Tension and Velocity: When Does the Object Return to its Initial Position?

[yayitsrobby]

Homework Statement

http://img141.imageshack.us/img141/9379/photo13ix6.jpg (sorry its backwards)


The graph above shows the velocity versus time for an object moving in a straight line. at what time after t = 0 does the object again pass through its initial position?

A) Between 0 and 1 s
B) 1s
C) Between 1 and 2s
D) 2s
E) Between 2 and 3s

The Attempt at a Solution

i picked B. it shows a velocity of -1 for 1/2s and then a velocity of 1 for 1/2s seconds. i just thought it would travel backwards first and then forwards and end up at the initial position after 1 second.


____ ____
[1kg]-----[2kg]----> F

When the frictionless system shown above is accelerated by an applied force of magnitude the tension in the string between the blocks is

A) 2F
B) F
C) 2/3F
D) 1/2F
E) 1/3F

i don't know where to begin for this


2)

 

[praharmitra]

i picked B. it shows a velocity of -1 for 1/2s and then a velocity of 1 for 1/2s seconds. i just thought it would travel backwards first and then forwards and end up at the initial position after 1 second.

First of all, the graph is a velocity v/s time graph and NOT displacement time graph. So in the first 1/2s velocity is not -1, acceleration is. The velocity is continuously increasing.

Secondly, remember displacement is the signed area under the velocity time graph. Find such a point, where total area is zero.

 

[praharmitra]

do you know how to draw a force-body diagram for such a system?

 

[yayitsrobby]

oh crap. i interpretted the graph wrong. i didn't see that throughout the first second the velocity was zero.

 

[yayitsrobby]

do you know how to draw a force-body diagram for such a system?

not sure

 

[praharmitra]

not sure

Well, an FBD, is a diagram in which you isolate each massive body and draw all the possible forces that are acting on it. This way, when you apply Newton's third law on them, it becomes much easier, as you don't have to bother with two bodies at once. Try to do that for this system.

Isolate both the bodies, and draw all the forces acting on them.

 

[praharmitra]

oh crap. i interpretted the graph wrong. i didn't see that throughout the first second the velocity was zero.

btw, "throughout" the first second, velocity is not zero. The velocity is zero only at time t = 1s.

 

[yayitsrobby]

negative was what i meant