I'm a big beginner having only taken Grade 11 high-school physics.

Imagine this situation: an object is accelerating north, and while it's accelerating, it's 'hit' by a northward force.

To me it would make sense that the acceleration of the object would increase - something like this:

aNew = aOld + (f/m)

I got the (f/m) from f=ma.

But then think of gravity - while an object is accelerating downward at 9.8m/s2, it has the force of gravity pulling it too. So then, based on my previous logic, it's acceleration would constantly be increasing!

aNew = aOld + (fGravity/mass)

And I know that you don't increase your acceleration while you fall, so now I'm confused.

In a nutshell: What exactly are the rules for how an already accelerating object reacts to forces? Am I wrong that your acceleration would increase?

EDIT: I realized I've made a big mistake; I never knew that the acceleration drops to 0 the second the net force is 0. This makes sense now.

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    $\begingroup$ No need to say you made a "big mistake". You had a misunderstanding and it got cleared up. That is what this site is for. Glad we could help you this time! Come back often... $\endgroup$ – Floris Jun 10 '14 at 1:57

The acceleration of an object is a result of the sum of all the forces. If it was accelerating before you hit it, there must have been a force. If an object is falling in air, there are forces of gravity and air friction on it. When all forces even out the object continues with the same velocity.

Simply take the vector sum of all forces - that will tell you what is going on.

  • $\begingroup$ So if there was no air friction, the acceleration would increase in both cases, but since there is, the air friction reduces the acceleration to 0 after enough speed is reached? $\endgroup$ – Aaron Jun 9 '14 at 23:17
  • $\begingroup$ It is possible that the acceleration becomes zero - certainly if you keep applying the same force you will eventually most likely reach an equilibrium (or reach escape velocity and disappear into space where there is essentially no friction...) "it depends". $\endgroup$ – Floris Jun 9 '14 at 23:50
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    $\begingroup$ @ForgiveGoto Remember that air friction is just another force as well. Calculating the force applied depends on the speed of the object, which makes a differential equation as you have both the acceleration and velocity of the same object in the equation. Terminal velocity in this case is the situation where the force of gravity, and the force applied by air friction equals out, and the object stops accelerating even though it's being pulled downward by gravity. Air friction is simply applying an equal but opposite force. $\endgroup$ – SplinterReality Jun 10 '14 at 2:22

You are right that a new force causes a new acceleration, and that this relationship is independent of the object's velocity. The reason why things don't constantly accelerate as they fall is because of air resistance. Something "falling" in a vacuum would continue to accelerate until it collided with the ground. Air resistance increases in relationship with the square of the object's velocity and provides a force opposite the direction of motion (in this case down). The force of gravity, however, basically remains constant as the object falls. At some point, the object will have accelerated to a velocity where the force of air resistance pushing upwards will have the same magnitude as the force of gravity pulling downwards. The forces being equal but opposite, they cancel each other out, making the object subject to no net force and thus no net acceleration. Thus the velocity of the object remains constant only due to air resistance.


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