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B An insect flying at 150 km/hr

  1. Feb 3, 2016 #1
    Given a scenario, where a car is going at a speed of 150 km/hr on a straight and level road. There is an insect flying inside the car. To a person inside the car, (I really do not know how to say this, but I hope you understand), the insect is flying naturally, basically, the speed of the car does not seem to have any effect on its motion.
    Now consider an observer outside the car. Relative to him, the car is going at 150 km/hr and so is the person inside the car. Does this mean that the insect is also flying at 150 km/hr relative to the observer outside the car? Is this not something which seems far from reality, an insect flying at 150 km/hr?
     
  2. jcsd
  3. Feb 3, 2016 #2

    Drakkith

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    The insect is certainly moving at 150 km/h with respect to an observer on the ground, yes.

    The insect is not flying through the air (moving with respect to the air directly surrounding and supporting itself) at 150 km/h, it is hovering or flying slowly through nearly stationary air this parcel of air itself is moving at 150 km/h with respect to the observer outside the car.
     
  4. Feb 3, 2016 #3

    russ_watters

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    You've discovered the Principle of Relativity: all velocities are relative. They are defined based on the motion between two objects -- any two objects, in any frame of reference.

    https://en.wikipedia.org/wiki/Principle_of_relativity
    Click through the history to where it describes the principle first outlined in a thought experiment by Galileo, describing cannonballs fired by ships on a rotating Earth.
     
  5. Feb 3, 2016 #4

    Drakkith

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    Note that it also seems ludicrous to think that you may be sitting and flying through the air at 600 mph on an aircraft. I mean, you're just sitting there doing nothing! How can you be moving at 600 mph? The answer is that the plane, and everything inside it, is being propelled through the external air by the plane's engines.
     
  6. Feb 3, 2016 #5
    Which further explains the fact that if at 150 km/hr, we open the window panes of the car, we tend to get blown away, or rather get squashed onto the car seat. So the controlled pressure inside the cabin of the car/plane also helps to maintain our state of rest relative to the car/plane.

    Wonderful concept!
     
  7. Feb 4, 2016 #6

    A.T.

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    But you have no problem with the humans in the car, moving at 150 km/h without even moving their legs?
     
  8. Feb 4, 2016 #7
    No, actually the human sitting in the car is in contact with the seat. So what I think is that the force produced by the engine carries the human along only because of the contact between them. Had there been no contact, the human, unlike an insect, would not have been able to travel along with the car.
    Now the insect is flying inside the car. There is no way the insect is in contact with any part inside the car. This is where the question arised.
     
  9. Feb 4, 2016 #8

    A.T.

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    When the car and human move at constant 150 km/h , the car doesn't apply any horizontal force to the
    human

    Unless you consider the air inside the car as part of the car. But here again, at constant and equal speed of car and insect there is no horizontal force by the air on the insect.
     
  10. Feb 4, 2016 #9
    Dude, the man moves along with the car iff there is a contact force (friction, in this case). Here the horizontal force is friction, although it is acting equally on both the car and the human. But it is definitely the force responsible for carrying the man along with the car.
     
  11. Feb 4, 2016 #10
    One thing is for sure, that there is a difference in pressures of air inside the car and outside the car. What you are saying is absolutely correct because the difference in pressures doesn't affect the motion of the insect in any way.So yes there is no external force on the insect by the air "outside".
     
  12. Feb 4, 2016 #11

    A.T.

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    There is no horizontal force on the man by the car, if the man and car move at constant speed.

    There is also no horizontal force on the insect by air inside, if the insect and car move at constant speed.
     
  13. Feb 4, 2016 #12
    Absolutely, but for "both" to acquire a constant speed, there must have been some initial force required, which was obviously friction. That is what I was mentioning.
     
  14. Feb 4, 2016 #13
    Why must there be?

    It didn't have to be all friction. It could have also been the normal force associated with the push of the back of the seat on the body.
     
  15. Feb 4, 2016 #14

    ZapperZ

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    Keep in mind that the insect, and everything else here on earth, be it on the ground or in the air, are moving at a significantly higher speed than 150 km/h according to some alien on another planet. 150 km/h is really nothing.

    Zz.
     
  16. Feb 4, 2016 #15
    My bad. There will not be any difference in pressures. Understood.

    What I probably meant was the drag of the air outside the car.
     
  17. Apr 23, 2016 #16
    I think the insect is flying at the speed of 150Kmph. According to the blog by a Pest Control Katonah NY, there are some insects such as Male Horsefly which can fly at the speed of 145.8 kmph.
     
  18. Apr 23, 2016 #17

    Drakkith

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    I answered this in the 2nd post. The insect is moving with respect to the ground at 150 km/hr, but it is not flying through the air at 150 km/hr. The air inside the car is stationary with respect to the car, and the insect is stationary with respect to this air, despite the car moving at such a high velocity.
     
  19. Apr 23, 2016 #18

    jbriggs444

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    Agreed. Further, there needn't have been any force from the car at all. The person could have parachuted into the seat like James Bond. Or could have been born in the car. But that is irrelevant in any case. When working a physics problem is not necessary to worry about how the given starting conditions came to be. If we do not ask for an explanation of how a person came to be at rest in a car that is at rest in a one reference frame why should we ask for an explanation of how a person came to be at rest in a car that is at rest in a different reference frame?

    There is no need to quibble about these statements. They are correct as they stand (modulo pedantic quibbles about speed versus velocity).
     
  20. Apr 23, 2016 #19
    With respect to a stationary observer, the insect is moving at a speed of 150 km/hr.
     
  21. Apr 23, 2016 #20
    Let's say the insect got into the car when it was stationary and parked itself on the rear window. Then the car accelerated to 150 km/hr. During the acceleration, the insect will feel the force of the rear window on it. Once the car reaches 150 km/hr and remains at that speed, the insect will continue to have that same speed. No further force is needed to maintain that speed.
     
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