M1 Moments Question Watch

php_cat_tsr
Badges: 0
#1
Report Thread starter 8 years ago
#1
Hello!



I cant do the last part of this question - even saw the answer but it didnt make sense!

a) 131 N
b) 163 N
c) Technically I think 20g+10g= 163
so what I did is taking moments about D: 20gx+10g= 163 (x being the distance I need to find)
I got x=0.33

The answer though is:
Moments about D: 20gx-10g=0

Why do both forces downwards equal 0? shouldnt they be equal to D - i.e. 163?
0
reply
Rohit93
Badges: 0
Rep:
?
#2
Report 8 years ago
#2
Im not sure if this is right, but, if its on the point of tilting about D then there is no reaction at D
0
reply
WCHS
Badges: 0
Rep:
?
#3
Report 8 years ago
#3
(Original post by Rohit93)
Im not sure if this is right, but, if its on the point of tilting about D then there is no reaction at D

If its on the point of tilting about D then reaction at B would equal zero...or at least thats wot i thought...
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#4
Report 8 years ago
#4
if it's about to tilt about D, the reaction at B would be zero...

I duno how much that helps lol i've been revising m1 for ages... was supposed be taking a break lol
0
reply
Rohit93
Badges: 0
Rep:
?
#5
Report 8 years ago
#5
Ah, right thanks guys
0
reply
php_cat_tsr
Badges: 0
#6
Report Thread starter 8 years ago
#6
yeh the reaction at B is zero, but I still dont understand it!
0
reply
WCHS
Badges: 0
Rep:
?
#7
Report 8 years ago
#7
(Original post by php_cat_tsr)
yeh the reaction at B is zero, but I still dont understand it!
Yuh sure the answers for A) and B) are write?
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#8
Report 8 years ago
#8
well it's in equilibrium so moments about D:

DF X 20g = 10g
cos if the plank is about to tilt, but hasn't, the child would now have to be on the other side to balance the weight (of the plank)
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#9
Report 8 years ago
#9
what you're missing is that the reaction at D changes completely once the reaction at B becomes zero...
0
reply
php_cat_tsr
Badges: 0
#10
Report Thread starter 8 years ago
#10
reaction at b is zero,
reaction at d we dont know.

its tilting about d - so how does this lead up to 20gx=10g (x being the length we want to find)?
0
reply
WCHS
Badges: 0
Rep:
?
#11
Report 8 years ago
#11
(Original post by php_cat_tsr)
reaction at b is zero,
reaction at d we dont know.

its tilting about d - so how does this lead up to 20gx=10g (x being the length we want to find)?

You do noe da reaction at D... Resolve vertically...D= 20g+10g...ryt(?)
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#12
Report 8 years ago
#12
(Original post by php_cat_tsr)
reaction at b is zero,
reaction at d we dont know.

its tilting about d - so how does this lead up to 20gx=10g (x being the length we want to find)?
It's in equilibrium so anticlockwise moments = clockwise moments

10g (weight of plank) X 1m (distance of center of mass from D)

=

20g (weight of child) X x (distance of child from D)
0
reply
php_cat_tsr
Badges: 0
#13
Report Thread starter 8 years ago
#13
(Original post by Insomniac_1901)
It's in equilibrium so anticlockwise moments = clockwise moments

10g (weight of plank) X 1m (distance of center of mass from D)

=

20g (weight of child) X x (distance of child from D)
ah right - that didnt click before!
thanks.
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#14
Report 8 years ago
#14
(Original post by php_cat_tsr)
ah right - that didnt click before!
thanks.

no problem

I am SO scared for this exam lol
0
reply
php_cat_tsr
Badges: 0
#15
Report Thread starter 8 years ago
#15
(Original post by Insomniac_1901)
no problem

I am SO scared for this exam lol
lol sarcastic!
0
reply
dontkillhannah
Badges: 0
Rep:
?
#16
Report 8 years ago
#16
may i just ask, what year is this paper from?
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#17
Report 8 years ago
#17
(Original post by php_cat_tsr)
lol sarcastic!

no really lol, for the first time in living memory, that's not sarcasm...
0
reply
anshul95
Badges: 11
Rep:
?
#18
Report 8 years ago
#18
If I got a negative answer because I assumed the child sat on the left of D, but then stated that it must therefore lie 0.5m to right of D would I still get the marks?
0
reply
Insomniac_1901
Badges: 2
Rep:
?
#19
Report 8 years ago
#19
(Original post by anshul95)
If I got a negative answer because I assumed the child sat on the left of D, but then stated that it must therefore lie 0.5m to right of D would I still get the marks?
yes. as long you make it clear that the -ve displacement only means its the opposite direction from what you assumed it would be, so 0.5 to the right instead of left.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • University of East Anglia
    All Departments Open 13:00-17:00. Find out more about our diverse range of subject areas and career progression in the Arts & Humanities, Social Sciences, Medicine & Health Sciences, and the Sciences. Postgraduate
    Wed, 30 Jan '19
  • Aston University
    Postgraduate Open Day Postgraduate
    Wed, 30 Jan '19
  • Solent University
    Careers in maritime Undergraduate
    Sat, 2 Feb '19

Brexit: Given the chance now, would you vote leave or remain?

Remain (1380)
79.49%
Leave (356)
20.51%

Watched Threads

View All