6.5 Stability

Question 1

If a body in stable equilibrium is displaced then released…

Answer 1

… it returns to its equilibrium position

Question 2

Why does an object in stable equilibrium return to equilibrium when it is displaced and then released?

Answer 2

The centre of mass of the object is directly below the point of support when the object is at rest. The weight is considered to act at its centre of mass, thus the support force and the weight are directly equal and opposite to each other when the object is in equilibrium. When it is displaced, the line of action of the weight no longer passes through the point of support so the weight returns it to equilibrium

Question 3

What is an example of unstable equilibrium

Answer 3

A plank balanced on a drum

Question 4

Why are skittles easy to knock over

Answer 4

They are tall so their centre of mass is high and the base is narrow

Question 5

What is tilting

Answer 5

Where an object at rest on a surface is acted on by a force that raises it up on one side

Question 6

Find moment of a tilted bookshelf

Answer 6

Moment=Wb/2
In which:
b is the width of the base
W is the weight

Question 7

For tilting to occur

Answer 7

Fd>Wb/2

Question 8

Why do tilted objects topple after a certain point

Answer 8

The line of action of its weight passes closer and closer to the pivot, if it is tilted so much that the line of action of its weight passes beyond, the object will topple over

Question 9

For equilibrium (for a tall object on a slope)

Answer 9

Parallel to slope
F=Wsinθ
Perpendicular to the slope
Sx+Sy=Wcosθ

Sx is greater than Sy because X is lower than Y