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48

A completely clean silica surface has a very high surface energy and a very high coefficient of friction. However silica surfaces adsorb pretty much anything at the least excuse, so glassware from your cupboard will have a layer of various molecules adsorbed from its environment, and this greatly reduces the coefficient of friction. Dishwash powder is ...


46

No, a car cannot steer on a frictionless surface. This has little to do with gyroscopic action and more to do with conservation of momentum: to turn, even when conserving its speed, the car needs to accelerate at right angles to its motion, which changes the total momentum of the motion. This change in momentum requires a force which, in normal roads, is ...


46

If the wheels had spun fast enough for a gyroscopic effect to become noticeable, the only result on a frictionless surface (which would be the same without a surface at all) is that when you turn the wheels, the rest of the car would rotate instead of just the front wheels :) You need some reaction force to alter the trajectory, like a sail or surface ...


38

Using the brakes on the front of the bike causes your weight to shift forward. Additional weight allows more force before the tire will slip (skid). If you brake hard enough the back tire of your bike will lift up and at that point all of the mass is distributed on the front tire. Remember the maximum force is $F_{max} = \mu F_{normal}$ and $F_{normal}$ ...


25

Yes you can It is actually possible with a real car, but you would have to be very patient to steer a little bit. Suppose you have built a car with power on the big front wheels to induce a gyroscopic effect. If you rotate the wheels, the direction in which the center of mass is going will not change directly, but the angle in which the rest of the body ...


10

Since there is no friction, then it will not affect any other forces that may act on the car. The direction of wind blowing on the car may change its trajectory, as any driver will attest when driving in high winds. Turning the car wheels may have a slight affect on the resultant direction of the force. If the car has curved roof, then it may acts as ...


9

The two lengths are given by: $$L_{normal}=L$$ $$L_{summer}=L+\alpha \Delta T L$$ With $\alpha$ the thermal expansion coefficient, $12.0 \times 10^{-6} \: m/m \: K$ Thus the two periods become: $$Period_{normal}=2 \pi \sqrt{\frac{L_{normal}}{g}}=2 \pi \sqrt{\frac{L}{g}}$$ $$Period_{summer}=2 \pi \sqrt{\frac{L_{summer}}{g}}=2 \pi \sqrt{\frac{L+\alpha ...


9

There are many, many factors that affect a pendulum's accuracy. There is a fairly famous pendulum clock in the bell tower of Trinity College, Cambridge, which is the subject of a fair bit of scientific analysis and monitoring (it keeps time to better than one second per month). The "keepers" of the clock wrote a detailed discussion about the factors ...


7

Gasoline / petrol / diesel engines run best at approximately 1000 to 5000 rpm (revolutions per minute) so at slow speeds and at high speeds different gears are needed to change the ratio of the number of revolutions of the car wheels to the number of revolutions in the engine. The shifter selects between different ratios. Electric engines work well over a ...


6

I can think of two possible reasons: first, you can have half your legs up in the air at one time (as in walking - two on one side and one on the other, then change) and still be perfectly stable (3 legs = most stable, like a tripod); and second, if a predator chews off a leg on either side, you still have two legs (so you can still walk). I think those ...


5

It may be worth pointing out that blankets also (surprisingly) act as (thermal) radiation shields. This is the reason that "emergency blankets" can sometimes be found in survival kits that appear to be nothing more than thin shiny plastic. But they really make a difference in the amount of heat lost by a warm (37 °C) body on a cold night (cloudless sky - ...


5

The braking force acts between the tyre and the road. The centre of mass is above this point so there is a rotational effect which increases the force going down through the front tyre and decreases the force going down through the rear tyre. Because the amount of braking force the tyre is able to produce is limited by the amount of force going down through ...


4

You could blame the laws of thermodynamics and say that cooling is much harder in our universe because of them. However, since we're in a dark energy-dominated universe that's expanding and cooling, it seems as though cooling is generally easier for the universe on the largest scales. Even on smaller scales, cooling is usually easier (I've of course ...


4

Friction is the only force that would cause the car to move along a different path. On a frictionless surface, the gyroscopic effect could change the orientation of the car a bit, but not the trajectory of the car. In other words, the front car would no longer point along the direction of travel, but would "skid". (That is, if you could call frictionless ...


4

For your 2 minute egg timer here on Earth it comes out to be 4 minutes 54 seconds on the Moon because: $t_{Moon} = t_{Earth} \sqrt{6}$ Full explanation below. Q: What is the relationship between hourglass flowrate and local gravity? As in the excellent answer to a related question (hourglass flowrate vs. sand grain size) and this published paper, the ...


4

I grew up on a farm. As the oldest child, I was given chores. Lots and lots of chores. All those chores (and neighbors with nicknames such as "Jimmy three fingers"; farming is hazardous) were part of what made me get a degree in physics. I drove one of these: Now that's a tractor! An old style tractor, that is, and it occasionally got stuck in the mud. ...


3

Not discounting other traits, is there something about six legs that has helped insects achieve this success? Spiders oftentimes have eight legs, mammals oftentimes have four. But centipedes have lots of legs, and millipedes have lots of legs. The reason mammals have four legs, and millipedes have lots and lots of legs isn't so much about optimality so ...


3

The fluid in the tube is not water as some might think but an organic solvent called Dichloromethane. The reason the bubbles form is due to the fact that the fluid is heated at the base of the tube to it's boiling point which is a low 103.3 F degrees. You can almost get it boiling by holding it in your hand. The bubble is actually the vapor form of the ...


3

The main considerations are grip and rolling efficiency. A tyre dissipates energy as it flexes, and any energy dissipated in the tyre means extra effort from the rider or motor and therefore fewer miles per gallon. The more you pump up the tyre the harder it becomes and the less it flexes, so higher pressures are more fuel efficient. However harder tyres ...


3

One of the reasons is that if the wheel axle is above the attachment point it drives the wheels downwards when pulling increasing traction. Think of the opposite, where the attachment point is really high it will force the front wheels off the ground limiting the pull force so the tractor does not flip. So what happens with the tall wheels and the ...


3

I could be wrong, but my guess would be it is caused by a change in temperature. At a very simple approximation, the air in the top portion of the jar can be modeled using the ideal gas law: $$PV=NkT$$ or $PV=nRT$ if you're a chemist. By simple inspection, you can see that if the amount of jam stays constant, and the glass doesn't budge, a drop in the ...


3

Lots of excellent answers here, but for fun, lets think about this backwards. Imagine you have the worlds first and only FRONT wheel drive motorcycle, and your rev it up and pop the clutch. What kind of launch do you think you would get with very little weight on the front tire? The reverse is true during braking when the deceleration shifts the weight of ...


3

Braking acts to stop the front tire. Friction acts at the contact patch under the front wheel to introduce a vector force directed towards the back of the bike. Since the force is not directed through the center of mass of the motorcycle/rider system, it introduces a moment or torque that acts to rotate the motorcycle and rider such that the back tire begins ...


3

In principle yes, but the effect is usually marginal. It also depends on how powerful your lights are compared to the size of the kitchen (a 1000 Watt flood light in a home kitchen will probably have a noticeable effect on the speed of drying). Bascially, the floor dries through evaporation, i.e. the water on the floor goes into the gaseous phase ('becomes ...


3

I was told a long time ago that the sound is from "twinning" - this is where a metal under large stress experiences a reorientation of grains to relieve stress. However I am not convinced this is the case - typically when the engine parts (catalytic converted being probably the hottest) cools down, it will shrink - and there is some "give" in the mountings ...


3

On a completely frictionless floor, with the absence of other external forces, the centre of mass of the car will continue in the same trajectory for ever. Hence no steering is possible. However, irrespective of whether the front wheels are rotating or not, turning of the front wheels will produce a counter torque changing the orientation of the car, albeit ...


2

I actually do some bow-hunting. First I have to tell you that you aren't completely right about the arc-shape: recursive bows have special curvings near the ends. It requires you to apply less force when you are holding the stretched bow and gives you more time to aim. Compound bows use levering systems to accomplish the same goal even more efficiently. ...


2

This question can't be answered from the picture alone without additional observations/data but @David Rose has given a good list of hypotheses. The size of the waves appear to be within the regime of 'capillary waves' Capillary waves are surface boundary waves with wavelengths on the order of millimeters up to a centimeter or two, where the energy restoring ...


2

Using large tires causes less compression of the soil. Tires with larger diameter and width have a larger area of contact to the ground. The weight of the tractor is spread over that area. (I assume that, in tractors with small front wheels, the center of gravity is near the large rear wheels, so that the small wheels do not support much more weight per ...


2

Leverage Often, a limiting factor in tasks required from a tractor is the amount of pulling force a tractor can apply without tipping over (the front rising up) - the engine is strong enough to do so. Having the driving axle be high from the ground helps by simple lever action - see the illustration; twice the height means twice the maximum pulling force ...



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