more.

1) Gravity pulls you down with an acceleration of 9.8 m/s squared. We have evolved to survive and not get squashed by this pull downwards. So we feel an mg (our weight) amount of force from the Earth and apply an equal and oposite force. Now lets pretend that you can only push back one half of mg. You would start to go squish. If it was no mg’s you would be a atom thin pancake moving your way towards the center of the earth. Bit of an over simplification, but that’s the general picture.

1. Now lets say you go into space, and take off your space suite. What would happen? Well, you have evolved to deal with the pressure of the earth. Your gasses and fluids would no start to expand without the pressure of the earth (or space suite) pushing in on you! The only think keeping them in is your skin and organs they are trapped in!

Key ideas to keep in mind

1. These laws of the crux of physics. Not important, the crux. If you have a force, you can determine, velocity, acceloration, displacement, energy, everything! Learning how to work with forces, will make the rest of physics easier.

2. The second law is not more important than the other two. It’s an equation sure, but the other two hold critical information that detail how to think about a problem, and how the world works. These three laws work in unison.

3. Get this in your head: the NET of the forces is equal to ma, NOT any old force. If you cannot, you are giong to have such a struggle