Korean Job Discussion Forums

[blynch]

a boy sits in a bus holding a balloon by a string. the bus accelerates forward. in which direction will the ballloon move? and explain why?

[blynch]

hint: think of the pressure gradient created by the motion of the bus...

[DanielP]

in brackets is easier english

As the bus is in constant motion, the bus and the balloon have the same velocity (both are moving at the same speed)

The bus experiences friction from the ground and air as it moves forward, the balloon, being in the air, only experiences the friction from the air. (bus has to deal with ground friction as it moves forward but the balloon doesnt).

As the bus accelerates, meaning you increase its velocity due to its engine, the balloon will not be able to keep up because it doesnt have that force created by the engine>>>>the balloon will fall back at the same acceleration as the bus is moving forward [+ the magnitude of the buses friction to the ground]. (bus is accelerating cauze it has an engine. balloon isn't accelerating. It's just sitting there. So the bus is gonna be moving faster than the balloon, duh).

What will happen is the balloon will fall back, and the boy will have to pull on the balloon (create tension) at the same magnitude as the force of the new acceleration of the bus. (to prevent the balloon from going away, the boy will have to pull on the balloon).

Hehe, did I get it right? what do I win =)

Daniel

here's a new question. If a skydiver is jumping out of a moving plane, will that skydiver be suddenly thrown back? or keep moving at the same speed of the plane?

[blynch]

thanks, daniel, for your input. and u win the "eslcafe science award".

our first guess is that the balloon moves backward as danielp reasoned. but the balloon actually moves forward. this occurs due to the pressure gradient created by the motion of the bus. when the bus is at reat, the air molecules undergo random motions. on the average the molecules (nitrogen/oxygen) remain in one position. as the bus accelerates, the back of the bus collects the air molecules. the front of the bus leaves the air molecules behind. the net result is an increase in air density at the back of the bus, and a decrease in air density at the front. just as a balloon rises due to the greater pressure at the lower end of the balloon than at the top of the balloon, similarly the greater pressure at the back of the balloon will cause it to move forward.

as for your question, initially the skydiver belongs to the inertial reference frame of the airplane, that it, the skydiver is flying at the same velocity of the airplane. even after the skydiver jumps out of a moving plane, the diver still remains in the same inertial reference frame of the plane, that is, the diver is still moving at the same velocity of the plane. but as the wind separates the diver from the plane and puts the diver into a new inertial frame in reference to the airplane. this new inertial frame is dictated by the earth's gravity, that is, free falling of the diver. so the answer is the skydiver will not be suddenly thrown back but keep moving at the same speed of the plane for a moment until the wind (air resistance) puts the diver into the new inertial frame called "free falling".

[poet13]

"as for your question, initially the skydiver belongs to the inertial reference frame of the airplane, that it, the skydiver is flying at the same velocity of the airplane. even after the skydiver jumps out of a moving plane, the diver still remains in the same inertial reference frame of the plane, that is, the diver is still moving at the same velocity of the plane. but as the wind separates the diver from the plane and puts the diver into a new inertial frame in reference to the airplane. this new inertial frame is dictated by the earth's gravity, that is, free falling of the diver. so the answer is the skydiver will not be suddenly thrown back but keep moving at the same speed of the plane for a moment until the wind puts the diver into the new inertial frame called "free falling"."

Uh huh. Think about when you see war movies, specifically napalm shots. You see a streak of flame along the ground. The only slowing of the projectile, whether it's a human out of a plane, or a free fall bomb, is from air friction. That's why jumpers appear to fall behind the jump vehicle. Neither the person or the bomb has any propulsion method, so for the sake of argument, if there air no air currents, call it +-0 friction influence. If the jumper has a "headwind", that is, an air current moving opposite to the trajectory of his exit vector, the jumper will reach a ground target point slightly shy than that of the +-0 jumper. A person with a "tailwind", will reach a ground target point slightly in front of the +-0 jumper as well. In reality though, there are always air currents, and from the higher you jump, the more different directions you have to deal with.