0
$\begingroup$

I am learning basics of physics, I have a doubt regarding air resistance.

Plane 1 and Plane 2 have same mass and surface area.

If plane 1 starts its flight by applying 500N force (numbers are rough and just for illustration), Then it would accelerate. But when speed increases - air resistance increases, if the plane applies the same 500N force throughout its Flight then soon acceleration decreases and the plane will reach terminal velocity due to the opposing force of air resistance. ( I have applied Newtons second Law )

If plane 2 starts its Flight with 1000N force, then it would accelerate more than Plane 1. As the speed of plane 2 increases faster than plane 1 - so will its air resistance. If the flight applies the same 1000N force throughout its flight - eventually its acceleration decreases and the plane will reach terminal velocity due to the opposing force of air resistance. ( I have applied Newtons second Law )

So how come two flights with different force reach their destination at the same time according to Newton's Law???

Improve this question
$\endgroup$
3
  • 2
    $\begingroup$ Well, they don't, so I suspect you would receive more helpful answers if you explain why you think that they do. $\endgroup$
    – J. Murray
    Commented May 4, 2021 at 3:49
  • $\begingroup$ @J.Murray .Lets say the mass of each plane is 100kg, so as f=m*a, acceleration would be 10m/s2 for plane 2 and 5m/s2 for plane 1. Thus plane 2 is twice as fast as plane 1. So having said that the speed and air resistance are directly proportional, with mass and surface area being the same for both the planes the air resistance would be twice as higher and faster ( increase from zero to 1000N ) .Thus air resistance from 0 to 1000N for plane 2 will take the same time as for 0 to 500N for plane 1, So both will each terminal velocity at the same time. Thus travel same distance in same time. $\endgroup$
    – Tim Lee
    Commented May 4, 2021 at 10:22
  • $\begingroup$ Terminal velocity depends on the force being applied by the engines. You are under the impression that the terminal velocities of the two planes are the same, which they are not. If you add this detail to your question body, you will be more likely to receive more helpful answers. $\endgroup$
    – J. Murray
    Commented May 4, 2021 at 13:46

1 Answer 1

Reset to default
0
$\begingroup$

I wouldn't say I am right but my answer is here:
Plane-1 and Plane-2 both reach terminal velocity, yes but terminal velocity for plane 2 is greater than that of Plane-1, much like if Jupiter had atmosphere, terminal velocity reached there would be greater than terminal velocity reached on Earth. Why would plane-2 have a greater terminal velocity? This is because air resistance is proportional to velocity and terminal velocity is reached when the forces acting are balanced. Now, for plane-1 terminal velocity is reached say at 100ms^-1, just an assumption. In that case, air resistance equals to thrust, that is, air resistance equals 500N. Now, for plane 2, when it travels at the same velocity of 100ms^-1, resistive force acting on it is 500N, but the thrust force of this plane is 1000N so there still is a net force accelerating it. Therefore, plane 2 reaches a greater velocity than plane 1. If any mistake is found in my answers please correct me. Thank you

Improve this answer
$\endgroup$
9
  • $\begingroup$ Dear Samip, I suppose you would be right If the two planes had same acceleration - thus same rate of increase in speed and same rate of increase in air resistance so air resistance would increase uniformly for the two planes and the 1000N thrust force plane would reach terminal velocity later than plane 1.I suppose your assumption is correct in case of free falling objects, but what about this??? $\endgroup$
    – Tim Lee
    Commented May 4, 2021 at 4:16
  • $\begingroup$ @dev. I do not think that would be necessary. As, we are talking about magnitude of force here not the rate in its increase. No matter the rate of increase in air resistance, air resistance is always proportional to velocity. So, for a certain air resistance a certain velocity NEEDS to be reached. This does not concern the rate of increase in air resistance. My understanding may yet be wrong so could you please justify your answer. $\endgroup$
    – Samip Gyawali
    Commented May 4, 2021 at 4:17
  • $\begingroup$ Dear Samip, my post was just a doubtful understanding which needs clarification and not an answer. My 1st comment was again a doubt in your justification. Again I would be more than happy to reiterate my doubt on your justification in my next comment $\endgroup$
    – Tim Lee
    Commented May 4, 2021 at 4:33
  • $\begingroup$ You said that velocity is proportional to air resistance, Lets say that plane 1 and plane 2 take off at the same time. Then obviously plane 2 would have more velocity on 1st second (time) of travel than plane 1 due to greater net force and acceleration, This higher velocity continues on every successive second of time of travel for plane 2 than plane 1. So higher velocity would mean higher air resistance for plane 2 than plane 1 as both planes have same surface area and mass. So the greater net force of plane 2 is offset by greater air resistance it experiences than plane 1, $\endgroup$
    – Tim Lee
    Commented May 4, 2021 at 4:49
  • $\begingroup$ continued .....If so how come plane 1 and plane 2 not travel a specific distance in a same time ?? Please Explain this scenario with Newton’s Law. $\endgroup$
    – Tim Lee
    Commented May 4, 2021 at 4:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.