Chapter 4 - Turning Effect of Forces

Question 1

Define “moment of a force (torque)”

Answer 1

The product of a force and the perpendicular distance from the line of action of the force to the pivot

Question 2

Formula for calculation of moment

Answer 2

M = F x d

Moments of a force = Force x Perpendicular distance from the line of action of the force to the pivot

Question 3

What is the SI unit for moment?

Answer 3

Newton metre (N m)

Question 4

Moment is a ________ (scalar/vector) quantity

Answer 4

vector

has both magnitude and direction

Question 5

The direction of a moment is either ________ or ________

Answer 5

clockwise

anticlockwise

Question 6

A pivot can also be called a ________ or a ________

Answer 6

turning point

fulcrum

Question 7

When is the moment due to a non-zero force zero?

Answer 7

When the line of action of the force passes through the pivot

Question 8

What is meant by the term “uniform”?

Answer 8

The centre of gravity is at the middle point of the object, through which the whole weight will act.

Question 9

State the principle of moments

Answer 9

When an object is at equilibrium, the sum of clockwise moments about any point is equal to the sum of anti-clockwise moments about the same point.

Question 10

When an object is at equilibrium, it is at a ________ state of motion where it is either moving ________ or ________

Answer 10

steady
at constant velocity
at rest

Question 11

What are the two conditions for an object to be in equilibrium?

Answer 11

1. The resultant of all forces acting on the object is equal to zero
2. The resultant moment about any point on the object is equal to zero

Question 12

Define “centre of gravity”

Answer 12

The point through which the entire weight of the object appears to act

Question 13

Define “stability”

Answer 13

A measure of a body’s ability to return to its original position after being tilted slightly

Question 14

How can an object be made more stable?

Answer 14

• by lowering its centre of gravity

- by increasing the area of its base

Question 15

What are the 3 types of equilibrium (for objects whose positions of C.G. are higher than the pivot)?

Answer 15

• stable equilibrium
• unstable equilibrium
• neutral equilibrium

Question 16

What happens when an object of stable equilibrium is slightly displaced?

Answer 16

• The C.G. of the object is raised
• The vertical line through its C.G. still falls within its base
• Its weight has a (clockwise/anticlockwise) moment about the pivot that causes the object to rotate back/return to its original position

Question 17

What happens when an object of unstable equilibrium is slightly displaced?

Answer 17

• The C.G. of the object is lowered
• The vertical line through its C.G. falls outside its base
• Its weight has a (clockwise/anticlockwise) moment about the pivot that causes the object to topple

Question 18

What happens when an object of neutral equilibrium is slightly displaced?

Answer 18

• The C.G. of the object remains at the same horizontal level
• The vertical line through its C.G. will always pass through its pivot
• Its weight has zero moment about the pivot and will not cause it to topple