The resistance a surface or object encounters when moving over another.
43
votes
6answers
4k views
What does it mean for two objects to “touch”?
If you've ever been annoyingly poked by a geek, you might be familiar with the semi-nerdy obnoxious response of
"I'm not actually touching you! The electrons in the atoms of my
skin are just ...
26
votes
6answers
1k views
Can I survive a free fall using a ramp and a rope?
Can I survive a free fall by carrying a very light and resistant ramp using a rope?
Note: lets assume the ramp is a little bit heavier at the bottom and I am very skilled at making it always land ...
22
votes
5answers
4k views
Why does a tire need to be filled with air?
The Mini 4WD's tires aren't full of air, and it can run. Also, the tank doesn't have tires with air. So, the question is: why do real cars on the road need to be filled with air? What is the idea ...
19
votes
3answers
532 views
Is gecko-like friction Coulombic? What is the highest known Coulombic $\mu_s$ for any combination of surfaces?
Materials with large coefficients of static friction would be cool and useful. Rubber on rough surfaces typically has $\mu_s\sim1-2$. When people talk about examples with very high friction, often ...
15
votes
2answers
439 views
How hot does the tip of a pencil get while writing?
When writing with a pencil, there seems to be quite a lot of friction - which seems like it would become heat. How hot would the tip of a #2 pencil get writing on normal copy paper?
14
votes
3answers
613 views
Why doesn't a fly fall off the wall?
Pretty simple question, but not an obvious answer at least not to me. I mean you can't just place a dead fly on the wall and expect it to stay there, he will fall off due to gravity. At first I ...
10
votes
3answers
704 views
Why does a curling rock curl?
In the game of curling, players "curl" a granite "rock" (of precise size and roughly a flattened cylinder) down a "sheet" of ice towards a target; the "rock" will curve in its path in the direction of ...
7
votes
4answers
5k views
Can the coefficient of static friction be less than that of kinetic friction?
I was recently wondering what would happen if the force sliding two surfaces against each other were somehow weaker than kinetic friction but stronger than static friction. Since the sliding force is ...
7
votes
3answers
1k views
Static Friction - Only thing that can accelerate a train?
I'm a computer programmer that never studied physics in school and now it's coming back to bite me a bit in some of the stuff I'm being asked to program. I'm trying to self study some physics and ...
7
votes
1answer
117 views
The quickest 6-3 play in baseball: to bounce or not to bounce?
What's the quickest way the get the ball (say from shortstop) into the first-baseman's glove, given some fixed initial (throwing) speed? Directly or with one bounce?
I'm fairly sure that the answer ...
6
votes
3answers
769 views
On a bicycle, why does my back tyre wear so much more quickly than the front?
This question is cross-posted from Bicycles.SE, but it is really one for those that know a bit about physics.
Why does the back tyre of a bicycle wear out quicker than the front tyre?
I have my ...
6
votes
2answers
3k views
Grip of the train wheels
How do the wheels of a train have sufficient grip on a metal track? I mean both of the surfaces are smooth (and not flexible) and it is okay if there is no inclination, but how about on an inclined ...
6
votes
2answers
625 views
Hydrostatic friction: why do water droplets stay at rest on an inclined glass surface?
Tjis is a non-expert question on a (seemingly simple) text-book topic.
The question is about "hydrostatic friction", defined as follows.
Consider a drop of water resting on a flat surface. If the ...
6
votes
0answers
140 views
What determines the angle of the cushion on a pool table?
If you look at the cushions (bumpers) on a pool table, you'll see that they're not vertical. They're tilted inwards. About 10 years ago, I came across a physics exam in which one of the problems ...
5
votes
4answers
1k views
After what speed air friction starts to heat up an object?
I understand that air friction cools off an object at low speeds. For example, if you blow on a spoon of hot soup, it cools off. Or if you swing a hot frying pan in the air, it cools off faster.
But ...
5
votes
1answer
2k views
Why do people recommend wider tyres in car for better road grip?
Tyre companies boast of their wider tires for better grip on road. Also, the F1 cars have broad tires for better grip. But as far as I know Friction does not depend on the surface area of contact ...
5
votes
2answers
152 views
Boy and box on ice
I have a troubles. Can you help me?
I have a task, called "slippery activity".
Boy (he has $m=45kg$) Stands at ice and tries to move a big box ($M=90kg$) with a string (rope). Boy's Slip ratio is ...
5
votes
3answers
945 views
Block on a block problem, with friction
Consider two blocks, one on top of the other on a frictionless table, with masses $m_1$ and $m_2$ respectively. There is appreciable friction between the blocks, with coefficients $\mu_s$ and $\mu_k$ ...
5
votes
1answer
282 views
What happens when two smooth surfaces touch?
I am wondering what will be the physics to explain how two neutral, chemically nonreactive objects stick. I know that using van der Waals formalism, we can treat neutral body electrodynamic forces and ...
5
votes
1answer
528 views
Dependence of Friction on Area
Is friction really independent of area? The friction force, $f_s = \mu_s N$. The equation says that friction only depends on the normal force, which is $ N = W = mg$, and nature of sliding surface, ...
5
votes
3answers
982 views
Best inflation pressure of car tire in the rain- higher, same, lower than normal?
If I am driving a car in the rain, and want to increase the available traction, should I:
Increase pressure in the tires
Decrease pressure
Leave the pressure set to optimum dry pressure
Increase or ...
4
votes
3answers
2k views
Why are bicycle pedal threads' handedness left on the left and right on the right?
I understand the reason that bicycle pedals are oppositely threaded on either side. What I don't understand is why it works because I'm missing something.
Take the right pedal for example. It's ...
4
votes
1answer
494 views
Magnetic fields and friction
If I was to pass a metal object through a magnetic field would there be any friction?
4
votes
2answers
5k views
Does the force of kinetic friction increase with the relative speed of the objects involved? If not, why not?
Does the force of kinetic friction increase with the relative speed of the objects involved?
I have heard and read that the answer is no. This is counter intuitive, and is a big part of why the ...
4
votes
2answers
111 views
Why is the damping force on a spring oscillator linearly dependent on velocity?
If you consider the damping force is friction like in:
then the force should be $$F=\mu N$$ where $\mu$ is the coefficient of kinetic friction. Why then is the damping force assumed to be linearly ...
4
votes
3answers
185 views
How can a vertical force cause motion at an angle?
I just started learning physics 3 days ago and am having trouble understanding what I am doing wrong. Can someone please explain my error(s)? Thanks!
We have a 1kg object on a plane at a 30 degree ...
4
votes
2answers
595 views
How does rubbing cause the transfer of electrons from one object to the other? [duplicate]
I have just learnt about electrostatics. Why would there be a transfer of electrons? Is it because of the difference of the materials (i.e. triboelectric series)? So in the case of two different ...
4
votes
1answer
918 views
finding angular velocity and regular velocity when bouncing off a surface with friction
Take the game of pong as a simple example. When you hit the ball with a paddle that has a frictional surface, the ball will spin as well as change direction according to the coefficient of kinetic ...
4
votes
1answer
56 views
Are there real life applications for Hausdorff dimensions, specifically crack formations?
I was curios about Hausdorff dimensions. They seem to neatly describe rough surfaces. So I was wondering if there are common applications of Hausdorff dimensions in things like complicated friction ...
4
votes
4answers
196 views
Friction at zero temperature?
By the fluctuation-dissipation theorem (detailed-balance for Langevin equation), $$\sigma^2 = 2 \gamma k_B T$$ where $\sigma$ is the variance of noise, $\gamma$ is a friction coefficient, $k_B$ is ...
4
votes
0answers
98 views
Why does the overhand knot jam but the figure-8 knot doesn't?
After tensioning a rope with an overhand knot in it, it is often very hard if not impossible to untie it; a figure-8 knot, on the other hand, still releases easily.
Why is that so? Most "knot and ...
3
votes
4answers
569 views
Why does a rotating tire use the static, rather than the dynamic coefficient of friction?
The explanation I have heard of the difference between static and dynamic friction is that static friction is stronger because bonds form when one object is put on top of another object and these have ...
3
votes
2answers
725 views
Wheel moving without sliding
Suppose we have a wheel moving on an horizontal surface, with constant velocity $v$, without sliding. This latter condition implies that the wheel rotates around its centre with angular speed $\omega ...
3
votes
2answers
86 views
Orbit in the vacuum
As the space is a vacuum and there is no friction in space,
Can we assume that, if we place an object in gravity in exactly the right distance from a planet with gravity and in the right acceleration, ...
3
votes
2answers
132 views
If you had two “perfectly” flat surfaces of the same material?
Let's say you had 2 nano-engineered surfaces of diamond which were as 'flat' as possible (of course considering the radii of each carbon atom in the lattice)... would there be any friction between ...
3
votes
1answer
581 views
Where does the lost energy go in a rubber band powering a rotating shaft?
Okay, I'm no physics whiz, and this has me stumped. You know those toy airplanes you can get with the rubber-band driven propellers? You twist the propeller a bunch of times, and this stores ...
3
votes
1answer
301 views
Change in appearance of liquid drop due to gravity
A liquid drop is spherical in shape due to surface tension. But why does it appear as a vertical line under the free-fall due to gravity? (E.g. During a rain - falling raindrop) Is there a specified ...
3
votes
2answers
377 views
How long will my fridge magnet stay attached to my fridge?
I have a 500 gram neodymium magnet. It is stuck to my fridge. With the constant pull of gravity trying to pull it off how long would it stay attached to fridge? For arguments sake lets say my fridge ...
3
votes
2answers
547 views
Is the wind's force on a stationary object proportional to $v^2$?
I am on a boat docked at Cape Charles, VA, about 30 or 40 miles from the center of Hurricane Irene. This understandably got me thinking about the force of wind on the boat. Since air friction is ...
3
votes
1answer
400 views
Does the friction force change directions with a change in reference frame?
In a basic friction problem with Block A sliding on top of Block B, the direction of the friction force is usually explained as being simply the opposite of the direction of motion. So if Block A is ...
3
votes
2answers
72 views
Does the slip-stick phenomenon have any application?
The slip-stick phenomenon is present all around us, be it the noise of car breaks or in earthquakes. But does it have any real-life application?
3
votes
2answers
259 views
Will two trains running along the equator in opposite direction experience same wear out?
Two identical trains, at the equator start travelling round the world in opposite directions. They start together, run at the same speed and are on different tracks. Which train will wear out its ...
3
votes
3answers
907 views
Does friction decrease as objects move faster against each other?
I was told that the faster two objects move against each other, the less the friction between them would be… compared to if they were moving slower. In physics class, we always use the coefficient of ...
3
votes
1answer
398 views
When is the principle of virtual work valid?
The principle of virtual work says that forces of constraint don't do net work under virtual displacements that are consistent with constraints.
Goldstein says something I don't understand. He says ...
3
votes
2answers
511 views
Why are there both Static and Kinetic Friction?
When dragging an object, there is a greater start-up force than the force it takes to keep it moving. Why is this? Why are there two different values for friction?
3
votes
3answers
274 views
Braking distances on a rainy road
I am curious to find the braking distance for a car on a road.
In attempting to find this out, I found that the braking distance for a car (on a flat road) is
$$
d = \frac{v^2}{2\mu g}
$$
where
...
3
votes
2answers
60 views
Frictional Forces
In the figure, blocks A and B have weights of 45 N and 23 N, respectively. (a) Determine the minimum weight of block C to keep A from sliding if μs between A and the table is 0.21. (b) Block C ...
3
votes
1answer
95 views
Calculating the path of a ball with spin moving across a table
A ping pong ball is rolling over a smooth (but not frictionless) table. During its travel, a clockwise spin is placed on the ball. The ball's path is changed to move to the right (in perspective from ...
3
votes
1answer
168 views
What is the function of the top point of a bouncing ball?
A ball is thrown away as parallel to x axis from M(0,h) point with speed V . After each jumping on x axis , it can reach half of previous height as shown in the figure.(Assume that no any air ...
3
votes
3answers
86 views
Static as opposed to Kinetic Friction in Rolling Motion
During analysis of rolling motion, why do we consider coefficient of friction as that of static friction and not kinetic friction?
