Homework Help: How much force is required to start a block moving on level ground?

grrer · Feb 28, 2010

1. The problem statement, all variables and given/known data
The Egyptian's sled system had a rather high coefficient of static friction (0.7) but a much lower coefficient of kinetic friction (0.3) How much force is required to start a block moving on level ground?
We know the block is 1980 kg

and after that:
Assume that one person is able to apply 100N the block. Using this value, how many people does it take to pull a block at constance velocity along flat ground?

2. Relevant equations

3. The attempt at a solution
Do i do Fs - Fk to find the answer to the first question? or is it just Fs?

aim1732 · Feb 28, 2010

Just Fs .

grrer · Feb 28, 2010

and is the second one just fk?

aim1732 · Feb 28, 2010

Well why do you think static and kinetic friction are different forces? They are just names of different phases of the same force. So use Fs when block does not move and Fk when it has started moving.

grrer · Feb 28, 2010

thanks!
Assume that a comfortable walking speed for an average person is 1.33m/s Assume that the workers can only walk at half this speed while they were pulling a block, but that they would walk at this speed on the trip from the pyramid back to the quarry. At these speeds, how many blocks can one crew of 59 people, move from the quarry (5km away) to the pyramid in one ten hour work day?

aim1732 · Feb 28, 2010

I am guessing that this one is related to the last. You can use simple kinematics for this- time for one round trip, how many stones for the crew in one round trip and then for the day.You have everything just go for it.

grrer · Feb 28, 2010

what forumula do I use? I'm confused what to do with the velocity

aim1732 · Feb 28, 2010

Well v=distance/t, what else?

grrer · Feb 28, 2010

:S and then?

aim1732 · Feb 28, 2010

Are you overcomplicating? Start with time for one round trip for one block. Since you have already calculated the people required for one block, you can calculate the no. of blocks a group of 59 would move in one round trip.The time for one trip divided by total working time and multiplied by no. of blocks in one trip is your answer.

grrer · Feb 28, 2010

wow, yes i was over complicating it, thank you so much!

grrer · Feb 28, 2010

sorry maybe i just dont understand what this question is asking but:
How long does it take a crew to accelerate a block to the walking speed?