Tonight's mythbuster

[Dave Gray]

Wouldn't initial velocity just be 0?  The ball is at a standstill when launched, more or less.

[Brian Fein]

no, initial velocity is the velocity of the ball the instant AFTER it is launched.  That's what the initial velocity in this equation needs.

Not only that but the 100N force is in the 45-dgree-angle direction, you have to break it up into horizontal and vertical components to find the time in flight (falling objects theory)

[BeefStewert]

distance is 1/2 a t^2 + Vo t

Finding Vo, a and t is most of the problem.  All three are different for each ball.

And, since you don't know the initial velocity, but you know a force, you have to use conservation of energy and impulse and momentum to get there.  You use F dt / m to get Vo.

Brian, if you keep assuming that the ball is flying in a vacuum, you will keep getting that the lighter ball flies furthur.  The key is that this is not a vacuum, you have other forces on the ball during flight.  

[Brian Fein]

I know, I was working out the Theoretical (in a vaccuum) stuff.  just for fun.

The wind drag is dependant upon the shape of the ball, inflation pressure, etc.  I don't see how air resistance would differ between the two cases.

But, what is different is the rate at which gravity acts upon the ball, since the helium ball has a buoyancy on it that the air ball doesn't have...

[raptorsfan29]

I AM an engineer - so I'll say that "THEORETICALLY" the ball should fly farther.  Mostly because the WEIGHT of the ball is less, thanks to the buoyancy of the helium.  However, due to the buoyancy of the helium in comparison to the weight of the ball itself, the difference is negligible and the distance wouldn't be much more (i'm thinking fractions of an inch).  Its that whole Newton's law of gravity being that all object fall at the same rate.

I could work out the kinematics for you, if you want, but its all theoretical values.  Real world behaves much different than theory.

As for your F=ma - if F is held the same, and mass decreases, accelleration would have to increase.  But, even if accelleration increases, that just means more time in flight, no necessarily more distance.

hey brian, got a question for you  I was watching speed the other day and wondering can a bus really jump 50 feet. My question for you is can it really happen and what elements have to be considered in finding the answer scientificly.

you seem like you know these complicated formulas

[BeefStewert]

hey brian, got a question for you  I was watching speed the other day and wondering can a bus really jump 50 feet. My question for you is can it really happen and what elements have to be considered in finding the answer scientificly.

you seem like you know these complicated formulas

velocity, acceleration, shape, and mass of the bus.  And the angle of the ramp it is jumping off of.

[BeefStewert]

I know, I was working out the Theoretical (in a vaccuum) stuff.  just for fun.

The wind drag is dependant upon the shape of the ball, inflation pressure, etc.  I don't see how air resistance would differ between the two cases.

But, what is different is the rate at which gravity acts upon the ball, since the helium ball has a buoyancy on it that the air ball doesn't have...

The amount of air resistance doesn't chage (assuming same shape and inflation pressure).  What does change is the effect on the flight of the ball of this constant reistance force.  If the ball is lighter, it will decelerate quicker. 

You are focusing on the initial force, not the forces throughout the flight of the ball. 

[BeefStewert]

I'm going back to my football shaped ping-pong ball.  If it was in a vacuum, it would fly much furtur than a real football.  But in real life, we know that it won't fly anywhere close to as far. 

And with a ping-pong ball, gravity has much less effect since the mass is so much lighter.

[Brian Fein]

I was thinking about this on my way down to get lunch.

While the ball's in flight, there are 4 things happening...
- The ball has momentum due to the initial force
- drag force acting on the ball
- gravity acting on the ball
- buoyant force acting on the ball (in the case of the air-filled ball, this is zero)

Now, the Helium ball has less mass, thus the force of gravity is less.  In addition, it has a buoant force.  These combined will add to the flight time of the ball.  But, the buoyant force is equal to the weight of the air displaced by the HELIUM (not the ball) which I'm guessing is so small, its negligible in this problem.  Therefore, the same forces acting on both balls, the flight path should be the same.

Beef, I'm not sure I'm getting what you're saying about decelerating quicker.  All objects fall to earth at the same rate, thanks to gravity (and Mr. Newton).

And raptor - that crap from Speed was just that - crap.  There is a way a bus can jump 50 feet, but not on an overpass like that.  They'd need to be going way faster than 55 MPH and need a much bigger ramp.  If you want I can try to solve that problem for you, too (in a vaccuum of course!).

[jtex316]

I don't understand a fucking thing in this entire thread.  That's what I know, Joe = ??

[Brian Fein]

Joe = ??

That's an easy one, vaccuum or not...

Joe  = dumbass

[bsfins]

And Sugarbush - that crap from Speed was just that - crap.  There is a way a bus can jump 50 feet, but not on an overpass like that.  They'd need to be going way faster than 55 MPH and need a much bigger ramp.  If you want I can try to solve that problem for you, too (in a vaccuum of course!).

Please tell me you didn't just call raptor ..Sugarbush? Lol

[Brian Fein]

Please tell me you didn't just call raptor ..Sugarbush? Lol

I have no clue what you're talking about...

I got nailed by the quick look at the avatar.  Not sure why I did that...  Damn...

My bad, raptor!

[Sunstroke]

I was watching speed the other day and wondering can a bus really jump 50 feet. My question for you is can it really happen and what elements have to be considered in finding the answer scientificly.

Critical elements to be considered for the bus-jumping equation:

* Drive: If I am at the wheel, that bus can fly. Period.
* Mass: Have I eaten a large meal before take-off?
* Momentum: Do I really need to use the restroom once I land?
* Drag Force: Did I forget to take the smiley-face ball off the bus's antenna prior to launch?
* Gravity: Am I taking my bus-jumping task seriously?
* Angle: If I fly more than 50 feet in the bus, will it help me score with the hotties in the crowd?

Who needs engineers...

[Phishfan]

Beef, I'm not sure I'm getting what you're saying about decelerating quicker.  All objects fall to earth at the same rate, thanks to gravity (and Mr. Newton).

Only in a vacuum which we are not using for this example (I assume). Outside of a vacuum a lighter object is affected by air drag more than a heavier object. Think of the example this way project a ping pong ball and an exact sized steel ball bearing. The ping pong ball will be affected by the air drag sooner and not travel as far.