Q-1: In order to open a particular bolt, a moment of 50 N.m is required. A

spanner of lenght D=0.20 m is used; how much force is required?

 

Solution:                                                    moment = force x distance of force

from pivot.

now plug in values in above formula               50        = force x 0.20 

Therefore,                                                 force = 50 / 0.20 = 250 N

So you have to apply a force of 250 N to open the bolt. However, if you are

not healthy enough to apply this much force to produce a moment of 50 N.m

then buy a larger spanner. Lets try same question with longer spanner.

Q-2: Same example (Q-1) but lets say spanner is 1 meter. Therefore,

                  moment = force x distance of force from pivot

                   50        =  force x 1         

                 force = 50 Newtons

With smaller spanner you need a heck of 250 N and now with longer spanner

you need only 50 N. So with a larger spanner you can still open the bolt even

when you are sick.

Example# 1:

A builder tried to remove a wooden post from the ground by pulling with a

rope.

(a) (i) The builder attached a rope to hole A, 0.8 m above the ground.

He pulled with a horizontal force of 300 N.

Calculate the turning moment about the pivot P.

Give the unit.

.................................................................................          300 x 0.8 =240 N.m

.................................................................................(2)

(ii) He then attached a rope to hole B, 1.6 m above the ground.He pulled with

a horizontal force.

What force would produce the same turning moment as before?

...........................................................N        (1)             120 N (Total 3 marks)

(b) state the principle of moments for a body in equilibrium.

Principle of moment states that

In equilibrium,

Total clock wise moment = Total anti-clock wise moment.

(c) make calculations using moment of a force = force × perpendicular

distance from the pivot and the principle of moments.

(d) describe how to verify the principle of moments.

Example#2:

Zena has a model plane attached to a rod as shown below.

The plane is balanced by a sliding counterweight.

(a) The rod is balanced horizontally.

(i) Calculate the turning moment produced by the counterweight about

the pivot. Give the unit.

______________________________________________________________

____________     (2)

______________________________________________________________

____________         Ans:100 N.cm

(ii) What is the turning moment produced by the plane about the pivot?

______________________________________________________________

____________      (1)

______________________________________________________________

____________          Ans:100 N.cm; since balanced moments should be

same.

(iii) Calculate the weight, W, of the plane.

______________________________________________________________

____________      (1)

______________________________________________________________

_____ N.                  Ans: 10 N.              

(e) describe how to determine the position of the centre of mass of a plane

lamina.

(f) describe qualitatively the effect of the position of the centre of mass on the

stability of simple objects.

Center of gravity is a point at which whole weight of the object appears to

acts. This point is also called center of mass.

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