Monday, September 21, 1998

Chapter 4 -- Frictional Force Tension

Hint: Be able to do the homework (both theproblems to turn in AND the recommended ones)you’ll do fine on the exam!

Friday, September 25, 1998 in classChapters 1 - 4 inclusive

You may bring one 3”X5” index card (hand-writtenon both sides), a pencil or pen, and a scientificcalculator with you.

I will put any constants and algebra & geometryformulas which you might need on a singlepage attached to the back of the exam.

I’ve finished grading the lab reports you turnedin last week Monday...

They weren’t so hot...

Could I have reproduced your results based on the descriptionyou provided in your lab report?

10 points

Did your abstract summarizesuccinctly your results and basically

how they were achieved?5 points

Did you have all your graphs?Were they labeled? Did they

have captions?5 points

Did you analyze the procedure?Your results? Provide some

estimate of the errors?

5 points

Laboratory #2-3Mean = 18.0 Stddev = 3.1

0

1

2

3

4

5

6

7

1 3 5 7 9 11 13 15 17 19 21 23 25

Scores

Nu

mb

er

So, there’s a lab report due today…However, we do not meet for lab this week…

If you’d like, therefore, reflect on this labreport, revise the one for last week’s lab…

This force is describedas the force of staticfriction, as it relates tothe force observed onan object that does notmove.

We find that experimentally, the magnitude of theforce of static friction is proportional to the normalforce exerted by the green block on the red block.

The constant of proportionality is known as thecoefficient of static friction (s) and is a propertyof the materials at the interface.

If I push hard enough,the force of staticfriction will be brokenand slipping will occurat the interface. Thered block will beginto accelerate.

When the red block slips, we note that it stillfeels a frictional force. The magnitude ofthat force, however, has changed.

We find experimentally that the new frictionalforce is still proportional to the normal forceexerted by the green block on the red block.

This force is describedas the force of kineticfriction, as it relates tothe force observed onan object that moves.

The constant of proportionality is known as thecoefficient of kinetic friction (k) and is a propertyof the materials at the interface.

F Ffs s Nmax

F Ffk k Nmax

Does friction always result in anobject slowing down?

Let’s look at a couple of examples to find out...

What’s going to happen to the black objectas I push the white book across the table?

Make a prediction. Consult with neighbors.

Does friction always result in anobject slowing down?

What’s going to happen to the tennis ballas I push the white book across the table?

Make a prediction. Consult with neighbors.

Our avant-guarde socialite pulls on the ropethat’s wrapped around the tree. Nothing happens.

What must be true about the forces acting...

here here

here

Let’s examine this piece more carefully...

Tension Tension

The forces balance -- The rope does not accelerate.

In fact, no matter which little segment of therope I examine in this case, the tension forcesbalance in either direction, and the rope remainsstationary.

Okay, let’s look at tension in a rope that resultsin the acceleration of an object...

Remember this one?

Frictionless pond of ice

Force Meter

What exactly is it that causesthe green block to accelerate?

Frictionless pond of ice

Let’s look at the free-body diagramfor the green block. What forcesare acting on the green block?

Weight Normalforce

Tension

The tension force of therope on the block resultsin the block accelerating.

What if we look at a piece of the rope in this case?

Remember, thewhole systemis accelerating

at the same rate, a.

Tension 1 Tension 2Some mass mr

F F F m anet T T r 2 1

Some mass mb