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Even Physics Textbooks Tend to Get Friction Slightly Wrong – WIRED

Even Physics Textbooks Tend to Get Friction Slightly Wrong – WIRED

Sometimes you think you have a complete understanding of something and then BOOM—a simple problem throws everything out the window. Let’s consider a very basic physics problem involving pushing a block with a frictional force. These kinds of problems are quite common in your introductory physics textbook—but they often miss some subtle details.I’m going to…

Once in some time you’re thinking that you maintain a entire working out of something after which BOOM—a uncomplicated mission throws all the pieces out the window. Let’s recall in mind a no doubt total physics mission interesting pushing a block with a frictional force. These invent of complications are moderately total for your introductory physics textbook—however they usually pass over some subtle details.

I’m going to slip over two major solutions in physics: the momentum precept and the work-vitality precept. Let’s exhaust these two solutions for some uncomplicated physics cases and peek what occurs. It’d be fun.

Momentum Thought

The momentum precept says that a find force on an object is equal to the replace in momentum (p) divided by (t), the replace in time (the time rate of replace of momentum). Oh, momentum (for so much of objects) will be outlined as the manufactured from mass (m) and velocity (v). I’m going to value you this with a 1 dimensional example merely so as that I will steer particular of using vector notation (it would possibly perchance well perhaps in all probability perchance commit it to memory uncomplicated). Right here is the momentum precept (in 1D).

Rhett Allain

Now let’s exhaust this. Relate I in actual fact maintain a no doubt low-friction cart with a constant-energy force pushing on it (on this case, it has a fan mounted on high). Since there is a force, the cart will velocity up. Right here’s what that appears love.

We can now exhaust the momentum precept to ranking the replace in velocity over a whereas interval. Listed below are some basically staunch values for the cart above (I made some small changes on account of measurement errors).

  • Cart mass = 0.85 kg
  • Fan force = 0.15 Newtons
  • Time interval = 3.0 seconds

With the force and the time interval, I salvage a replace in momentum (Fdt) of 0.Forty five kgm/s. Dividing this replace in momentum by the mass, I salvage a closing velocity (assuming it starts from relaxation) of 0.53 m/s. Yay.

OK, let’s originate all of it once more. This time with TWO followers. Right here’s a cart with two equal forces pushing in reverse directions. After turning on the 2 followers, I give the cart a push so as that it moves to the ultimate.

In this case, the find force on the cart is zero Newtons for the rationale that force pushing to the ultimate has the an analogous magnitude as the force pushing to the left. With zero find force, there’s zero replace in momentum and the cart moves alongside at a constant velocity.

One extra case. Relate I recall a field with some hundreds and pull it alongside the table at a constant velocity. In this case, there is a force pulling to the ultimate (the string) and a frictional force pulling to the left.

All over again, for the rationale that find force is zero, there’s not the sort of thing as a replace in momentum. Every thing is handsome.

Work-Vitality Thought

This shouldn’t be at all times no doubt entirely new. In fact, it’s doubtless you’ll perchance additionally obtain this belief from the momentum precept. The work-vitality precept says that the work (w) carried out on a level mass is equal to its replace in kinetic vitality. Work is performed by a force transferring a obvious distance. In actual fact, it’s only the force within the direction of movement that matters. As an equation, it appears to be like love this.

Rhett Allain

Right here θ is the angle between the force and the displacement. If the force is “pushing backwards” it’s doubtless you’ll perchance additionally maintain unfavorable work. For the kinetic vitality, it’s far determined by the mass and the rate.

Rhett Allain

OK, let’s slip abet to the fan cart from above. Relate I desire to stare at this mission using the work-vitality precept as a substitute of the momentum precept. If that is so, I need one extra element—the gap over which the force is utilized. From that identical fan video, the force pushes the cart over a distance of about 0.Seventy nine meters. Now I will calculate the work (the angle is zero levels) with a stamp of 0.11 Joules. If I plot this equal to the closing kinetic vitality, I will solve for the closing velocity and I salvage 0.528 m/s. Stutter. That’s in actual fact the an analogous element as with the momentum precept.

What about the case with the 2 followers pushing in reverse directions? In this case, one fan does some work—let’s merely instruct it does 0.11 Joules. The varied fan has the an analogous force for the an analogous distance, however it’s far pushing within the reverse direction. For the backwards-pushing force, the angle between the force and the displacement is 180 levels. On memoir of the cosine of 180 levels is unfavorable 1, the work carried out by this force is -0.11 Joules. That makes the overall work equal to zero Joules and a replace in kinetic vitality of zero Joules. The single plan for that to happen is for the cart to switch alongside at a constant velocity. Substantial.

What about the block being pulled alongside the table with friction? All over again, the 2 forces are the force from the string pulling to the ultimate and friction pulling to the left. The full work on the block would possibly perchance be zero and it would possibly perchance well perhaps in all probability perchance switch at a constant velocity.

BUT WAIT! There would possibly perchance be a mission. What within the event you measure the temperature of this block sooner than and after you pull it? Listed below are two thermal images—additionally, I build a portion of styrofoam on the bottom so as that it’s doubtless you’ll perchance additionally peek the temperature replace.

Rhett Allain

It is not a worthy raise in temperature, however it did certainly warm up. If I dash the block over a bigger distance (or abet and forth) it’s doubtless you’ll perchance additionally peek a realistic traipse on the bottom. That is an house where the table will increase in temperature—the block additionally gets hotter.

However if the block gets hotter, which formulation that it will increase in vitality. In this case, it would possibly perchance well perhaps in all probability perchance be an raise in thermal vitality. So, how can the block raise in vitality if there’s zero work carried out on the thing? That is certainly a thriller. How is it imaginable for there to be zero work AND an raise in vitality.

Right here is the answer. You would possibly perchance well also peek this with a unfamiliar example. Relate that I rub two brushes together as a substitute of a block and a table. Look what occurs.

Look that as the brush is pulled, there are two forces that originate work. My hand does work (obvious work) and the brushes originate work (unfavorable work). However stare closely. Look that as the brush (and my hand) switch to the left a obvious distance, the brushes bend. This formulation that the force the bottom brush exerts on the end brush moves over a shorter distance than the hand moves. Even supposing the force of the brush is the an analogous magnitude as the force of my hand, the brush does less work ensuing from it moves over a shorter distance. Which formulation the overall work carried out on the brush is NOT zero Joules however some obvious amount.

Surely the brush is an analogy for friction. We love to imagine friction as this nice and uncomplicated interplay, however it’s not at all times. For the block sliding on a table, the frictional force is an interplay between the bottom atoms within the block and the bottom atoms on the table. It is not so uncomplicated. Physics textbooks love to treat a block as a level object—however it’s far not a level object. It is a cultured object manufactured from countless atoms. Within the case of friction, it’s doubtless you’ll perchance additionally’t put out of your mind that and merely treat a block as a level object. It doesn’t work.

Work Done by Friction

Let’s merely be particular. If a physics textbook asks you to calculate the “work carried out by friction”—merely instruct no. Exact instruct no. You would possibly perchance not no doubt calculate that. Yes, we desire to have confidence physics as uncomplicated as imaginable—however not so uncomplicated that it gets you into not doubtless cases love the one with a block sliding at a constant velocity.

Oh, however wait. There are so much of physics textbooks that no doubt demand about work carried out by friction. The first e-book I grabbed had an example that changed into once something love this:

Jake pulls a field with a mass of twenty-two kg. The rope makes an angle of 25 levels with respect to the horizontal. The coefficient of kinetic friction is 0.1. Accumulate the work carried out by Jake and the work carried out by friction for the case where the sphere moves alongside the bottom a distance of 144 meters.

Unfriendly. Unfriendly quiz. You would possibly perchance well also certainly calculate the force of friction—however it’s doubtless you’ll perchance additionally’t calculate the work carried out (unless you additionally know some stuff about the changes in thermal vitality). Whenever you calculated the work carried out by friction as the frictional force multiplied by the gap the block moves, how would you memoir for the raise in thermal vitality of the block (and floor)? Oh, however it’s doubtless you’ll perchance additionally originate this mission with the momentum precept and it would possibly perchance well perhaps in all probability perchance not be a mission. Be aware that the momentum precept offers with forces and TIME, not DISTANCE. So even supposing the frictional force acts over a unfamiliar distance, the time is the an analogous for both the frictional force and the force pulling the string.

What then?

Then what are we to originate? If we are in a position to not originate work carried out by friction, how are we presupposed to coach physics? Wisely, here is the mission. The critical blueprint in physics is to manufacture fashions that agree with staunch-existence experiences. These fashions is in most cases a huge belief love the work-vitality precept—and that is the rationale huge. Let’s recall in mind an example with one other mannequin. What a pair of globe? It is a mannequin of the Earth. It even reveals the positioning of the continents and all the pieces. However what if I desire to make exhaust of this globe and measure its mass and quantity so as that I will decide the density of the staunch (fat-sized) Earth? That would possibly perchance not work for the rationale that globe shouldn’t be at all times no doubt no doubt Earth. The identical is ultimate with the work-vitality precept. It is huge for some things, however it’s doubtless you’ll perchance additionally’t merely exhaust it wherever you love.

At last, let me point out that I only know about these complications with work and friction on account of my genuine colleagues Bruce Sherwood and Ruth Chabay (yes, the authors of my favourite physics textbook Matter and Interactions). It changed into once at some level of an informal aspect dialog on the new assembly of the American Association of Physics Lecturers (AAPT). In actual fact, there are this type of lot of educators at this conference that maintain a huge influence on the fashion that I imagine physics. It is repeatedly huge to peer them.


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