DAY 25 LETS GO!

TODAY AT A GLANCE:
1) You try a math example and learn about pulleys.
 2) We look at pulleys in a different way.
 3) LOTS of learning/graphing and questions for
pulleys

LETS GET OUT THE HW

We will have a HW turn in next class, possible
HW are Day 23, Day 24, Day 25.

Questions?

So we have 1 more HW due for tomorrow.
TENSION AS A FORCE

Tension is the force from a rope or string. A rope
can only have 1 tension on it.


Put equal masses on each side. Now draw a FBD
for each mass under your general picture.
.


Put unequal masses on each side. Now draw a
FBD for each mass under your general picture.
Remember to figure out which way the imbalance
is!
.

As one side gets more and more massive, what
will happen to the acceleration?
Is there a limit to the value?

MY THINKING:
“The weights oppose each other. As one gets
heavier, its like the other weight is not really
there.
So its going be like its not opposed. Which would
be free fall.
LETS DO AN EXAMPLE TO START US OFF


Here is a massless
pulley that rotates
without friction.
What does that mean?
3
kg
7
kg
MAIN GOAL: What is the acceleration of these
masses?
 SIDE GOAL: How much tension is there?



1st: Draw a FBD on each
Circle forces, label imbalance/balanced
Next: Lets CREATE net force equations for this
situation.
Lets solve for our:
 Main Goal:


Side Goal:

Now for a new, fresh look:
Draw this in your
notebook front and
center.
 CALL IT: Pulley
on table


Observe the setup
IN CLASS NOTEBOOK

The graph:

The reasoning:
IN CLASS NOTEBOOK

A)
B)
 The reasoning:

IN CLASS NOTEBOOK

The result:

The reasoning:
IN CLASS NOTEBOOK

The result:

The reasoning:

1) What do you observe?

2) What can you measure?

3) What can you change?
GOALS:
 1) Determine how the acceleration relates to the
force making them move.


2) Determine how the acceleration relates to the
total mass of the system (but not changing the
hanging mass).
GOALS:
 1) Determine how the acceleration relates to the
force making them move. Get actual trials
 More hanging weight = more acceleration in a
directly proportional amount
 2) Determine how the acceleration relates to the
total mass of the system (but not changing the
hanging mass). Get actual trials
 More mass on top = less acceleration because
more mass is being moved by the hanging thing.

LETS SHOW IT

Assume no friction, and the pulley and rope are
mass less (WHY?)
DRAW A FBD FOR EACH


Hanging Mass
Table Mass
Write the imbalance/etc, circle, create a net force
equation for each:
DO MORE
Lets now solve for TENSION for each
 Hanging:
Table


And now set the Tensions equal to each other to
solve for acceleration:
QUESTION:

Try the 1st HW question for me. On the boards!

1) Any volunteers to tell me what they think?

Possible Answers:

Block B’s weight pulls it down. The imbalance is
down, so the down pull MUST be larger than the up
pull on B. The up pull is the tension, so it must be
less.

The amount of imbalance is the weight of B. That
weight moves it and A. Since the tension is just
moving A and NOT B, the tension is less than B’s
weight.