Constructing Physics

Elementary Physics Course Notes

Adam Johnston
Department of Physics
Weber State University

 

"It's been so long since I've had a good physics lecture," noted one woman.  "I don't know how I made it through the weekend," her boyfriend responded.  "I just can't get enough physics."

Fortunately, this lecture did not disappoint.  As a class, we got to witness objects both sitting still AND in motion.  Motion, of course, comes in different qualities.  It can be uniform and constant, like a ball rolling across a table or a cart rolling across a track or a book just sitting on a desk.  (Yes, rest is a state of motion, as far as we're concerned.)  However, motion can also change.  A change in a velocity (Remember, velocity is a speed with a specific direction.) over some time is known as an acceleration.  This can be a speeding up (ball rolling down hill), slowing down (cart rolling up a hill), or a change in direction (object going in circles).

We discussed how a ball tossed straight up into the air has a changing velocity, but a constant acceleration (because the velocity is changing consistently).  In fact, even when the ball's velocity is 0 at the top of its path, it still has an acceleration.  If it didn't, then it would have never fallen back down!  An opposite situation is that of a ball rolling across a flat table.  In that case, the velocity is constant, so the acceleration is zero (because there is 0 change in velocity).

Aristotle would have stated that any motion must be caused, but we have seen that only accelerated motion has a cause.  (Constant motion just happens, without any kind of intervention from the outside.)  Such a cause is known as a force.  In fact, a force is any action which has the potential to cause an acceleration.  More simply put, any push or pull is a force.

Still, even without forces, we've seen that motion continues just fine on its own.  This is known as the law of inertia.  This was initially described by Galileo, and later incorporated by Newton as his first law of motion.  We demonstrated this by looking again at the cart rolling across a level surface, an object at rest, the spinning of a wheel, the tendencies of a pencil resting on a hoop resting on a bottle (you had to be there), and the tendency of a ball launched up into the air from a moving cart (similar to the tendency of a person jumping up and down on a moving airplane).

You can re-create the ball being launched from a moving cart with the ball catcher animation.  It is found on the projectile motion animation list.  (We'll cover more details of projectile motion later.)

People were still wondering about these things we call forces and what they do for us and what they do for motion.

Want to see some more examples of linear motion?  Take a look here.

It is important to review Newton's 1st law over and over; but, since it was stolen from Galileo, and since Johnston keeps rolling the cart back and forth on the track, over and over, we tend to get this well engrained into our heads. (If only someone had done the same for Aristotle!)

Some applications of Newton's laws can be seen in the linear motion demos found here.

Newton's second law could be most quickly described as an equation: a=F/m, where 'a' is acceleration, 'F' is the net force, and 'm' is the mass.  Mass, and we mentioned last time, is the amount of matter in an object, or a measure of the object's inertia (its tendency to maintain a constant motion).  A net force is the resulting, total force after all forces have been added up and/or canceled out.  It is important to keep straight the net force from any individual force.

Anyway, N's 2nd simply states that the harder something gets pushed, the more its motion will change (accelerate).  However, the more massive an object is, the more resistive it is to such change.  This was demonstrated in multiple ways in class, and is seen in everyday life quite a bit.  N's 2nd also is used to solve a plethora of physics problems, from how to launch a probe to Saturn to where a rubber ball will land after it bounces off a table.

Newton's 3rd law states that you can't get away with anything: Anytime you push on something, it automatically pushes back on you with an equal but oppositely directed force.  Thus, you can walk forward because you push backward on the ground.  Rockets lurch upward because they push exhaust backwards.  In this sense, we say that there is always a reaction to every action, or that forces always come in pairs.