General Engineering Stuff... (1 Viewer)

[inasero]

This is a question from Schlenker/McKern Engineering Studies book:

Ok, theres a boat of mass 2 tonnes resting on a ramp inclined at 20 degrees to the horizontal...coefficient of friction is 0.3.....calculate the force required to push the boat DOWN the ramp and keep it moving at a constant velocity. Sounds Simple? Not quite...I calculated the answer to be NEGATIVE (?!) 1.2 kN, whereas the answer is positive 1.2 kN...can someone explain what's going on here?

 

[tieki]

well, if it weighs that much, and with that coefficient, it won't go anywhere unless you push it downhill, obviously...lol

Use a force triangle with 2000 x 9.8 in the vertical pointing down, using ur angle of static friction (tan-1 0.3), and the third force...haven't got my textbook on me atm. It sounds like a problem with the direction of your friction or pushing force...which direction was the negative force pushing, btw? if it was pushing uphill, then ur answer's fine...tell me how it goes

 

[inasero]

I thought it was pretty much self explanatory?! Oh well...
Check out the attachment and you'll see~

 

[inasero]

k here tis

 

[tieki]

I think I found ur problem from the diargam...isn't N (the vertical component of the weight force) the wrong way around...? See the diagram I uploaded
I'm just doing it now...upload it in a minute

 

[tieki]

Um...I won't upload it, coz I came out with the same answer as you...it makes sense, though, coz the parallel component of the weight force is bigger than the frictional force going uphill, thus requiring P to be pushing uphill to stop it sliding...if the textbook says it's 1.2kN, then I'm stumped, because it should be -1.2kN, otherwise that boat's gonna go down the hill pretty fast...lol

 

[tieki]

Um...force is a vector, isn't it? Therefore we can't have a negative force, can we? Just a positive force in a certain direction...? So the answer is not -1.2kN down the hill, it's 1.2kN up the hill...did the answers specify a direction for the force, or just a value? Because if they only told a value, then they're wrong...they need to say a direction as well...

 

[inasero]

yeh I'm tending to think along your lines...
anyways what's ur real name? Is it Tieki?!

 

[tieki]

no, it's Kieran Tie...I just write tieki coz it's my login name on the network at school...last name then first 2 letters of first name...lol