Physics Help

loopstah

loopstah

loopstah

Soldato
Joined
10 Nov 2004
Posts
2,878
Location
My secret mountain base!
I'm currently working through the "Physics for You" GCSE textbook to refresh my physics knowledge for my course and I'm stumped at one of the questions as I just can't figure out what I should be doing.

Can anyone shed some light on the subject?

A uniform metre rule of mass 100g balances at the 40cm mark when a mass X is placed at the 10cm mark. What is the value of X?

All the other questions have had weights on each side so I'm stumped with only 1 weight. Mechanics never was my strong point even though it is only GCSE level. :o

The answer is apparently 33.3g but I don't know how to get it.
 
Whoops re-reading the question I thought there were two masses balanced on one half of the ruler. The ruler itself weighs 100g!

So if its uniform and balanced at 40cm, then thats 40g on one side, and 60g on the other.

That's as far as my maths takes me.
 
loopstah said:
All the other questions have had weights on each side so I'm stumped with only 1 weight. Mechanics never was my strong point even though it is only GCSE level. :o
Click to expand...
You can treat the ruler as if all its mass were concentrated at the center of mass. So it's the same as a weightless ruler with a weight of 100g at the 50cm mark. You now have weights on each side...
 
Weight acts through centre of section so (distances from pivot)

(40g x 20cm) + (X x 30cm) = (60g x 30cm)

solve this X=33.3

QED?
 
Ahh. That makes it all clearer now. I didn't think of putting the whole mass in the centre although it makes sense now I think of it. Doh!

Thanks for the help. :D
 
You must log in or register to reply here.
Back
Top Bottom