Physics help please...

Miscellaneous and Personal Stuff I Must Share
1

Two pretty simple questions that I just can’t find help for on the net or . First, in this two pulley system: http://www.webassign.net/hrw/07_32.gif
what’s the relationship between how far you pull the free section of rope down and how far the weight raises. Like, say you pull the rope down 3 meters, how far would the weight rise? I assumed that it would be 3m, but that was incorrect. I couldn’t think of any other possibilities :frowning: , and there’s no other info given on the system, so I know it’s a simple answer.

The second I’ve asked already, but my answer is incorrect so I must have misunderstood.
“A floating ice block is pushed through a displacement **d = (20 m)i - (10 m)j[B/] along a straight embankment by rushing water, which exerts a force F = (156 N)i - (157 N)j on the block.”

I simply don’t understand the notation. Does this mean that there’s a 156N force pushing the block 20m, say, north, and a 157N force pushing the block 10m another direction? I’m supposed to find total work, but I can’t use the Pythag since I don’t know that it’s a right angle, and I can’t use the usual cos or sin theta since yet again I have no angle between the two directions of force.

Thanks for any help that you may give my ignorant self.

2

The weight would rise 1.5m i.e. It would rise half of what you would pull the rope.

You have the right idea in the second one. Think of i and j as the x- & y-axis or north & west axis.

Work = Force * Displacement.

3

Thank you, that simple info about the pulleys helps me to solve 4 easy questions.
OK, my question about the second problem, do I assume it’s a right triangle? What I did was multiply the i force to the i distance, likewise with the j’s, and then used the pythag theorem to get the total work. Ended up with 15680Joules (obviously way off). Any hints would be appreciated, regardless of how general.

4

Yes, you can consider them to be at right angles to each other. You worked it out the right way, you’ve just got the wrong answer.

Try again and post you working so I can see where you’ve gone wrong.

5

10 length x 157 force= 1570
20 length x 156 force= 3120
1570^2 + 3120^2= 3492.74

Hope that’s right, I only get one more chance to enter answers.