Mechanics Problem with I/J vectors

  • Thread starter FaraDazed
  • Start date
  • #1
347
2

Homework Statement


Two particles P and Q move in an x-y plane. At time t seconds the position vector of P is [itex](t^2\hat{i}+4t\hat{j})m[/itex] and Q is [itex](2t\hat{i}+(t+1)\hat{j})m[/itex].

A: Prove the particle never collide
B: Show that the velocity of Q is constant, and calcualte the magnitude and direction
C: Find the value for t when P and Q have paralell velocities and find the distance between them at this point.


Homework Equations


?


The Attempt at a Solution


Its part C that is confusing me but incase I messed the others up I will post them as well.

Part A:
For the particles to collide their position vectors will equal each other at the same time so setting their I an J components to equal eachother should produce any times they are equal. First their I components.
[tex]
2t=t^2 \\
0=t^2 - 2t \\
0=t(t-2)
[/tex]
Therefore when t=0 and t=2 the I components are the same. And now the J components.
[tex]
t+1=4t \\
1=3t \\
t=\frac{1}{3}
[/tex]
Therefore their J components are only equal when t=1/3, therefore they never collide as the I and J's are never the same at the same time.

Part B:
The velocity of Q will be constant if neither of the components are functions of time so differentiation will find me the velocity vector
[tex]
\dot{r_Q}=2\hat{i}+1\hat{j}
[/tex]
Therefore the velocity is constant as neither are functions of time. To find the magnitude next
[tex]
\sqrt{2^2+1^2}=\sqrt{5}m/s
[/tex]
And the direction
[tex]
tan^{-1}(\frac{1}{2})=26.57°
[/tex]

Part C:
To find the time when P and Q have parallel velocities first P's velocity vector needs to be found
[tex]
\dot{r_P}=2t\hat{i}+4\hat{j}
[/tex]
It can be seen that both P and Q have constant J components therefore they will be parallel when the I components are the same.
[tex]
2t=2 \\
t=1
[/tex]

However the answer to part C is apparently 4 not 1. Any help is appreciated :)
 

Answers and Replies

  • #2
73
1

Homework Statement


Two particles P and Q move in an x-y plane. At time t seconds the position vector of P is [itex](t^2\hat{i}+4t\hat{j})m[/itex] and Q is [itex](2t\hat{i}+(t+1)\hat{j})m[/itex].

A: Prove the particle never collide
B: Show that the velocity of Q is constant, and calcualte the magnitude and direction
C: Find the value for t when P and Q have paralell velocities and find the distance between them at this point.


Homework Equations


?


The Attempt at a Solution


Its part C that is confusing me but incase I messed the others up I will post them as well.

Part A:
For the particles to collide their position vectors will equal each other at the same time so setting their I an J components to equal eachother should produce any times they are equal. First their I components.
[tex]
2t=t^2 \\
0=t^2 - 2t \\
0=t(t-2)
[/tex]
Therefore when t=0 and t=2 the I components are the same. And now the J components.
[tex]
t+1=4t \\
1=3t \\
t=\frac{1}{3}
[/tex]
Therefore their J components are only equal when t=1/3, therefore they never collide as the I and J's are never the same at the same time.

Part B:
The velocity of Q will be constant if neither of the components are functions of time so differentiation will find me the velocity vector
[tex]
\dot{r_Q}=2\hat{i}+1\hat{j}
[/tex]
Therefore the velocity is constant as neither are functions of time. To find the magnitude next
[tex]
\sqrt{2^2+1^2}=\sqrt{5}m/s
[/tex]
And the direction
[tex]
tan^{-1}(\frac{1}{2})=26.57°
[/tex]

Part C:
To find the time when P and Q have parallel velocities first P's velocity vector needs to be found
[tex]
\dot{r_P}=2t\hat{i}+4\hat{j}
[/tex]
It can be seen that both P and Q have constant J components therefore they will be parallel when the I components are the same.
[tex]
2t=2 \\
t=1
[/tex]

However the answer to part C is apparently 4 not 1. Any help is appreciated :)
Hello FaraDazed,
For part C you are asked to find t for parallel vectors.What you are doing is making them equal (by equating both components ).When two vectors are parallel their components are proportional. I think this should give you the correct answer.
If you use t=1 as you got in C
v(p)=[itex](2\hat{i}+4\hat{j})[/itex]
and v(q)=[itex](2\hat{i}+\hat{j})[/itex]
They are neither equal nor parallel.
Just try making the velocity components proportional.
Regards
Yukoel
 
  • #3
CAF123
Gold Member
2,915
88
If two vectors are parallel, then one can be expressed as a scalar multiple of the other. i.e we can express this as $$\begin{pmatrix} {2}\\{1} \end{pmatrix} = \alpha \begin{pmatrix} {2t}\\{4} \end{pmatrix}$$ Solve this system for ##\alpha## and hence for ##t##.
 
  • #4
347
2
Thank you guys I get it now, thanks :)
 

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