4. Centre of Mass and Moments

Question 1

Define the centre of mass

Answer 1

The single point in an object that you can consider its whole weight to act through (whatever the orientation) and through which a single force has no turning effect

Question 2

Where is the centre of mass for a uniform regular solid?

Answer 2

At the centre of the object

Question 3

Is the centre of mass always on the body?

Answer 3

No - it can fall outside the body

Question 4

Describe the method of finding the centre of mass of an object

Answer 4

1. Hang object freely from a point (e.g. a corner)
2. Draw a vertical line downwards from the point of suspension by using a plumb bob to get your line exactly vertical
3. Hang the object from a different point and draw another vertical line
4. The centre of mass is where the two lines cross

Question 5

When will an object topple over?

Answer 5

If the line of action of its weight falls outside its base area because a resultant moment occurs

Question 6

What two conditions allow for a stable object?

Answer 6

1. Low centre of mass

2. Wide base area

Question 7

Name an example of an object that is stable

Answer 7

Racing cars

Question 8

Describe the method of finding the forces on supports when an object is on a plank held up by 2 supports

Answer 8

1. Treat one of the supports as the pivot point
2. Find out how much force is needed to balance moments using the principle of moments and M = F x d
3. The two moments to balance are from the weight of the plank (acting upwards) and the weight of the object (acting downwards)
4. Treat the other support as the pivot point and do the same thing