I'm starting Physics, and I don't understand why the acceleration along the x-axis is zero for an object thrown near the surface of the Earth. This may be problem specific, but I wouldn't know since I am just beginning.
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$\begingroup$ This makes 0 sense to me. How can you accelerate an axis? $\endgroup$– ApoorvCommented Jun 17, 2011 at 19:08
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2$\begingroup$ Are talking about an object thrown near the surface of the earth? $\endgroup$– MBNCommented Jun 17, 2011 at 19:13
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1$\begingroup$ @MBN Yes I am, I should have been more specific. $\endgroup$– StrawberryCommented Jun 17, 2011 at 19:15
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3 Answers
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Because gravity points purely along the y-axis. If you ignore air drag or thrusters or engines (as you would in a projectile problem), there are no forces along the x-direction, and hence no acceleration along the x-direction.
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The acceleration in the x-axis can be non-zero.
In most 2-D physics problems using Cartesian coordinates, the convention is that the y-axis is vertical and the x-axis is horizontal (doesn't need to be that way; it is just the convention used).
So, the y-axis is TYPICALLY the name given to the vertical axis. And gravity on a free falling or stationary object TYPICALLY works only in this vertical plane.
If you were to modify your physics problems, say, to make a ball roll freely down a ramp in the earth's gravitational field, you would find that you get acceleration in both the x and y axes.
When you get to 3-D physics problems, the convention changes: Typically x and y are the assigned names for the two horizontal axes of the Cartesian coordinate system, and the z-axis is the vertical one.
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The acceleration along the axis in this case depend upon the choice of axis. If you have chosen vertical as y axis then there will be only acceleration along y axis as it cannot have any component along x axis.
I think you have problems with why it only affect the motion along the vertical. It is because tho orthogonal vectors doesn't affect each other as they have no component along each other. Therefore motion along vertical is independent from motion along horizontal. But they have a resultant. As in your case the resultant motion is parabolic.
Also I have assumed that you are talking about projectile motion.
