PHYSICS - Basic Force Analysis

OPENING QUESTION:

Please recite Newton's Laws to your team. Make sure you do that in a way that reflects how we will use those as a tool, and not just something to remember.

CALENDAR:

Vertical Motion Retake/Make Ups: This Friday (12/1/2023). Please arrive before 2:05

WORDS O' THE DAY:

  • Force ("Push" or "Pull")
  • Newton (kgm/s2)
  • Force Diagram (A sketch showing ALL forces acting on an object)
  • Net Force Diagram (A sketch showing only the resulting (net) forces acting on an object)

═══════════════════════════

We'll REVISIT THESE LATER IN THE WEEK

  • Newton's 1st Law ("An object will continue whatever it is currently doing unless acted on by an unbalanced force")
  • Newton's 3rd Law
    • (Short Version: "Forces ALWAYS come in pairs")
    • (Quick memory aid version: Noun1 Verb Noun2, Noun2 Verb Noun1)
    • (Formal Version: For every force acting on one object, there is an equal and opposite force acting on a second object)

FORMULAE OBJECTUS:

  • F = ma (Newton's 2nd Law)

═══════════════════════════

WORK O' THE DAY

We'll continue here tomorrow-ish

═══════════════════════════

Let us please now evaluate the following situations:

1) Your kid brother sits in his little red wagon (total mass = 31 kg). You pull that wagon to the right with 100.N of force.

  • Sketch that situation showing all initial conditions
  • Now please use Newton's 2nd to determine how fast your kid brother (and his little red wagon) will accelerate.

2) Your kid brother sits in his little red wagon (total mass = 31 kg). You pull that wagon to the right with 100.N of force while your cousin pushes on the little red wagon (with your brother still inside) also to the right with 51 N of force

  • Sketch that situation showing all initial conditions
  • Now please use Newton's 2nd to determine how fast your kid brother (and his little red wagon) will accelerate.

Those sketches are what we call "Force Diagrams". We show all forces acting on an object

However, when we do the 2nd problem, we add up the forces using algebra -- so as always, a force to the right is positive and an opposing force to the left is negative. The resulting force in this case represented in a NET FORCE DIAGRAM is 100.N + 51 N = 151 N of force to the right.

 

═══════════════════════════

Let's move along to Newton's 1st Law:

I usually think of Newton's 1st this way: An object will continue to do whatever it is currently doing until it is acted on by an unbalanced force

A more formal version:

An object in rest stays at rest until acted on by an unbalanced force. An object in motion stays in motion until it is acted upon by an unbalanced force

Either version works just fine for me... but PLEASE keep in mind these are not just words to memorize but TOOLS to help us explore some increasingly difficult physics situations!

To wit:

 

Example #1: Imagine we have a 501 kg mass being pulled to the right by four people while at the same time being pulled to the left by four different folks.

The 501 kg mass is NOT in motion.

Using Newton's First Law we can analyze that situation to show that since the object is at rest, the forces acting on it must be either:

1) Equal and opposite

2) Absent

In this case the answer is obvious #1 ==> equal an opposite forces.

Example #2: Imagine we have a 501 kg mass begging pulled to the right by four people while at the same time being pulled to the left by four different folks.

The 501 kg mass accelerates for a moment and then continues moving at a constant velocity to the left.

Using Newton's First Law we can analyze that situation to show that since the object is at rest, the forces acting on it must be either:

1) Equal and opposite

2) Absent

In this case the answer is not at all obvious

  • Please draw a force diagram for the time that the object is accelerating
  • Now please show a NET force diagram for that time period
  • Now please draw a separate force diagram for the time the object is moving at a constant velocity to the left
  • Now please draw a NET FORCE diagram for the time the object is moving at a constant velocity to the left

Notice that a smaller force MUST exist on one side. The object accelerates in the direction of that unbalanced force (Newton's 1st)

The second situation is a bit more nuanced:

The object is in motion but it is NOT accelerating. By Newton's first law that MUST mean the forces acting on it are balanced, even though the object is in motion!

Consider Newton's 2nd in that situation:

F= ma

The forces are balanced which means the net force acting on that object is Zero. The mass is NOT zero which means the acceleration MUST be zero.

But hold the bus!!!! recall from our dim dark days of motion that an object very definitely can be in CONSTANT motion (constant velocity) and have an acceleration of zero.

So... that means the object can also be moving at a constant velocity and still be represented by that same NET force diagram... *WHEW*

If time permits:

 

 

Speaking of the effects of acceleration -- let's take a gander at THIS rather astounding video of human endurance.