Practical Daily question [about jumping off a building]

Factual Questions
81

Back up a second. More mass does not equal more drag–in fact, the mass has no effect whatsoever on the drag force. But we’re not talking about the drag force, we’re talking about the motion of the ping-pong ball–its position, velocity, and acceleration.

I’m sure you’ll agree with the following equation:

F = ma

Now, if the force stays the same, and the mass changes, what happens to the acceleration?

That’s what’s going on here–changing the mass doesn’t change the drag, it changes the effect of the drag.

82

It doesn’t, nor does it have to. The object with more mass has more downward force to overcome the air resistance. Imagine holding a balloon over your head and blowing upwards. The air resistance easily overcomes the weight of the balloon and propels it upwards. Now try to imagine doing that with the same balloon filled with water.

83

Perfect analogy, Boyo Jim, and thanks for the equation, zut. Got it now!

84

As others have noted, the essential point is that other forces aren’t absent. In particular, aerodynamic drag is present.

The important point is that more mass doesn’t equal more drag. The drag of a ping-pong ball is simply a function of its velocity (assuming its size and the air density are constant).

More complete equations:
If F = downward force, W = weight, M = mass, G = gravitational acceleration, D = drag and A = acceleration (actual change in vertical velocity), we can write:
F = W - D
W = M * G
F = M * G - D
F = M * A
A = F / M
A = (M * G - D) / M
A = G - D/M

From the final line we see that at any given speed, the greater the mass, the lower the acceleration penalty due to drag.
ETA: Sorry - it took a long time for this to post.

85

Thanks, Xema–the problem was, I was picturing drag as a force acting equally on both objects and therefore might-as-well-be-absent.

86

Oh honey! You know I love you!

87

Well? Don’t keep us in suspense, OP. Did your friend hit her?

88

Not if it took you almost a week to reply, you don’t! :smiley:

89

Sorry! Been alittle bit busy making up to the sweetest and most amazing woman in the world :slight_smile:

90

Not if a certain someone keeps messing around on here when they should be studying:mad: then that woman ceases to be sweet.

91

Get a room, guys, we’re trying to employ the laws of physics in illegal and unethical ways here.

92

I dunno. From 600 feet away, I’m supposed to know when she’s exactly 26.2 feet from the impact point? I might need better glasses…

93

Well, I’m assuming the OP has some sort of system in mind to alert him when the woman in question passes the point where he should jump. Otherwise it would be silly to ask the question, wouldn’t it?

94

We get all kinds 'round here. Never assume anything.

95

How do figure this out is a separate thread.

96

The simplest way would be to just mark it out, have a spotter on the ground, and have the spotter let you know when she steps on the mark. Of course, you’ll have to compensate for any lag in the transmission.

97

Yes, you’ll have to jump just before the transmission. :stuck_out_tongue:

98

Why, how clumsy of me to land on you… I was aiming for my car… Say, you have the most beautiful eyes, would you like to get some coffee sometime? How about 7:30 ish? Could you call an ambulance? Thanks

99

I believe you meant, “Oh shiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii-- GLORP.”

100

I thought homework questions in Physics 101 usually allowed you to ignore air resistance…