Posts Tagged ‘Velocity’

In learning about Fluid Mechanics, I couldn’t help but recall an interesting episode over the summer involving a water bottle.  My family and I were at the Water Park and my brother, for reasons unknown, decided to poke a hole in the side of his plastic water bottle.  Perhaps a physics deity coerced him into doing it. 


If that weren’t foolish enough, he also decided to lose the cap meaning that he had to start drinking, quickly!  Which he decided to do the slow way, though the teensy little hole.  Assuming that the velocity of the slowly sinking level of water was 0 m/s and with measurements of the distance from the surface of the water to the ground and from the punctured hole to the ground, it’s possible to calculate the velocity of the spouting water with Bernoulli’s equation.  The pressure would be equal to atmospheric pressure at a value of 101.3e5 Pa.

P1 + ½ ρv12 + ρgh1 = P2 + ½ ρv22 + ρgh2

I imagine that it would take an extremely long time to finish a bottle of water this way.  Very painful for the neck too.  And not very efficient if you were dying of thirst.


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Hello!  I wrote this post earlier than usual because this weekend is a hectic one with the psat…

Today we were playing dodgeball in my PE class.  I got “out” pretty early in one round due to my absolute lack of athletic skills.  So I was just sitting there on the floor of the wrestling room thinking about all the sweat and other bodily fluids that must have covered the surface at some point in time when…  I noticed a black squishy “Rhinoskin” ball at rest.  Soon, a blue ball of equal mass rolled towards it and there was an amazing occurence. 

Right there in front of me, the balls collided, sending the black one off at an angle.  The blue ball kept moving but at a different velocity and in a direction that looked perpendicular to the direction of the black ball (characteristic of elastic collisions).  Clearly, this could have represented a game of pool in which a cue ball hits a target ball in a elastic collision.  The collision is not head on so the target ball travels in the direction of the line connecting the centers of the two balls when they are next to each other and not all kinetic energy is transferred.  The system’s overall momentum and kinetic energy would have been conserved.  Of course, I would have had many calculations for you if I had known the initial velocity of the blue ball.

Awesome, yes?  But the environment definitely did not include a pool table…soon the “playing” got more intense and I had to move out of the war zone to avoid a major concussion.  I’d hate to make myself the victim of such a vicious, bloodthirsty beast.

Blue…     Hits    Black!!!

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Physics of Rain

Knowledge I’ve gained after spending Friday and Saturday learning about policy debate and Sub-Saharan Africa: “The Malaria epidemic is like loading up seven Boeing 747 airliners each day, then deliberately crashing them into Mt. Kilimanjaro.”  : (  How can such a statistic not be depressing?

As for physics…it started pouring on my way home on Saturday.  (Which was actually quite a liberating experience.) 

Let’s take the hypothetical average raindrop to be a few millimeters in diameter.  With only gravity, the drop would accelerate towards the Earth at 9.8 m/s².  That’s pretty scary considering that it falls from thousands of feet above sea level.  Luckily for us, there’s air resistance, an upward force which opposes motion in air.  Drag will increase as the velocity of the raindrop increases.  When that force is equal to the downward force of gravity, the raindrop reaches its maximum or terminal velocity.  Since a raindrop is so small, it reaches its terminal velocity rather quickly and therefore won’t cause too much physical harm to those out in the rain.

Furthermore, we could even calculate the change in potential energy as rain (it’s probably too difficult to find the mass in kg of a single droplet) falls to the ground with the handy equation below! 

ΔPE = mgΔh

But we would expect the actual kinetic energy released from rainfall to be a much smaller number due to the opposing drag force. 

Anyways, throughout this whole experience I kept remembering the famous question, “Do you get wetter from walking or running in the rain?”  I’m sure the answer is physics-related… maybe Doc could answer this. : )

Rain.jpg Rain picture by xtaintedwatersx

FYI: In the time it’s taken you to read this post, 4 more children have died from malaria. 

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