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Models of Forces

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A note on usage: The clicker slides in this booklet are meant to be used as stimuli to encourage class discussion. They are intended for use in a class that attempts to help students develop a coherent and sophisticated understanding of scientific thinking. They are NOT intended as items to test whether students are “right or wrong” or “know” the correct answer by one-step recall if enough cues are given. This has a number of instructional implications that are reviewed in general on the next four slides. The individual slides also contain annotations discussing their intended use.

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Usage: 1 Feedback One of the most important values of a clicker-response system is to provide instructors with some understanding of what students are thinking. Good clicker questions can be highly revealing (and surprising). But the critical fact is not that the students make mistakes but to use those mistakes to probe their thinking and find out why. This raises the importance of a rich subsequent discussion well above “letting the students know what the right answer is.”

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**Usage 2: Student-student interactions**

The critical value for student learning occurs in what happens after a clicker question has obtained a mixed response from the students. The standard next cue is, “Find someone who disagreed with the answer you chose and see if you can convince them.” After a minute or two of discussion, a second click may show students having moved dramatically towards the correct answer. A brief call for who changed their answer and why can lead to a useful exchange. When they have not moved significantly, more discussion is called for.

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**Usage: 3 Incompletely specified questions**

Some items have questions that are simple if idealized assumptions are made, subtler if they are not. Part of the discussion of these items are intended to include issues of modeling, idealizations, and hidden assumptions. Questions where answers are not provided. In these items, the intent is to have students come up with potential answers and have the instructor collect them and write them on the board. Occasionally, especially at the beginning of a class, it may take some time before students are willing to contribute answers. It can help if you have some prepared answers ready, walk around the class, and put up the answers as if they came from the students. This can help students get more comfortable with contributing.

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**Usage: 4 Cluster questions Problem solving items**

Some questions are meant to be used as part of a group of questions. In this case, resolving the answers to individual questions is better left until the entire group is completed. The value of the questions are often in the comparison of the different items and in having students think about what changes lead to what differences and why. Problem solving items In these items (indicated by a pencil cluster logo), the intent is to have students work together to solve some small problem. After a few minutes, ask the groups to share their answers, vote on the different answers obtained, and have a discussion.

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Two springs, are linked together and pulled from opposite ends by equal tension forces T. The spring constants are NOT the same: k1 >> k2. The system is at rest. How do the forces that the springs exert on each other compare? They are equal but not equal to T. They are equal and equal to T. Spring 1 exerts a larger force on spring 2, than 2 does on 1. Spring 2 exerts a larger force on spring 1, than 1 does on 2. Something else. 1 2

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Two springs, are linked together and pulled from opposite ends by equal tension forces T. The spring constants are NOT the same: k1 >> k2. The system is at rest. How do the the amounts that the springs stretch compare and how do you know ? They are equal because they exert equal forces on each other. They are equal because the larger k of spring 1 is compensated for by its larger mass. Spring 1 stretches more because it has a larger k than spring 2. Spring 2 stretches more because it has a smaller k than spring 1. Spring 2 stretches more because it feels a larger force from spring 1 than spring 1 feels from spring 2. 1 2

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In the figure is shown the force needed to stretch an uncoiled DNA molecule. Suppose we measure the spring constant of DNA at three points: When it was 5%, 75%, and 125% longer than its unstretched length; Which measurement would yield the largest spring constant? 5% 75% 125% They would all be the same

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Consider two blocks made of the same material with the same cross sectional area but one twice as long as the other. If the same compressional forces are exerted on both blocks They will compress by the same amount. The short block will compress by twice as much. The long block will compress by twice as much. There is not sufficient information given to tell.

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Consider two blocks made of the same material with the same length but one twice cross sectional area as the other. If the same compressional forces are exerted on both blocks They will compress by the same amount. The wide block will compress by twice as much. The narrow block will compress by twice as much. There is not sufficient information given to tell.

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Suppose I start pushing a box along a table that has a box sitting on top of it. The boxes slowly start moving and the top one doesn’t slip. If there is a friction force from box A on box B, in what direction does it point? There is no friction between the boxes. Left Right Up Down You can’t tell from the information given.

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Suppose I start pushing a box along a table that has a box sitting on top of it. The boxes slowly start moving and the top one doesn’t slip. Box B is accelerating to the right. What unbalanced force is responsible for this? The push of the finger. The normal force of box A on B. The friction force of box A on B. The weight of box B. Something else.

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Physics 131 4/12/2017 If I drop a light object (a wooden ball) and a heavy object (a steel ball) from 4 m, which will hit the ground first? The light one (by a lot) The heavy one (by a lot) About the same You can’t tell from the information given. Prof. E. F. Redish

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Physics 131 4/12/2017 If I drop a light object (a paper ball) and a heavy object (a steel ball) from 4 m, which will hit the ground first? The light one (by a lot) The heavy one (by a lot) About the same You can’t tell from the information given. Prof. E. F. Redish

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**Which ball will hit first?**

Physics 131 4/12/2017 Which ball will hit first? The shot one The dropped one They’ll hit at the same time You can’t tell from the information given. Prof. E. F. Redish

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Physics 121 4/12/2017 Two dense objects (so air drag can be ignored) are shot straight up at the same time from the same height. Object A is shot with a speed of 1 m/s, object B with a speed of 2 m/s. Which takes longer to come back to its starting point? Object A Object B Both take the same. I can’t tell since you didn’t give me the masses. I can’t tell for some other reason. Prof. E. F. Redish

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Physics 121 4/12/2017 Two dense objects (so air drag can be ignored) are shot up at the different angles at same time from the same height. They follow the trajectories shown. Which will hit its target first?? Object A Object B Both the same. I can’t tell since you didn’t give the masses I can’t tell for some other reason. A B Prof. E. F. Redish

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Physics 131 4/12/2017 The cart runs along a horizontal track. Part way along it strikes a small hook that causes the cart to throw the ball straight upward. Will the cart catch the ball? No. The ball will fall behind the cart. No. The ball will fall ahead of the cart. Yes It depends on how fast the cart is traveling. You can’t predict. Prof. E. F. Redish

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Physics 131 4/12/2017 Now the cart is attached to a string that runs over a pulley to a weight. This exerts a constant horizontal tension force on the cart. Will the cart catch the ball? No. The ball will fall behind the cart. No. The ball will fall ahead of the cart. Yes It depends on how fast the cart is traveling. You can’t predict. Prof. E. F. Redish

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Physics 131 4/12/2017 Now the string is removed but the track is tilted down. Will the cart catch the ball? No. The ball will fall behind the cart. No. The ball will fall ahead of the cart. Yes It depends on how fast the cart is traveling. You can’t predict. Prof. E. F. Redish

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**a. Situation I only. e. In two or more of the situations. **

In the situations below, a mover pushes two crates on a horizontal surface, and they move together with a constant a. In which situations are the forces that the two crates exert on each other equal in magnitude? a. Situation I only. e. In two or more of the situations. b. Situation II only. f. In all of the situations. c. Situation III only. g. You can’t tell from what’s given. d. Situation IV only. From Jerry Touger’s collection

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**F2 -- the force that crate B exerts on crate A **

The mover is pushing two crates along a frictionless horizontal surface and the crates are slowly increasing their speed. Consider the following four forces: F1 -- the force that the mover exerts on crate A F2 -- the force that crate B exerts on crate A F3 -- the force that crate A exerts on crate B F4 -- the force that crate A exerts on the mover Which of the following correctly compares the magnitudes of these forces? From Jerry Touger’s collection

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A paramecium swimming through a fluid is moving at approximately a constant velocity as a result of wiggling its cilia. What can you say about the net force that is being exerted on it while it is doing this? It is significantly greater than zero It is a little bit greater than zero It is equal to zero It cannot be determined from the information given.

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A paramecium swimming through a fluid is moving at approximately a constant velocity as a result of wiggling its cilia. What can you say about the magnitude of the normal force that the paramecium’s cilia exert on the water (Ncw) compared to the magnitude of the normal force that the water exerts back on the cilia (Nwc) ? Ncw is significantly greater than Nwc Ncw is a little bit greater than Nwc Ncw = Nwc Ncw is significantly less than Nwc Ncw is a little bit less than Nwc It cannot be determined from the information given.

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A paramecium swimming through a fluid is moving at approximately a constant velocity as a result of wiggling its cilia. What can you say about the magnitude of the normal force that the paramecium’s cilia exert on the water (Ncw) compared to the magnitude of the viscous force that the water exerts on the paramecium (Fwp) ? Ncw is significantly greater than Fwp Ncw is a little bit greater than Fwp Ncw = Fwp Ncw is significantly less than Fwp Ncw is a little bit less than Fwp It cannot be determined from the information given.

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