Moments Flashcards
(40 cards)
what is centre of mass (or gravity)
the position where the weight of an object appears to act from
it always wants to be in the lowest possible position
talk about humans and centre of mass
humans have to do work to raise the centre of mass
e.g. sit up straight and not slouch
what is the equation for moments?
moments (Nm) = force (N) x perpendicular distance from the pivot to the force (m)
M = Fd
what happened when Cici (more mass) and Tess (less mass) stand on different ends of a balance?
Why?

the balance tipped so that Cici went down to the ground and Tess was raised
Cici and Tess are the same distance away from the pivot but Cici has a greater force han Tess and a thus a greater moment to Tess
the plank is unbalanced
CIci’s moment is anti-clockwise and Tess’ clockwise from the pivot thus the plank rotates anti-clockwise
Explain why the see-saw became balanced with Tess and Nikita in this position relative to Cici

the sum of Tess and Nikita’s force is greater than Cici’s but their distance from the pivot is smaller thus their moments are equal
Cici’s moment is 1200Nm anti-clockwise and Tess’ and Nikita’s is clockwise from the pivot
what is the principle of moments?
the sum of the clockwise moments is equal to the sum of the anti-clockwise moments when the system is in equilibrium
moments are caused by …, but they are not … themselves
they cause objects to turn or …
like forces, they are … quantities and have a direction; these are usually restricted to … or …
moments are caused by forces, but they are not forces themselves
they cause objects to turn or rotate
like forces, they are vector quantities and have a direction; these are usually restricted to clockwise or anti-clockwise
what are moments measured in?
Nm
a door handle is 0.75m from the hinge
the door is closed and a person pulls on the handle with a force of 20N
calculate the moment produced by the person
M = f x d
= 20 x 0.75
= 15Nm
a bus driver uses his right hand to turn his steering wheel
the diameter of the wheel is 40cm and she pulls down on the wheel with a force of 15N
calculate the moment
M = f x d
= 15 x 40
= 300Nm
which direction will the wheel turn?

clockwise
describe two ways she could increase the moment on the wheel

pull down with a greater force (force)
make the wheel bigger (distance)
how do you find the centre of mass of an object?
suspend it from two different points (see figure)

what is the method for finding the centre of mass of a piece of flat card?
suspend the flat card from a pin and let it swing freely
when you do this, the centre of mass of the card is directly below the pin
you can use a ‘plumbline’ to draw a veritcal line on the card from the pin downwards
now repeat this procedure with the card suspended from a different point to give another line
the centre of mass of the card is where the two lines cross

What are A and B?

pivots
A unifrom bridge has a weight of 20,000N. Two supports are placed near the ends, 7.5m from the centre of mass of the bridge. State the value of the support forces at A and B

A = 10,000 N
B = 10,000 N
A car with a weight of 10,000N now parks at the centre of the bridge. State the extra force exerted at A and B

Extra force at A = 5,000 N
Extra force at B = 5,000 N
A similar car drives from A to B at a constant speed. Describe the changes in the support forces acting on A and B, stating values of significant positions of the car along the bridge. You can state the total forces (which include the weight of the bridge), or extra forces (ignoring the weight of the bridge)

as the car moves away from A, the force acting on A decreases as the force acting on B increases in a directly proportional way in order for the bridge to stay in equilibrium
when the car has moves 1/4 of the way across the bridge the force at A would be 7500N and at B it would be 2500N (exluding the weight of the bridge)
Fa + Fb = the weight of the car when ignoring the weight of the bridge
how do you calculate the mass of ruler?
Wm x d1 = Wr x d2

Explain why Fb increases as the car approaches it

the bridge is in equilibrium
the anti-clockwise and clock-wise moments are equal
Fa + Fb = weight of the car as the resultant force is zero (the bridge is stationary)
the car moving away from A increases the distance from A to car and therefore the anti-clockwise moment about A increases
the car moving closer to B decreases the distance from B to the car but increases the force about B and therefore the clockwise moment increases
Calulate the force of F

M = f x d
10 x 2
= 20 Nm
M = f x d
20 = F x 4
F = 5N
Calculate the distance of d

M= f x d
= 18 x 30
=540 Nm
M= f x d
540 = 36 x d
d = 15
Calculate the force of F (assume cm is m instead)

M = f x d
= 100 x 40
= 4000 Nm
M = f x d
= 40 x 20
= 800 Nm
4000 - 800 = 3200
3200 = 50 x F
F = 64 N
Explain why the ruler is equilibrium. Remember to refer to moments.
“A student investigated equilibrium by using the equiptment in the diagram. The 2N wieght is 60cm from the pivot and the Newton meter is 10.5cm from the pivot.”

the sum of the clockwise moments is equal to the sum of the anti-clockwise moments
(principle of moments)









