# Tagged Questions

The resistance a surface or object encounters when moving over another.

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### Why is friction force negative in ice skater problem?

A 68.5 kg skater moving initially at 2.40m/s on rough horizontal ice comes to rest uniformly in 3.52s due to friction from the ice. What force does friction exert on the skater? I am not really ...
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The equation of motion of a damped oscillator $$\frac{d^2x}{dt^2}+\gamma\frac{dx}{dt}+\omega_0^2x=0$$ which is invariant under time-translation $t\rightarrow t+a$, but not under time reversal $t\... 5answers 3k 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 ... 1answer 919 views ### Why do you hear an acute sound when you rub your shoe on the floor? Sorry for the silly question, but I've been wondering about this for quite some time. After raining when I get home I rub my shoe's wet outsole on the floor and I hear an extremely acute (high pitch) ... 1answer 1k 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 ... 2answers 11k 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 ... 3answers 10k views ### why making a surface “super” smooth increases the coefficient of friction? I read that: If you take a rough surface and make it smooth, the coefficient of friction decreases. But if you make it super smooth, then the coefficient of friction increases. How come? 5answers 178 views ### A conceptual doubt regarding Forced Oscillations and Resonance While studying about the Resonance and Forced Oscillations, I came across a graph in my textbook that is given below:- Now, the author writes As the amount of damping increases, the peak shifts ... 2answers 2k 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 ... 1answer 640 views ### Lagrangian and Hamiltonian EOM with dissipative force I am trying to write the Lagrangian and Hamiltonian for the forced Harmonic oscillator before quantizing it to get to the quantum picture. For EOM $$m\ddot{q}+\beta\dot{q}+kq=f(t),$$ I write the ... 1answer 2k 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 ... 1answer 598 views ### Analysis of a card pyramid Here's an interesting question I thought of. Firstly, picture one of those card pyramids. Hopefully you know what I'm taking about when I say card pyramid, but basically you balance two cards up ... 2answers 691 views ### Why is it easier to glide on sharp ice skates than on dull skates? There have been previous questions (e.g. here and here) on Physics.SE about the mechanism that makes ice skating possible. Reviewing these, as well external references, it seems pretty clear that the ... 1answer 364 views ### Why does chalk dust stick to a chalkboard? I understand that friction causes the chaos dust to come off the stick of chalk, but what exactly is causing the chalk dust to stay on the chalk board until rubbed off? 2answers 237 views ### Free body diagram with forces of friction I'm reallizing that I'm very confused on how to make a free body diagram of forces in some situations where there is friction between two objects. I'll make an example where I cannot figure out what's ... 1answer 569 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 ... 1answer 223 views ### Distance a curveball travels? I've seen some discussions regarding the movement of a spinning object, say a curveball. However, all have been largely qualitative. I was wondering if anyone has seen or worked through a ... 5answers 1k views ### Zero gravity means zero friction? The frictional force acting on a body placed on a horizontal plane is$F=\mu{R}$where$R$is the normal reaction and is equal to weight of a body in this case. And$\mu$is the coefficient of ... 4answers 9k views ### Would a pendulum swing indefinitely in a frictionless vacuum? I am attempting to settle a friendly bet. Would a pendulum swing indefinitely in a hypothetical vacuum (i.e. no air resistance) having a hypothetical frictionless bearing (i.e. no energy lost due to ... 3answers 2k views ### Paradox in the two block problem [closed] UPDATE (regarding duplicate) : This question is not a duplicate of another question. Sure, the situation in both the questions is same and, yes, both questions ultimately provide a methodology to ... 5answers 3k views ### Why do we take small steps while walking on ice? When we walk on ice we should take small steps. Small steps ensure: a.)larger friction. b.)small friction. c.)larger normal force. d.)smaller normal force. The correct answer ... 3answers 8k 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. So does friction decrease when the body is moving ... 2answers 506 views ### Does a tire need to slip to generate force? Recently, I have been doing some research on racing and tire modelling. While I was doing this, I encountered many curves like those shown below. While I understand the need of slip angles to ... 2answers 10k views ### What role does static friction force play for a rolling object? How can I know what direction it points? Fundamentally, what does a friction force do for a rolling object? I am very confused by the way my textbook explains it. In my textbook , it says: A wheel rolling on a horizontal flat surface at a ... 2answers 5k views ### Determining the direction of friction I had a question which has always been of slight confusion to me. Say you are dealing with your typical block-on-an-angled-plane setup. You have a block with mass m that is initially at rest. The ... 2answers 177 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 ... 3answers 117 views ### Model of contact friction Consider the standard model of kinetic contact friction taught in introductory physics courses: $$F_f=\mu_k*F_N$$ According to this model, for a block with initial speed$v$moving on a horizontal ... 4answers 15k views ### Does a car use friction to move? When a car's engine injects fuel into the cylinder chambers, the reaction creates a force that generates rotational momentum to the shaft and over the transmission, it translates that power to the ... 1answer 182 views ### Superhydrophobic coating and water friction If you were to apply super-hydrophobic coating to the hull of ships or even submarines, would you get more or less water friction? If the coating completely repels the water you could argue there ... 1answer 488 views ### How to get rotational speed after disk-disk collision with friction? Assume two circular disks A and B collide. They have both initial linear momentum and angular momentum. If their surface has no friction, their angular velocity does not change after collision, so I ... 8answers 9k views ### Simple friction formula for a car I am making a 2D driving video game, and I would like to know the "simple" formula for calculating the friction force between the car and the road. I have read lots of friction diagrams involving ... 1answer 627 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 ... 1answer 227 views ### How can a reel of thread move in opposite direction? There is a nice puzzle about reel of thread. I wonder why the solution works. The puzzle: You take a reel with threads on it. Inner radius r, outer radius R. You put it on a table. If you pull ... 0answers 154 views ### Particle sliding on a semisphere - solving the differential equations analytically [closed] I have the following problem: A particle of mass$m$is at rest at the top of a semispherical surface of radius$R$. After an impulse, it starts to slide along a meridian with speed$v_0$. The ... 0answers 331 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 ... 4answers 1k 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 ... 5answers 6k views ### Force required to drive car From what I know of Newtonian Mechanics, if an object is moving at a constant velocity, the net force acting on that object is equal to zero. If there is friction, then the applied force required to ... 2answers 7k views ### Friction as a Centripetal Force When an object is moving in a circle, won't the frictional force oppose it's motion? And if the velocity is always tangential to the path, there is no component of it towards the center, so there is ... 2answers 2k 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 ... 5answers 10k views ### Physics behind Wheel Slipping Lets say that I'm in a car and I apply full acceleration suddenly. Now, the wheels would slip and hence the car doesn't displace much. But If I start with some constant acceleration, slipping doesn't ... 2answers 2k 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 ... 4answers 1k views ### Why doesn't centrifugal force appear on an FBD for a car on a banked track? Suppose you wish to find the max speed at which a car can travel on a banked curve of radius R and angle theta to the horizontal and coeff. of static friction mu. I don't understand the relevant ... 3answers 330 views ### Why do sand dunes, hills and mountains assume the shape they normally do? What about something taken in a tablespoon? This shape, as I understand, could be explained by gravity, friction and similar concepts, but why the shape isn't cubical or cylindrical couldn't be ... 1answer 3k views ### Magnetic fields and friction If I was to pass a metal object through a magnetic field would there be any friction? 2answers 2k 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 ...
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### Why does friction play the role of centripetal force during the turning of a car?

What makes frictional force the centripetal force of a car turning along a curve? As friction is the opposing force and acts anti-parallel so there is no component of frictional force towards the ...
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### 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?
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### 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 ...