# A level Maths, "easy" Moments questions

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#1
I have just learn about moments and am struggling to even draw the diagram for this question let alone answer it
"A uniform horizontal bar of mass 5kg has length 30cm and rests on two vertical supports,10cm and 22cm from its left-hand side. Find the magnitude of the reaction force at each of the supports"
0
1 year ago
#2
(Original post by Mermaidqueen)
I have just learn about moments and am struggling to even draw the diagram for this question let alone answer it
"A uniform horizontal bar of mass 5kg has length 30cm and rests on two vertical supports,10cm and 22cm from its left-hand side. Find the magnitude of the reaction force at each of the supports"
Draw a thin bar - mark the middle - that's where it's weight acts as the bar is uniform. Draw a small triangle to represent a support - put one 10 cm from the left and draw an arrow upwards to represent the reaction. Put the other 22cm from the left ie the other side of the middle.

Now resolve to get one equation and take moments about on of the supports to get the reaction. Post your working ...
0
1 year ago
#3
(Original post by Mermaidqueen)
I have just learn about moments and am struggling to even draw the diagram for this question let alone answer it
"A uniform horizontal bar of mass 5kg has length 30cm and rests on two vertical supports,10cm and 22cm from its left-hand side. Find the magnitude of the reaction force at each of the supports"
First off, draw the diagram, you will not be able to do anything in mechanics if you dont have a solid diagram to help answer it.
Once you have this completed be very cautious with the word choice in the question. "uniform" for example means that the rod has a centre of mass located in the centre so it is not unevenly distributed about the rod. You can draw a 5g force going down in the middle of the rod. This also tells you the 5kg acts 15cm away from the left side as its in the centre of the 30cm rod.
Secondly, make sure you have your supports or 'pivots' these will each have their own "Reaction force" acting upwards.
Thirdly "take moments" about each of the pivots, taking into account their individual "clockwise" and "anticlockwise" moments, (moments being = to force x perpendicular distance). This should give you simultaneous equations that you use to find the individual Reaction forces going upwards from each pivot.
1
#4
But to draw the little triangles, if I mark the middle so that there is 15cm to left of it and 15cm to the right. If i then want to draw the first small triangle does it mean that it is 5cm away from the middle (towards left side) and 7cm away from the middle (on the right side)?
(Original post by Muttley79)
Draw a thin bar - mark the middle - that's where it's weight acts as the bar is uniform. Draw a small triangle to represent a support - put one 10 cm from the left and draw an arrow upwards to represent the reaction. Put the other 22cm from the left ie the other side of the middle.

Now resolve to get one equation and take moments about on of the supports to get the reaction. Post your working ...
0
#5
Yes, I tried to draw the supports but I'm struggling to draw them I don't understand how far they are from the centre of the bar

(Original post by thoque)
First off, draw the diagram, you will not be able to do anything in mechanics if you dont have a solid diagram to help answer it.
Once you have this completed be very cautious with the word choice in the question. "uniform" for example means that the rod has a centre of mass located in the centre so it is not unevenly distributed about the rod. You can draw a 5g force going down in the middle of the rod. This also tells you the 5kg acts 15cm away from the left side as its in the centre of the 30cm rod.
Secondly, make sure you have your supports or 'pivots' these will each have their own "Reaction force" acting upwards.
Thirdly "take moments" about each of the pivots, taking into account their individual "clockwise" and "anticlockwise" moments, (moments being = to force x perpendicular distance). This should give you simultaneous equations that you use to find the individual Reaction forces going upwards from each pivot.
0
1 year ago
#6
One support is 10 from the left, the other is 22 from the left.
Easy to work out the equivalent distances from the center?
(Original post by Mermaidqueen)
Yes, I tried to draw the supports but I'm struggling to draw them I don't understand how far they are from the centre of the bar
0
1 year ago
#7
(Original post by Mermaidqueen)
But to draw the little triangles, if I mark the middle so that there is 15cm to left of it and 15cm to the right. If i then want to draw the first small triangle does it mean that it is 5cm away from the middle (towards left side) and 7cm away from the middle (on the right side)?
Yes - sorry I've been marking books ..
1
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