New answers tagged

2

The ground reaction force is the reaction of the ground acting upward opposite to the person's contact force (weight) acting on the ground, which is essentially the person's weight. When the person upper body quickly drops during actions B through D the downward force on the ground (his/her weight) drops and, per Newton's third law, the equal and opposite ...


0

Evaporation causes cooling! Lesser water to pump to the pads $\Rightarrow$ Lesser evaporation by taking heat from hot air or lesser cooling of air.


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If the water in the cooler's reservoir falls too low, then the pump in it that draws water from the reservoir to keep the porous pads wet will start sucking air instead of water and it will be unable to keep them all wet- and the cooling capacity of the device will fall suddenly. This is why desert coolers have an automatic float valve in the reservoir to ...


1

loose sand has no shear strength. This is why bike wheels skid on sand: the sand adheres to the tire, but that sand shears loose from the rest of the sand. Then you fall down go BOOM.


3

The pressure in a pressure cooker (that is the excess pressure over atmospheric) is due to water vapour (steam). The hotter the water the greater the evaporation from it and the greater the vapour pressure. "Why does reducing the temperature on the surface of cooker help me accelerate the reduction of pressure inside the cooker?" Cooling the ...


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I think you may be referring to the partial vacuum that develops when cooling a sealed pressure cooker, as evidenced by the Ideal Gas Law: $$pV=nRT$$ As $V$, $n$ and $R$ are all constants, this reduces to: $$\frac{p}{T}=\text{constant}$$ So reducing $T$ demands $p$ also decreases. However, pressure cookers are built to withstand (modest) over-pressures and ...


3

I had this issue when I was an architecture student living in college. Oddly, the sounds came not from the bedsit above but the next one along, diagonally up to one side. Assuming that it really is the flat above that is your problem, there are several reasons why the sound is coming through: Inadequate structural mass. A heavy floor vibrates less than a ...


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two things cause this, as follows. First, note that the ceiling of your flat is the floor of the penthouse. If that floor/ceiling is too flexible (i.e., insufficiently rigid) then even slow walking sounds will be readily broadcast by it right into your flat. Second, if the floor of the penthouse is insufficiently dense (for example, plywood sheeting over ...


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But there is a net friction force on me which is making me walk. Now according to me there are no other forces acting. Shouldn't that mean that I should be accelerating but I am not accelerating? As @FakeMod pointed out its answer, you are alternatively accelerating and decelerating in such a way that your overall average velocity is constant. Consider when ...


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When we walk, the friction with the ground is important not only to move ahead. When people fall down because of a wet floor, they can fall forwards or backwards. Friction is essential to avoid our foot ahead to slip. The friction force is then backward. When we walk at a constant speed, (negleting air drag), friction forces acting in our feet should be ...


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Mathematical analysis The net momentum imparted by the friction force on your body is zero. Mathematically, $$\Delta \mathbf p =\int_0^T \mathbf F(t)\: \mathrm dt =0\tag{1}$$ where $\mathbf F(t)$ is the friction force acting on you at any time $t$. Now, since you have the same velocity at every instant, thus the integral in equation $(1)$ should evaluate to $...


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I like the comment from tfb. The larger object will experience a larger buoyant force from the atmosphere.


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The volume doesn't determine the speed of sound. The volume is lower because some sound waves get absorbed and reflected back when they hit the wall which makes the amplitude smaller and therefore the volume is lower.


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Maybe because those same objects are in different places. If one of them is on earth and the other one on the ISS (International Space Station), lifting them both for the same height will take different efforts.


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for the west wind case we assume the door faces west i.e., the wind is blowing towards the door. this means the wind is applying a small pressure to the outside of the door which is not balanced by an equal pressure on the inside, thereby creating a pressure difference across the thickness of the door. pressure is in pounds of force per square inch, and ...


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If the two objects have the same mass, the work done in raising both by a set distance is the same. However, if a human is the thing doing the lifting, biomechanical efficiency needs to be taken into account. For example, larger objects may require more awkward grips or be held further away from the body, resulting in a longer lever created by the arm and ...


5

A fan can keep a person cool by two methods: (1) increasing the rate of evaporation of sweat off the skin, and (2) moving air warmed by the person's body away from their skin. See this answer for more details. A bottle of vitamins does not sweat nor maintain homeostasis, so blowing air on it does not affect its temperature. The plastic bottle is a decent ...


3

because the aerodynamic forces applied to it by its travel through the air, which tend to break it up onto droplets, greatly exceed those from its surface tension, which would otherwise draw the body of water into a spherical glob.


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What happens is that the water forms a seal between the steel glass and the glass table. Due to the slightly higher pressure $p$ inside the glass it's kept afloat while the inside pressure is maintained because of the seal. The slight over-pressure arises when the glass 'sinks' into the water layer and is caused by the weight of the glass, which slightly ...


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No. In this case the moisture sweat from your hand causes the near side of the paper to expand, which makes the paper curve upwards. This is because one side of the paper has expanded while the other is still dry and has not. "Capillary forces" is not a term I would use, but essentially it refers to force of surface tension, which depend on whether ...


1

This happens because paper is hydrophilic and absorbs moisture emitted from your skin. As the paper absorbs moisture it swells. The side nearer your skin absorbs moisture faster than the side farther from your skin, so the side nearer your skin swells more rapidly and this causes the paper to bend. The actual motion can be quite complicated because paper is ...


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Actually , evaporation increases with increase in surface area i.e actually your hair should dry faster when long. But, we have not considered that longer the hair more water gets accumulated and the hair traps the water from evaporating. Hence ,conceptually water from shorter hair should take more time to evaporate. But then why does it take less time? It ...


1

One of the reasons stems from extreme value statistics. Objects break at their least resistant (call it softest) spot. The probability of having a softer spot is larger in a larger object. You could think of a chain with $N$ links. Each link has a maximum force it can bear, $F$. Since links are not all the same, $F$ comes from a probability distribution, $P(...


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Big objects break because they are heavier than small objects, so they hit the ground harder. You might think that a big object is also stronger than a small object. That's true, but it's not enough to compensate for the heaviness. To see why, imagine two objects of the same shape, one twice as long as the other. Since the big object has twice the height, ...


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An answer wrote by user Enthalpy found on https://www.chemicalforums.com/index.php?topic=103339.msg363521#msg363521: This question is still debated in mechanical engineering where unsound loads on ball bearings serve to measure a life expectancy which is then extrapolated to normal load. Books give formulas for that, but SKF, the biggest manufacturer, tells &...


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The bigger (and longer) the object, more will be the torque experienced by it. Let's say the length of the chalk we have is $\frac{L}{2}$ (Chalk 1) and $L$ (Chalk 2). When the chalk falls on the floor, it's most likely to hit on one of its edges. Given that it is dropped from the same height, the force on the heavier mass (Chalk 2) will be more than the one ...


1

Yes, you might find falling objects harder to push, if you factor in the drag force. Initially, before you apply any force, the direction of the velocity is1 downwards ($\downarrow$) and the corresponding drag force acts upward ($\uparrow$), because almost all the drag forces experienced in daily life can be written as $$\mathbf F_{\text{drag}}=-g(|\mathbf v|...


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Newton's second law is a vector equation $$\mathbf F=m\mathbf a$$ With two dimensional motion this gives us two equations $$F_x=ma_x$$ $$F_y=ma_y$$ Therefore, the only way the scenario you propose can happen is if there is some horizontal force that depends on the vertical velocity $\mathbf f=f(v_y)\,\hat x$. For falling objects I don't think this is the ...


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As Galileo discovered some 400 years ago, a body's horizontal motion is independent of the vertical motion. A given resultant horizontal force acting on a falling body will give it the same horizontal acceleration (while the force is acting) as if the body had no vertical motion.


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In a fluid like water or air, the pressure $P$ and velocity $v$ depend on the height $h$ and density $\rho$ in a way described as the Bernoulli equation, $$ P + \frac12 \rho v^2 + \rho g h = \text{constant} $$ In your setup, the opening of the straw and the upper liquid surface at the base of the bottle are both open to the air, whose pressure doesn't really ...


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This experiment (with tea and milk) seems to show that immediate addition of the milk gives the warmest drink: But the experiment is far from perfect, especially because of the lack of replication. And this simplified derivation, based on Newton's Law of Cooling finds the same. But it too is very open to well-reasoned critiques. It certainly relies on one ...


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Very nice DC motors can be easily removed for salvage from cordless electric drills or cordless weed trimmers. These are plentiful and cheap in second-hand stores.


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I've taught hands-on workshops to ftfth graders in which they built electric motors from wire, paper clips, a few nails, a magnet, and a small block of wood. I think that several lectures on E&M could build on that foundation. Optics is easy: so many ways that Nature produces color, builds eyes; so many optical phenomena that we can see just looking at ...


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Probably not a popular opinion, but I think it works best if you treat them as a dedicated physics class in terms of how you want to motivate them. Also: what is your field of expertise? Are there no connections to the lecture? If you find something really exciting, its easier to spread enthusiasm. Assuming the wave equation is part of the course (and not ...


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The question is an old conundrum and can be found in various guises on the Internet and in handbooks. One can summarise it as follows: If I add milk to my coffee and wait 5 minutes before drinking it and another person waits 5 minutes and then adds the milk to his/her coffee, who is drinking the hottest coffee? A theoretical derivation of the end ...


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But the lighter vehicle has a lower mass, so lower inertia. If the friction forces on each vehicle are proportional to its weight, and hence proportional to its mass then neither vehicle has an advantage on a slope (assuming by "advantage" you mean a greater acceleration due to gravity). On flat ground, if the same force is applied to both vehicles, then ...


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