Module 3 Moments

Question 1

What is the centre of mass/gravity of an object?

Answer 1

single point through which the whole weight of the object acts through

Question 2

What can be considered about the weight of a uniform object?

Answer 2

acts in the symmetrical centre of the object

Question 3

Describe how you would find the centre of gravity of an irregular object?

Answer 3

• Hand object freely from a point
• Use a plumb line(bob) to draw a vertical line down the object
• Hang the object from a different point and repeat two more times
• The point where all the lines intersect is the centre of gravity

Question 4

What is a plumb bob?

Answer 4

a string with a weight attached at one end

Question 5

Define a moment

Answer 5

force times the perpendicular distance from line of action of the force to the pivot

Question 6

What is the unit for moments?

Answer 6

Nm (newton meters)

Question 7

How to find the perpendicular distance of a force from pivot to calculate a moment?

Answer 7

Draw the line along the direction that the force acts

Go to the pivot

Draw a line through the pivot that meets the line of force at right angles

Question 8

What is the principal of moments?

Answer 8

when an object is in equilibrium, the sum of anticlockwise moments about any point is equal to the sum of clockwise moments about the same point

Question 9

When calculating moments about a point when an object is in equilibrium, what must be stated?

Answer 9

principal of moments - sum of clockwise moments = sum of anticlockwise about the same point

Question 10

Re moments
When there are two unknown forces acting on an object, how do you find the magnitude of the forces?

Answer 10

use the point where one of the forces acts as a pivot
(moment of this force = 0Nm)

apply the principle of moments

calculate the magnitude of the other unknown force

Question 11

Why can the moment of a weight acting through the centre of rotation be considered 0Nm?

Answer 11

perpendicular distance to centre of rotation is 0 m

Moment = force x perpendicular distance so moment = 0Nm

Question 12

What is a couple?

Answer 12

a pair of forces equal in size (magnitude) acting parallel to each other but in opposite direction

Question 13

What is torque?

Answer 13

the turning effect/moment of a couple

Question 14

Definition of a torque

Answer 14

size of one force times the perpendicular distance between the forces in a couple

Question 15

What is the same as the torque of a couple?

Answer 15

resultant moment

taking clockwise as positive and anticlockwise as negative

Question 16

Describe in full how to find which direction of force would give the largest moment?

Answer 16

• Continue the lines the lines of forces in a direction
• Find the perpendicular between pivot and line of force (right angle on the line of force)