HDE Section 2: Moments

Question 1

What is a moment?

Answer 1

The moment is the turning effect of a force.

Question 2

What is the principle of moments?

Answer 2

The principle of moments is that the sum of the clockwise moments must be equal to the sum of anticlockwise moments if an object is in equilibrium (not accelerating).

Question 3

A delivery woman holds a wardrobe prior to pushing it into position by applying a force, P, at right angles to the wardrobe door. The mass of the wardrobe is 40 kg and she applies the force at a point 1.60 m from the bottom of the wardrobe. In this equilibrium position, the centre of gravity, G, is vertically above a point 12 cm to the left of the pivot.

Use the principle of moments to calculate the size of the force that she applies to keep the wardrobe in this position
Determine the value of the upward force, N, of the floor on the corner of the base

Answer 3

29N

380N

Question 4

Cranes lift heavy building materials using a horizontal arm called a jib. A hook moves up and down the jib and can lift loads from different points. Cranes have a counterweight balanced at the other end of the jib.

Use your understanding of moments to explain why cranes need to have a counterbalance. Why is it useful to be able to lift loads from different points on the jib?

Answer 4

You have stated that without the counterweight, the crane would topple over.

You have stated that the counter-weight creates a moment that opposes the moment from the load.

You have explained that in order to balance the crane, you need to make the clockwise moment, and anticlockwise moment, equal.

You have stated that heavier masses can be lifted closer to the pivot in order to equal the moment of smaller masses lifted further away from the pivot. This will ensure that the clockwise and anticlockwise moments are equal.

Question 5

What is a couple?

What is torque?

Answer 5

We often apply two moments to an object - for example when turning a tap. Together these are known as a couple. The overall turning effect is known as the torque.

Question 6

A 10m ladder is leaning against a wall. Its centre of mass is at the centre of the ladder. If the 5 kg ladder is leaning at 30o above the horizontal, what is the force exerted by the wall to keep the ladder in its position?

Answer 6

Anti-clockwise moments = (5 x 9.81) x 5 cos(30)
= 212.39 N

Clockwise moments = F x 10 sin(30)
= 5F

Principle of moments:
anticlockwise moments = clockwise moments

Therefore 212.3 = 5F
F = 42.5N