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B Jumping on a boat that is moving

  1. Dec 29, 2017 #1
    Hello PF, I am a junior in high school who has yet to take physics and is currently in Honors Chem. I recently saw a video on social media, where a group of guys is on a big boat and this one guy was standing relatively close to the bow. And they hit a big wave and he jumps in the air and lands almost exactly where he started. And i have been sifting through other threads and information but can't seem to find an answer. Why does this happen, isn't there air resistance and such. But he is on the boat so he must be going the same speed as it. I am absolutely baffled and confused, so i turned to you people of great intelligence to help me.

    Here is the video if you were curious (https://www.instagram.com/p/BdRWcVTj_Em/?hl=en&taken-by=barstoolsports)

    Thank you
    -Eric
     
  2. jcsd
  3. Dec 29, 2017 #2
    You are correct. He is going the same speed as the boat, and he keeps that speed when he is up in the air. The air resistance is pushing him back, just not enough to notice. Your observations are good, and the rest will make sense when you take physics.
     
  4. Dec 29, 2017 #3

    tech99

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    Think of the up-and-down movement as being separate to the horizontal movement.
    Newton's First Law says that objects keep moving with the same velocity unless acted on by a force. So if air resistance is negligible, he will continue at boat speed when he is in the air.
    Of course, air resistance will slow him down a little, but if we assume a boat speed of 5m/s, the force is too small to slow him significantly during the brief interval he is airborne.
    Regarding the up-and-down movement, it is no different to making the jump ashore, and he will be in the air for the same time. The horizontal speed of the boat has no effect, although I agree the boat pushed him up at just the right moment, which would not happen ashore.
     
  5. Dec 29, 2017 #4

    tech99

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    Sorry Eric, I did not spot your reply.
     
  6. Dec 30, 2017 #5

    sophiecentaur

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    You can 'feel' the force of a 5m/s wind against your face so the same pressure is acting over the whole area of your body facing the wind, which would correspond to a small but significant force. Remember that sailing boats (albeit with large sails) will sail happily in an 11mph wind. A though experiment on a boat adds several extra factors that can get in the way of understanding what's going on. If you imagine hanging on a bar with a 5m/s wind blowing on you (preferably a stationary dummy.). The feet are 0.5m above the ground. In the time taken to fall (about 0.2s) that force will move you a small amount - enough to measure a movement along the ground.
     
  7. Dec 30, 2017 #6

    ZapperZ

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    This is no different than jumping on the earth, which is moving with respect to other celestial bodies.

    Zz.
     
  8. Dec 30, 2017 #7
    The wind starts to blow him back when his feet leave the boat. The distance he is blown back can be estimated, given some assumptions:
    Estimated wind speed: 20 MPH
    Estimated frontal area of the boy: 5 ft^2 (he's skinny)
    Estimated Cd of the boy: 1.0
    Estimated weight: 150 lbs
    Estimated time in the air: 0.9 seconds

    Velocity pressure of air at 100 MPH is 25 lbs/ft^2, so the velocity pressure at 20 MPH is (20/100)^2 X 25 = 1.0 lbs/ft^2.
    Total force of wind on the boy is 1.0 lbs/ft^2 X 1.0 Cd X 5 ft^2 = 5 lbs.
    Acceleration = 5 / 150 X 32.2 = 1.07 ft/sec^2.
    Distance = 0.5 X 1.07 ft/sec^2 X 0.9^2 sec^2 = 0.4 feet = 5 inches.

    If he jumps slightly forward on his way up, he will land exactly where he takes off.
     
  9. Dec 30, 2017 #8

    jbriggs444

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    And by no coincidence, he will already be leaning forward at the right angle to accomplish just that.
     
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