Easy Free Body Diagram Question

[mohabitar]

Whats wrong with my answer? I've tried just about everything, but its not accepting it. I've tried +N too, even though I think what I have in there should be right.

 

[collinsmark]

Whats wrong with my answer? I've tried just about everything, but its not accepting it. I've tried +N too, even though I think what I have in there should be right.

Look at the definition of the axes below the free body diagram. y is pointed perpendicular (away) from the surface (i.e. normal to the surface). x points parallel to the surface (down the slope).

The question is asking you to find a_y. Is the object accelerating into or away from the surface plane?

 

[thrill3rnit3]

Shouldn't up the y-axis be positive and down the y-axis be negative?

 

[collinsmark]

Shouldn't up the y-axis be positive and down the y-axis be negative?

Sure, if you want to define them that way, and if you are given the freedom to do so. But in this problem, the x-axis and y-axis are defined for you. One doesn't have a choice on how to define them, for this particular problem.

[Edit: and yes, in this particular problem, up "the y-axis" does represent a positive y, v_y and a_y. But my point is that in this particular problem, the y-axis does not point up. It points up and to the right. The problem statement doesn't give you a choice to define it differently.]

 

[mohabitar]

I don't get it. It should just be the sum of all the forces in the y direction, and that's simply the Normal force and the y component of mg, which is mgcos30?

 

[collinsmark]

I don't get it. It should just be the sum of all the forces in the y direction, and that's simply the Normal force and the y component of mg, which is mgcos30?

Yes, okay I see what you're saying now.

(My previous line of reasoning is that these two force components add up to something very trivial, and that trivial thing is what is equal to ma_y. But maybe that's not the form of what this online question is asking.)

Try thrill3rnit3's advice. It might be just what this online program thing is looking for. Given how the y-axis as it is defined for you, which term gets the negative sign, the N or the mgcos(30)?

 

[mohabitar]

I think I've tried all possible combinations of signs (+,-) to see if it works, but it wouldn't accept anything. It should be N-mgcos30 then right?

 

[thrill3rnit3]

(My previous line of reasoning is that these two force components add up to something very trivial, and that trivial thing is what is equal to ma_y. But maybe that's not the form of what this online question is asking.)

Ah, I see what you're saying. I thought the program thing was asking for an expression for ma_y, not a numerical value.

 

[mohabitar]

Ya it is-it wants an expression not a numerical value

 

[thrill3rnit3]

So which should be positive based from the axes given, N, or mgcos30^o?

 

[collinsmark]

I think I've tried all possible combinations of signs (+,-) to see if it works, but it wouldn't accept anything. It should be N-mgcos30 then right?

Well, yes I suppose. If I had to use something of that form, "N - mgcos30" would be better.

But if you've tried that and different combinations, maybe my original idea isn't so off the mark (maybe, that is. It depends on the authors of the program). So maybe try this: Besides summing up the individual force components along the y-axis, you already know something about what ma_y is (assuming the object stays on the x-axis and doesn't accelerate away from it). Maybe the program is looking for that, maybe.

 

[collinsmark]

Or maybe the program is barfing because you are inputting "N - mgcos30" instead of "N - mgcos(30)" or something like that.

[Edit Oops. Forgot about the "*" symbols. Make that "N - m*g*cos(30)". My point is that maybe the program is expecting the parentheses around the "30" in the sine function.]

 

[thrill3rnit3]

Or maybe the program is barfing because you are inputting "N - mgcos30" instead of "N - mgcos(30)" or something like that.

haha I also think it's an issue with parentheses :rofl:

 

[PhanthomJay]

If the problem is asking you to find ma_y, where y is the axis perpendicular to the plane, what is the magnitude of a_y? Hint: Think about applying one of Newton's laws in the given y direction.