dessskris Posted February 18, 2012 Report Share Posted February 18, 2012 how does air resistance affect the motion of an object?how will the speed of the object change over time if there is air resistance?how will the acceleration of the object change over time if there is air resistance?is air resistance a constant deceleration?please, I need to understand this. thank you!! Reply Link to post Share on other sites More sharing options...
nametaken Posted February 18, 2012 Report Share Posted February 18, 2012 Okay. I'll attempt to go through your questions, step by step.1) I'm assuming you mean 'falling' objects. Air resistance in essence simply refers to the application of a force in opposition to the direction of motion of an object. It is dependent on the current velocity of the object. Let's consider projectile motion. The air will resist the motion of a ball as it is kicked upwards for example, with a force. With air resistance factored in, the optimum height that the ball can reach is less as is the range. Without air resistance, there is a bigger height it can reach.3) In the real world, because of air resistance, objects do not fall indefinitely with constant acceleration. 4) I think that air resistance is a little more complex than that. It would vary, and so there perhaps wouldn't be a constant deceleration since air resistance is dependent on the density of air (which would vary at different temperatures), and the drag coefficient which again varies depending on the type of object it is. I dunno...hope that somehow helped. To be honest, I'm feeling slightly baffled now myself. Reply Link to post Share on other sites More sharing options...
dessskris Posted February 18, 2012 Author Report Share Posted February 18, 2012 thank you so much!!so then, if I were to throw a ball upwards, how will the velocity change over time until it reaches the ground as affected by air resistance?a graph of V versus t would be lovely! Reply Link to post Share on other sites More sharing options...
nametaken Posted February 18, 2012 Report Share Posted February 18, 2012 Pfffffffffffft. It becomes difficult when air resistance is factored in.Okay. Well, I think the velocity of the ball will be fastest the instant it leaves your hand. When it is thrown up, it decreases constantly as does air resistance since air resistance is the square of velocity till it gets to zero (at the maximum height). The acceleration of the ball downwards now increases, and the air resistance gets bigger, which prevents the ball from getting faster. This is where the ball has terminal (constant) velocity.I think that's generally what happens. I could be wrong, but makes some sense to me,Couldn't include a graph, since I don't have the software or technical knowhow at my disposal at the moment. :/But the graph would obviously be in the positive set of axes and negative, since velocity is a vector. 2 Reply Link to post Share on other sites More sharing options...
Grassroot Posted February 21, 2012 Report Share Posted February 21, 2012 BTW... I dont think numberical and quantitative calculation of air resistence is required for exam though Reply Link to post Share on other sites More sharing options...
Slovakov Posted February 23, 2012 Report Share Posted February 23, 2012 (edited) The acceleration (deceleration) of an object acted upon by drag force will surely not be constant, as this force (and hence acceleration due to it) changes at every instant with the velocity of an object. When we assume parameters such as drag coefficient or air density to be constant and put them under mutual constant k, and then set an equation kv2 = ma (from Newton's 2nd law), then the acceleration appears to be kv2/m.However the above assumption is a bit useless, because when we throw the ball upwards, and it reaches a significant height, slight differences in air density must occur, and what's more, the drag coefficient rises rapidly at very low speeds (for Re<~100).Morevoer in case of very light objects of high volume (eh. baloons), the buoyancy force will also have a significant impact on the acceleration of a body.Having said all that wierd stuff, now it's time for some good news. For an IB exam all you need to know is that when air drag acts on a body thrown upwards, the max. height will be smaller as well as the final velocity of a body. Edited February 23, 2012 by Slovakov 1 Reply Link to post Share on other sites More sharing options...
omri Posted April 23, 2012 Report Share Posted April 23, 2012 You also should know that air resistance increases and velocity increases! Reply Link to post Share on other sites More sharing options...
SerUmbras Posted April 28, 2012 Report Share Posted April 28, 2012 thank you so much!!so then, if I were to throw a ball upwards, how will the velocity change over time until it reaches the ground as affected by air resistance?a graph of V versus t would be lovely!That's really hard to say. It depends on the amount of air you have above you. The lower down you are, the more air is on top of you, the more it will be resisted, the lower it goes. But, in short, a V over t graph would show V decreasing at a decreasing rate over time (hard to get, yes. But please, bear with me)Basically, the air gets in the way of a moving object. Imagine throwing a ball in water - the water gets in the way. Same for air resistance, except the air is considerably less dense, so has a much smaller effect.As the ball goes faster, it hits more air every second. If there's x amount of air per metre, and you go 5 metres a second, you get 5x litres of air slowing you down a second. if you go 20m/s, you get 20x litres slowing you down every second etc etc...Now, because the air is slowing you down, there is a reduction in the ball's forward acceleration. BUT, the acceleration change depends on the speed, because the speed dictates how much air is slowing it down. So, no, it's not a constant deceleration, it decreases as you slow down further.I hope that helped you get the general idea... Reply Link to post Share on other sites More sharing options...
HiggsHunter Posted May 2, 2012 Report Share Posted May 2, 2012 so then, if I were to throw a ball upwards, how will the velocity change over time until it reaches the ground as affected by air resistance?As you know, if air resistance is neglected the horizontal component of the velocity of a projectile remains constant during flight, whereas the vertical component decreases linearly from a positive value at launch to a negative value just before it returns to ground.Although the relationship depends on many factors, for a ball that is thrown by hand the air resistance is roughly proportional to the square of its speed. So taking air resistance into account, the horizontal component of the velocity decreases exponentially towards zero, whereas the vertical component of the velocity decreases exponentially from a positive value at launch towards a constant negative value called the terminal velocity. (Of course it doesn't actually reach the terminal velocity if it hits the ground first!) Reply Link to post Share on other sites More sharing options...
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