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Rope pulling a block problem

by kaotic
Tags: block, rope
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Nov4-03, 03:53 PM
P: n/a
This is the first time I've posted here. I am having a problem with an assigned problem. I know the policy here is to not give out answers, and I'm not looking for that. I just need a starting point. Here's the problem:

A 5.00 kg block is pulled along a horizontal frictionless floor by a cord that exerts a force of magnitude 12.0 N at an angle of 25 degrees above horizontal. a) make a free-body diagram and determine the magnitude of the block's velocity b) the F magnitude is slowly increased. What is the value (of the force, I'm assuming) just before the block is lifted (completely) off the floor? c) What is the magnitude of the block's acceleration just before it is lifted (completely) off the floor.

OK, I have done part a), drawing the free body diagram and figured out the magnitude of the acceleration (+2.17m/s^2). I also know that to do b), I need to find the value of the force when the normal force is equal to zero. From there, I can't figure out where to go.

If someone could help me out with an equation to use and/or other tips, it would be much appreciated.

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Nov4-03, 04:15 PM
Sci Advisor
PF Gold
P: 478
I'm assuming that you don't have to worry about the block pivoting about its connection point to the string for this problem, so:

You know that the normal force will be zero in part b. The only other forces acting on the block are its weight and the tension of the string. This means that the vertical component of the tension will exactly balance out the weight. Since you know one component of the tension and the angle the string is at, you can use that to find F = Fy/sin(25).

The block's acceleration is then found the same way as part a: there is 0 acceleration in the y direction and the x-component of the tension provides an acceleration in that direction.

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