Help me with this problem....

Prakash Panneer · 2007-01-04 00:36:32

An acrobat is shot out of a cannon and lands in a safety net that is 10 feet above the ground. Before being shot out of the cannon, she was 4 feet above the ground. She left the cannon with an initial upward velocity of 50 feet per second. Find the time t (in seconds) it takes for her to reach the net. Explain why only one of the two solutions is reasonable.

Thanks in advance

gnitsuk · 2007-01-04 03:37:52

One can use the equation of motion:

All we want to know is which values of t will make s = 6 (when s = 6 the acrobat is 6 + 4 = 10ft above the ground - and 10ft above the ground is where the net is).

We only need consider the vertical motion and a = -32 (gravity acting downwards) so rearranging for t we have:

Using the quadratic formula gives:

which gives:

The first time is not the answer, it is the time when the acrobat is first at the same height above the ground as the net (but not yet at the net). Of course the net could be placed here, but she would be travelling upwards into it at this time and so this is not reasonable! The second time is the answer.

Last edited by gnitsuk (2007-01-04 05:25:48)

Prakash Panneer · 2007-01-04 05:09:56

Here, we have to take a = g = -32 feet/sec. Right?

gnitsuk · 2007-01-04 05:23:24

Absolutely.

Oops! I was suprised that the numbers didn't come out nice but more than that, I was very unhappy with the flight time of over 9 seconds. I've corrected it.

Even the big boys made this mistake - http://edition.cnn.com/TECH/space/9909/ … metric.02/

Thanks,
Mitch.

Last edited by gnitsuk (2007-01-04 05:27:38)

Math Is Fun Forum

#1 2007-01-04 00:36:32

Help me with this problem....

#2 2007-01-04 03:37:52

Re: Help me with this problem....

#3 2007-01-04 05:09:56

Re: Help me with this problem....

#4 2007-01-04 05:23:24

Re: Help me with this problem....

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