Some stuff you should know

Physics of the Mundane

Adam Johnston

 

Given the fact that you might not have some of this in your notes, and Feynman doesn’t give it to you quite as clearly as you might like, and your instructor just smiles at you as you try to figure things out . . . here are some things that might be helpful in terms of understanding the physics of rockets, tree branches, and other mundanes:

 

Newton has three laws of motion:

1.     The law of inertia (inherited from Galileo):

a.  An object remains in motion unless acted upon.

b.  This means that an object’s acceleration is zero, unless there is an unbalanced force exerted upon it.

c.  E.g.: When you’re in the elevator and your motion is constant (you’re either stopped or you are moving at a constant rate) the scale will read exactly what is required to support your weight.

2.     “Newton’s 2nd” (this is so well known, that to just call it “the 2nd law” is descriptive enough)

a.  This can be written in a variety of different and lengthy ways, but most physicists would write it as:

 

 

 

 

 

 

b.   The 2nd law describes what happens when the 1st law no longer applies – when there is more force in one direction than in another.

c.  This applies to you in an elevator that is changing its motion; for example:

 

 

 

 

 

 

 

 

 

3.     “Equal & Opposite Reaction” law

a.  Whenever object A pushes on object B, object B automatically and instantly pushes on object A with an equal amount of force that is oppositely directed.

b.  Examples of this range from your ability to walk, the chair pushing up on your backside right now, and the fact that we can detect planets orbiting around distant stars by looking at how the star gets wobbled around, rather than at the planet itself.

 

 

Torques:

While forces are great to analyze, sometimes it’s not enough to just look at the amount of force.  Force will get some leverage if it is applied via a teeter totter or a wrench.  This is known as torque, which is defined as the length of the lever arm times the amount of force applied:

 

 

 

 

 

 

 

In the same way that Newton’s 2nd applies to forces, torques can be “balanced” so that there will be no change in motion of a system that might otherwise rotate.  This might be true in the case of a teeter totter:

 

 

 

 

 

 

 

 

 

 

 

Or for a tree branch: