Physics - Vertical Motion - Basic Falling Object Problems-- How are we doing now?

OPENING QUESTION: Today we'll do a couple of timed exercises to see how we're doing with these pesky falling object problems.

#1) VERY basic: You're lying on your back in a pretty meadow on a nice calm spring day with your trusty wrist-rocket sling shot and a supply of frozen grapes.

You launch a grape upwards with an initial velocity of 39.50 m/s.

a) How high does it go

b) How long does it take to get there.

I'll set the timer for 5 minutes - Go.

We'll peer grade these

#2) Intermediate: You drop a frozen grape from the top of a 550 m structure.

a) How long does it take before it goes *splat* ?

b) How fast is it going the tiniest fraction of a second before it hits the ground?

I'll set the timer for 10 minutes - Go.

ARNING OBJECTIVES:

  • I will evaluate my ability to solve falling object problems during today's class

CALENDAR:

Vertical motion test on Thursday (NOT to include projectile motion)

WORDS O' THE DAY:

  • gravity! gravity! gravity!

FORMULAE OBJECTUS:

    • a = (vf - vi)/(tf - ti) (definition of acceleration)

    • g = 9.81 m/s2 (acceleration an object experience on Earth) ONLY present in vertical motion (Y axis) problems

      1) vfy = viy +agt

      2) yf = yi + viyt + 1/2agt2

      3) vfy2 - viy2 = 2ag∆y

WORK O' THE DAY:

Let's go back to this MOST informative (AND COMPLEX) graphic of a falling object problem:

 

There are a couple of things to keep in mind when we evaluate motion in 1 dim (vertically):

  • gravity is ALWAYS present and is ALWAYS pulling an object towards the earth @ 9.81 m/s/s. What evidence is present on the graphic that reminds us of that?

  • when an object is dropped, thrown, kicked, shot or otherwise launched upwards in vertical motion, we ALWAYS evaluate the motion as directly upwards and/or directly downwards. There is no horizontal (x) motion at all. (However, there will be when we get to projectile motion next). If we look at the graphic, that doesn't appear to be the case. Why has the author shown motion that seems to be <slightly> in error?

Here's a pretty complex graphic. Look at JUST parts A and B.

What can you determine from the values shown there?

 

Let's take a look at a couple of practice problems in our book: #40, 42 and 43.