Monday, September 21, 1998 Chapter 4 -- Frictional Force Tension

Hint: Be able to do the homework (both the problems to turn in AND the recommended ones) you’ll do fine on the exam! Friday, September 25, 1998 in class Chapters inclusive You may bring one 3”X5” index card (hand-written on both sides), a pencil or pen, and a scientific calculator with you. I will put any constants and algebra & geometry formulas which you might need on a single page attached to the back of the exam.

I’ve finished grading the lab reports you turned in last week Monday... They weren’t so hot...

Could I have reproduced your results based on the description you provided in your lab report? 10 points Did your abstract summarize succinctly 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

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 lab report, revise the one for last week’s lab…

This force is described as the force of static friction, as it relates to the force observed on an object that does not move. We find that experimentally, the magnitude of the force of static friction is proportional to the normal force exerted by the green block on the red block. The constant of proportionality is known as the coefficient of static friction (  s ) and is a property of the materials at the interface.

If I push hard enough, the force of static friction will be broken and slipping will occur at the interface. The red block will begin to accelerate. When the red block slips, we note that it still feels a frictional force. The magnitude of that force, however, has changed. We find experimentally that the new frictional force is still proportional to the normal force exerted by the green block on the red block.

This force is described as the force of kinetic friction, as it relates to the force observed on an object that moves. The constant of proportionality is known as the coefficient of kinetic friction (  k ) and is a property of the materials at the interface.

Does friction always result in an object slowing down? Let’s look at a couple of examples to find out... What’s going to happen to the black object as I push the white book across the table? Make a prediction.Consult with neighbors.

Does friction always result in an object slowing down? What’s going to happen to the tennis ball as I push the white book across the table? Make a prediction.Consult with neighbors.

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

What must be true about the forces acting... here

Let’s examine this piece more carefully... Tension The forces balance -- The rope does not accelerate.

In fact, no matter which little segment of the rope I examine in this case, the tension forces balance in either direction, and the rope remains stationary. Okay, let’s look at tension in a rope that results in the acceleration of an object...

Remember this one? Frictionless pond of ice Force Meter

What exactly is it that causes the green block to accelerate? Frictionless pond of ice

Let’s look at the free-body diagram for the green block. What forces are acting on the green block? Weight Normal force Tension The tension force of the rope on the block results in the block accelerating.

What if we look at a piece of the rope in this case? Remember, the whole system is accelerating at the same rate, a. Tension 1 Tension 2 Some mass m r Some mass m b