Need help understanding this problem!

  • #1
confusedaboutphysics
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so i understand part a, but I don't understand how to do part b. please help!

You are given a vector in the xy plane that has a magnitude of 90.0 units and a y component of -40.0 units.

(a) What are the two possibilities for its x component?
I got 80.6 and -80.6.

(b) Assuming the x component is known to be positive, specify the vector which, if you add it to the original one would give a resultant vector that is 90.0 units long and points entirely in the -x direction. What is the magnitude and direction?
 

Answers and Replies

  • #2
Doc Al
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Think of your original vector in terms of its components. Now think of the resultant vector in terms of its components. (What are those components?)

What do you have to add to the original components to end up with the resultant components? What you have to add are the components of the vector that you need to find. Once you have the components, then figure out its magnitude and direction.
 
  • #3
quantumdude
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confusedaboutphysics said:
(a) What are the two possibilities for its x component?
I got 80.6 and -80.6.

That's right.

(b) Assuming the x component is known to be positive, specify the vector which, if you add it to the original one would give a resultant vector that is 90.0 units long and points entirely in the -x direction. What is the magnitude and direction?

OK, so your vector can be written as [itex]\vec{v}=80.6\hat{i}-40.0\hat{j}[/itex]. What vector would you have to add to that to get a result that has no y-component, and a magnitude of 90? Remember that vectors add componentwise.
 
  • #4
confusedaboutphysics
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what are my original components and my resultant components?
 
  • #5
quantumdude
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Look at the following equation:

[itex]9+x=14[/itex].

You wouldn't have any problem finding the solution to that, right? Well, this problem is basically the same. Let [itex]\vec{v}=80.6\hat{i}-40.0\hat{j}[/itex], let [itex]\vec{w}=x\hat{i}+y\hat{j}[/itex], and let [itex]\vec{r}[/itex] be their sum. Then set up the vector equation:

[itex]\vec{v}+\vec{w}=\vec{r}[/itex].

You know [itex]\vec{v}[/itex] and you have enough information to determine [itex]\vec{r}[/itex]. All you have to do is find [itex]\vec{w}[/itex] by subtraction.
 
  • #6
Doc Al
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confusedaboutphysics said:
what are my original components and my resultant components?
The original vector is the one you started with. You were given its y-component as -40.0; you found the x-component. So, you should have the components of this one.

The resultant is given. Hint: It only has an x-component (the y-component is zero). The magnitude is 90.0 units; figure out the components based on the information given in the problem.
 
  • #7
confusedaboutphysics
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so V1y = -40 and V1x = 80.6 and V2y = 0, right? so Vy = -40 +0 = -40? so how do i find V2x?
 
  • #8
quantumdude
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Well, if the magnitude of the resultant is supposed to be 90, and if the resultant only has one component, then how long must that component be?
 

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