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Document Title
Fundamentals of Stress and Vibration Chapter Title
[A Practical guide for aspiring Designers / Analysts] 2. Engineering Mechanics

Moment due to normal force is given by:

M normal force = N ∗ 0.25 = 200 ∗ 0.25 = 50 Nm

It can be observed that the moment due to weight of the bracket is far less compared to moment
due to normal force. However, if it were the other way around, then, the bracket would have rotated
(pivoting at the corner) or the contact pressure with the wall would have varied linearly. In the
worst case, the bracket would have lost contact with the wall.

In this case, the bracket is in equilibrium and maintains uniform contact pressure with the wall.

Example 2: a block is given a push on a rough table, as shown in [Fig 2.55], with an initial
velocity of 20m/s. The friction coefficient between the block and the table is 0.5. Find the length of
2
the table to keep the block from falling off. Assume the acceleration due to gravity to be 10 m s .

[Fig 2.55: Block travelling on a table]

Solution: the deceleration of the block due to friction is given by:

Friction force μmg
2
deceleration (−a) = = = μg = 0.5 ∗ 10 = 5 N/m
mass of the block m
The distance travelled by the block is given by:

v = U + 2as = 0 = 20 + 2 −5 ∗ s = s (length of table) = 40m
2
2
2
Neglecting the dimensions of the block, the length of the table must be at least 40m to prevent the
block from falling off.

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