monday, september 21, 1998 chapter 4 -- frictional force tension
TRANSCRIPT
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.
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
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Scores
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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.
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...
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.